MASTER OF SCIENCE

IN

Amy M. Baylor

Thesis Committee:

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HAWN.

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SEISMICITY OF AN INTRAPLATE SUBMARINE VOLCANO:

LOIHI SEAMOUNT, HAWAI'I

A THESIS SUBMITTED TO THE GRADUATE DIVISION OF THEUNIVERSITY OF HAWAII IN PARTIAL FULFILLMENT OF THE

REQUIREMENTS FOR THE DEGREE OF

OCEANOGRAPHY

MAY 1995

Alexander Malahoff, ChairpersonGary M. McMurtry

Frederick K. Duennebier

Theses for the degree of Master of Science University of Hawaii at Manoa!

I thank my parents most of all for their continual encouragement and praise, evenfor my smallest accomplishments. I also thank them for suggesting that I go on to gradschool; otherwise I never would have dreamed of living in Paradise. To Judi and Bill onthe KQK, who have been my second set of parents, goes a heartfelt thanks for alwaysbeing there for me and for pushing me to get done with my work no wooden spoons!!.

Finally, appreciation goes to my best source of moral support: the Bellows BrewCrew... Doug mahalo for the fun times!, Kevin, Scott & Jacquie, George & Vicki,Skinney, Lizette & Scott, Richard & Denise, Ben & Tracy, Prabash & Radha, Buzzy,Lauren, Justin, Bo, and the umpteen others. I thmk them or maybe curse them?! foralways providing me with ample alternatives ta studying... there was never a dullmoment. I thank my ohana: Elarka, Kristl, and Megan Yuen, for allowing me to live intheir home for my first 9 months in Hawaii and for making me feel so welcome andcomfortable. I sincerely thank Sophia Asghar for the years of mutual griping, letting mevent when I needed it, and always understanding me. To Jacquie MacDonald, my bestfriend, bunjee-jumping buddy, and partner-in-crime: a special mahalo nui loa to her forabsolutely everything.

I would like to share a statement that sort of puts the whole ~duate student ordeal

into perspective and suggests a purpose:

"... Obviously there are no well qualified students of the

Earth, and all of us, in different degrees, dig our own smallspecialised holes and sit in them." -- Bullard, E. C. �960!Response to award of Arthur L. Day medal, Proc. Vol. for

1959, Geol. Soc. Am., p, 92.

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ACKNOWLEDGMENTS

A huge thanks goes to my committee: Alex Malahoff, Gary McMurtry, and FredDuennebier. This group of professors saw me pick up a subject relatively off thebackground I had, let me run with it with total freedom, and encouraged me as much aspossible, Alex, my committee chairman, continuously provided crucial positive feedback and equally crucial funding!. I also appreciate the confidence that he has shown in mesince day one here at UH. Gary was very helpful througli with his insightful suggestionsand his constructive reviews of tny thesis proposal attd drafts and reviews and questions.Fred, the seismologist on the committee, played a major role in answering specificquestions regarding seismology and instrumentation. His and Gary's encouragement was

also very appreciated.

At the Hawaiian Volcano Observatory HVO! I thank Paul Okubo for his assistance

and for allowing me to be a "data raider" at the observatory, even though he was extremelybusy. I could not have pulled it together without his help. Laura Kong, Alice Gripp, andAlvin Tomori were also very helpful, especially duritig the last several months of finishingup the thesis. Renee Ellorda also deserves my thanks in particular for allowing me to crashat her house at Volcano during one of my HVO visits.

I thank my roommates Janie and Andrew for all their encouragement, In the UHoceanography department I thank my officemates, Kimo Zaiger and John Smith. When Imoved into their space, they not only got a new officemate, they got an inquisitive soulwho never seemed to stop asking for advice or help since they and Kevin Kelly were theveterans here, they seemed like the logical trio to bombard with questions!. A specialthanks also goes to Cheryl Komenaka; without her in Alex's office, it would shut down,and Alex and many grad students myself at the top of the list! would suffer.

TABLE OF CONTENTS

ACKNOWLEDGEMENTS ..

LIST OF TABLES ..

LIST OF FIGURES ..

LIST OF PLATES....

CHAPTER 1; INTRODUCTION .

Geologic Setting of Loihi

Previous Work on Loihi...

CHAPTER 2; 1991 OBS DATA

Instrumentation .

Data .

Data Set Selection .

CHAPTER 3: LOIHI EVENTS

OBS Recording of Loihi Activity ..

Background Noise, .

Short-Period Events,

Long-Period Events .

Spindles

Paper Rustling

Pele's Canaries

Discussion .

Conclusions

CHAPTER 4: HAWAII EVENTS,

Examples of Hawaii Events ..

HEA Earthquakes,,

12

12

13

16

23

23

23

25

31

35

. 39

49

52

56

56

UKF Earthquakes

UER/POL Earthquakes

Travel Time Data.......

Conclusions

APPENDIX....

REFERENCES,,

57

.58

62

65

66

.. 77

1. Loihi event families: numbers, durations, frequencies, and percentages ..2. Hawaii Island events: locations, longitudes, and travel time data ..

~T1

LIST OF TABLES

38

.. 54

LIST OF FIGURES

P~ae~Fi re

1. Bathymetric map of the Hawaiian Islands

2. Seabeam bathymetry map of Loihi Summit and South Rift Area .,

3. Loihi and Hawaii earthquake epicenters: 1970-1981

4. Cumulative numbers of Loihi earthquakes: 1961-1994 ..

5. 1986 QBS array and Loihi earthquakes...

6. HVO earthquake identification regions on Hawaii, 19

7. HVO seismic network on Hawaii 20

8. Example of background noise

9. Seismograms and power spectrum of a typical short-period earthquake .,

10. Seismograms and power spectrum of a typical short-period earthquake ..

27

28

11. Seisrnograms and power spectrum of a typical long-period event .,

12. Seismograms and power spectrum of a typical long-period event ..

32

13. Seismograms and power spectrum of a typical spindle event ..

14. Seismograms and power spectrum of a typical spindle event .. .. 37

15. Example of Paper Rustling . 40

16. Example of Pele's Canaries

17. Total number of Loihi earthquakes per hour .

43

46

18. Total number of Loihi earthquakes per day by family 47

19. Total number of minutes of PR and PC per day,

53

21. HEA earthquake; 4.5 HEA

22. UKF earthquake: 1.5 UKF, .

59

60

VI11

20. Map of Hawaii and Loihi with earthquakes recorded by OBS and HVO, .

23, UER/POL earthquake: 2.5 PO ..

24. Theoretical and observed travel time curves for shallow earthquakes ..

25. Theoretical and observed travel time curves for deep earthquakes ..

LIST OF PLATKS

Plate

l. Image of the ocean bottom observatory .

CHAPTER 1: INTRODUCTION

The Hawaiian Island Chain consists of extinct, active, and dormant volcanoes. The

chain's youngest is Loihi seamount, a seismically active submarine volcano located about30 km southeast off the coast of the island of Hawaii. It marks the southernmost position

of the hot spot that produced the >70-million-year-aid Hawaiian-Emperor intraplatevolcanic chain. If its powth continues, it could become the next Hawaiian Island; untilthen, Loihi provides an ideal natural research laboratory for various aspects of volcanicdevelopment. Research in fields such as volcanology, bialogy, chemistry, geology, andseismology is feasible given Loihi's level of activity and accessibility relative to othersubmarine volcanoes. Knowledge of volcanic processes in particular is essential in the

Hawaiian Islands, primarily because the active Kilauea, Mauna I.oa, and Hualalai

volcanoes pose a serious hazard to people and structures on the island of Hawaii. Insightas to what processes occur on Loihi would provide significant inforination an the dynamics

of the islands' creation and growth.

To this date, because of its inaccessibility relative to Kilauea, Loihi has not been as

extensively monitored as Kilauea. However, the available Loihi seismic data provideuseful information once various types of analyses are performed, such as first arrival

determination, waveform correlation, and spectral analysis. The primary goal of the thesisproject was to take a close look at Loihi's seismic activity as recorded by an ocean-bottomseismometer OBS! September 1-7, 1991 Julian Day 244-251!, The OBS, deployed onLoihi's summit by Dr. Alexander Malahaff, continuously recorded seismic signalsproduced by sources on the island of Hawaii, Loihi, and adjacent regions. The mostsignificant factor of the data is that it was obtained during a quiescent period during whichno swarms were detected, a period which represents 4e majority of Loihi's 'normal'

activity. A comprehensive analysis of the Loihi events was performed in an effort to

classify and describe the events and to suggest their possible source mechanisms. Theresults were partially based upon comparison with seismic activity recorded on Kilauea andLoihi by the U.S. Geological Survey USGS! Hawaiian Volcano Observatory's HVQ!seismic network. The secondary goal of the project was to investigate the quality of OBS-recorded signals from events originating off Loihi. It was also hoped that sisal qualitywould allow for accurate determination of amval times, so that revisioiis to or suggestions

could be made on the existing crustal velocity model of Loihi.

Geologic Setting of Loihi

Loihi seamount is located about 30 km southeast of the island of Hawaii at 18'56'

N., 155'15.9' W. Figure 1!. It has a rift and crater system structurally simi1ar to that ofKilauea and Mauna Loa volcanoes. The relatively flat summit sits at a depth of 969 m and

is characterized by a depressed area 2.8 km wide and 3.7 km long, possibly representing asummit caldera Malahoff et al., 1982!. A 128-m-deep caldera-like depression contains

two pit craters, east and west, with depths of 73 m and 146 m, respectively. The edificealso possesses an approximately 30-km-long north to southeast rift zone Malahoff; 1987!.The seamount's elongate shape gave rise to K. O. Emery's naming the edifice "Loihi",which means "to be long" or "to extend" in Hawaiian Moore et al., 1982!.

Located at the southern portion of the summit is an active, low-temperature

hydrothermal vent field called Pele's Vents, named in reference to Pele, the mythicalHawaiian goddess of volcanoes. This location is significant due to its role as thedeployment site for the ocean-bottom seismometer used in this thesis project. Figure 2shows Loihi's sutnmit in more detail, including the two pit craters and the pinnacle

containing Pele's Vent field. The field covers an area of roughly 250-300 m~ Karl et al.,1988; Sedwick et aL, 1992!. The vents emit hydrothermal fluids previously thought to be

>$5' I$vw23'4

Figure l. Bathymetric map of the Hawaiian Islands. The contour intervalis 1000 m, Heavy dashed lines connect volcanic centers along a doubletrace. Loihi lies on the hot spot track connecting Mauna Loa, Hualalai,Kahoolawe, and Lanai Moore et al., 1982!.

no more than about 35'C, typically considered "low temperature". However, recent

evidence suggests that individual vents may emit waters of over 1000C A. Malahoff, pers.comm., 1994!. The hydrothermal venting processes at Pele's Vents 6eld may affect theseismicity in that localized region, which will be discussed later in this thesis.

The volcano's base lies 1,900 m below sea level at its northern end and 4,755 m

below sea level at its southern end Malahoff, 1987!. Bottom photo~phy, bathymetric

15'155 16' 155*18'18'

56'

!8;

Figure 2. Seabeam bathymetry map of Loihi summit and South Rift area.The contour interval is 50 m. Loihi's two pit craters are located north andnortheast of Pele's vents after Sedwick er aL, 1992!.

mapping and observations from the Hawaii Undersea Research Laboratory's HURL!Pisces V submersible expeditions provide evidence of large-scale landslides along the steep

eastern and western flanks. This evidence includes the presence of volcanic talus, the most

common rock type found on the summit through photo interpretation by Malahoff �987!.Malahof'f describes talus to be the direct result of downslope mass wasting, created throughbreakup of pillow lava and sheet and lobate flows. The presence of these and pahoehoe

flows exposed on the summit and upper rift zones indicate recent volcanic activity,although most of the recent activity seems to have occurred along the southern edge, of thesummit Malahoff, 1987!. Flank extrusions are possible, as intact flows have been

observed deep near the southern rift zone; however, lack of significant flank cones alsosuggests that activity is pritriarily restricted to the summit and upper rift zones Cooper and

Duennebier, 1988!.

Previous VVork on Loihi

The suggestion of submarine volcanism off the southeastern coast of Hawaii came

from G. A. Macdonald after a swarm of over 4,000 shallow earthquakes was detected in

that area in 1952. It is thought that the swarm occurred on Loihi or its north flank Klein,

1982!. Data collected since have shown Loihi to be an active site for small shallow earth-quakes Moore et al., 1982; Macdonald et at., 1983!. In 1978 the U.S. Geological Surveyresearch vessel S.P. Lee dredged and photographed Loihi's summit and found evidence for

recent volcanisrn: fresh-appearing pillow lava flows and pillow fragments with glassy

crusts Moore et at., 1982; Macdonald et aL., 1983!. In addition to echo-sounding, ageophysical survey was also conducted by the S.P. Lee that included magnetics, gravity,and high-resolution profiling of Loihi's upper flanks. Loihi's youth and activity became,undisputed when a 1980-81 study provided over 10,000 bottom photos and samples from17 dredges obtained with the University of Hawaii's R/V Kana Keoki and bathymetric datafrom the U.S. Navy ship H. H. Hess Moore et al., 1982; Macdonald er al., 1983!, From

the data it appeared that recent eruptions had occtnred along the summit crater's edge.Surprisingly, analyses of the dredge samples showed the presence of alkalic basalts inaddition to the expected tholeiitic basalts, leading to the suggestion that perhaps all

Hawaiian volcanoes initially produce alkalic basalt, continue growth with tholeiitic basalts

as the volcanoes mature, and return to a final alkalic phase while activity declines Moore et

al., 1982!.

Figure 3 shows the epicenters of earthquakes originating on Hawaii Island andLoihi between 1970 and 1981 Klein, 1982!. Figure 3a shows those events less than 20km deep, and those deeper than 20 km are plotted in Figure 3b. Although deep events dooccur on Loihi, as seen in these plots, shallower earthquakes are more prevalent, Thisappears to be the case on Kilauea as well. Klein �982! also found that the deep events atabout 50-60 km appear to link Loihi with the deep source below Kilauea's southwest riftzone Figure 3c!. This source is almost equidistant between Loihi, Kilauea, and MaunaLoa, suggesting that magma is supplied to all three volcanoes through this zone.

Klein's 1982 study focused primarily on two particular Eoihi swarms: the 1971-72 and 1975 swarms. These swarms termed "swarms" because neither began with a

dominating mainshock! were both volcanic in nature and unlike Kilauea swarms, they wereunaccompanied by shallow harinonic tremor Klein, 1982!. However, Klein does note thatthis might only be due to the HVO seismic stations' distances from Loihi, since onlynearby stations detected tremor during Kilauea eruptions.

The 1971-72 swarm started at depth about a month before the shallow earthquakes,

implying verncal magma migration towards the surface of the edifice Klein, 1982!. Theswarm's earthquakes changed location from near the summit and uppermost rift areas, tothe southwest flank of Loihi, and back to near the summit. Rifting at the summit causedthe flank earthquakes, which suggests a mobile, active southwest flank. From the

earthquake pattern and the shifts in earthquake location, occurrence mode, and b value�om 2.13+ 0.23 to 1.19+ 0.08!, it appears that Loihi is a smaller version of Kilauea, asthese changes are also seen in Kilauea seismic activity gGein, 1982!. The b value, avaluable statistic in characterizing a rey'on's seismicity, is the slope of the, log earthquakefrequency versus magnitude distribution. Therefore, a high b value suggests a higher

htAUNa L Oa

l9' 20k'

KILAVEA North

2

20VJ

Yd0

0 lUa!E~ toZ

lUO

Figure 3. Loihi and Hawaii Island earthquake epicenters: 1970-1981.Events were located by HVO. a! Shallow events 0-20 km; b! Deepevents 20-60 km; c! North-south cross section from Kilauea to Loihiwith events of 0-60 km depth. Note how Loihi's deep events appear to linkwith Kilauea's seismic 'root' at depth of rough1y 40-50 m Klein, 1982!.

instance of small earthquakes and lower probability of a large event, while a low b valuesuggests a higher probability of a large event. En Klein's �982! study of Loihi, thedecrease in the b value indicates that events' magnitudes increased with the shift in locationfrom the summit to the southwest flank. A b value of 2.13 was obtained using the 89summit earthquakes from the 1971-72 swarm, while a lower value of 1.19 was obtainedfrom the 239 lank earthquakes. Klein and Koyanagi �979! report an average overall b

value of 1.5. These values were obtained using earthquakes with magnitudes of 2.5 and

larger. However, the average compositely calculated b value for Loihi summit and flankevents greater than a magnitude of roughly 1.3 from 1962 to 1992 was 1.16 P. Okubo,

pers. comm.!.

Generally, Kilauea and Loihi have two classes of seismicity: volcanic and tectonic

Klein, 1982; Malahoff, 1989!. The former is characterized by rift and summit earthquakesthat occur during intrusions and eruptions. They occur in short bursts, are smaller in

magnitude, are concentrated spatially, have higher b values, and seem to result from lowand. concentrated sources of stress. The latter class is marked by flank earthquakes that

typically increase after rift activity. They occur continuously, often have largermagnitudes, are spatially diffuse, have lower b values, and. appear to result ft'om higher and

more diffuse stresses Klein, 1982; Shibuya er aI, 1989!.

Interestingly, the 1975 swarm, although confined to the rift axis and summit

region, consisted of activity similar to that of the 1971-1972 swarm's flank events,

suggesting that the '75 swarm resulted from relatively higher and more diffuse stresses,

regardless of location. It was also smaller and shorter in duration than the 1971-72 swarm

and triggered no flank events Klein, 1982!.

Seismic data collected at the Hawaiian Volcano Observatory HVO! and by

scientists at rhe University of Hawaii indicate that Loihi has been continuously active since

it's discovery. Cumulative numbers of Loihi earthquakes over the period from 1961-1994

have been plotted in Figure 4 Paul Okubo HVO!, pers. comm, 1995!, Sharp increases

coincide with the earthquake swarms and are interpreted to be submarine eruptions Klein,

1982; Macdonald et aL, 1983!. Until roughly ten years ago, swarms occurred infrequently

and lasted up to several months, long in duration by Hawaiian standards since Kilauea andMauna Loa swarms typically last only a few days Klein, 1982!. However, activity

appears to have changed recently. The seismic data show that, within the last ten years,

1900

8GG

1:00'16GG

:500

130C

1200

u 1100

1000

900

600

900

600

600

400

300

200

100 Figure 4. Cumulative numbers of Loihi earthquakes: 1961-1994. Arrowsmark rapid increases in number and are inferred to be earthquake swarms.The events are of magnitude greater than 1.25-1.30 P. Okubo, writtencomm., 1995!.

swarms have occurred every one to two years and lasted from hours to several weeks. It

should be mentioned that the HVO location of Loihi events is generally limited to those

with magnitudes above roughly 1.25-1.30. The ability of the network to "meaningfullylocate" events is lost for events with smaller magnitudes P. Okubo, pers. comm., 1995!.

Another swarm detected by HVO in the fall of 1986 prompted Hawaii Institute of

Geophysics BIG! scientists Frederick Duennebier and. Patricia Cooper to deploy a seismic

axray consisting of five 3-component ocean-bottom seismometers. They were placed in a

Y-shaped pattern on the summit and flanks for 28 days Figure 5!, during which periodover 1700 events were detected with at least 200 earthquakes located on Loihi, mostly

along the summit Cooper and Dunnebier, 1988; Bryan, 1992!. Like the 1971-72 and

1975 swarms, no harmonic tremor was observed Cooper and Duennebier, 1988!,

implying that either tremor-producing mechanisms do not operate at this site or magma was

not moving at that time Duennebier, 1989!. Poor coupling of the instruments to the

edifice's surface may also have precluded tremor detection. The OBS array recorded

several flank earthquakes and the rockfalls which they induced; visual observations and

bottom photographs of flank-covering pillow and flow fragments support this conclusion

Cooper and Duennebier, 1988!. Near the seamount's base at the deeper part of the south

rift zone, intact flows have been seen, although Cooper and Dunnebier �988! found few

events located in this region during the 1986 swarm.

Unfortunately, poor site locations and poor coupling in 1986 prevented locations

from being well-constrained. However, their results suggested that Loihi velocity models

were too slow Bryan, 1992; Bryan and Cooper, 1995, in press!; therefore, a new Loihi-

specific crustal velocity model is necessary for accurately determining and locating Loihiearthquakes. Bryan �992! gives references discussing other swarms occurring in 1984,

1985, 1989, 1990, and 1991. The '90 and '9l swarms revealed seismic activity on Loihi's

eastern flank, unlike previous swarms located at the summit and southwestern flank seen

in Figure 5! and the '93 swarm which occurred in the southern summit region P. Okubo,

written comm., 1993!. This demonstrates that Loihi's seismic activity frequently shifts

location.

10

155'15' 155'10'155'

19 00'N dePIhS, km0 0+

ss 5+

eI 13+

+ 20+

ststloAIocssons

18'55'

18'50

18'4

Figure 5. 1986 OBS array and Loihi earthquakes. Stars mark locations ofindividual OBSs. Depths of the Loihi earthquakes are represented byshapes given in the legend. Note that the majority of the events are locatedin the western to southwestern summit region after Bryan, 1992!,

11

CHAPTER 2: 1991 OBS DATA

Instrumentation

In 1991 two OBS deployments were performed, one continuously recording

seisrriicity during the period from September I through October 6 Julian Day 244-279!and the other from October 25 through December 7 Julian Day 298-341!. Dr. Alexander

Malahoff at the University of Hawaii at Manoa deployed a two-component OBS as part of

an ocean-bottom observatory OBO! project to obtain data via time-lapse video, pressure

and temperature sensors, a hydrophone, and the OBS. The OBO and a few of its various

elements are seen in Figure 6, which is an image taken from HURL's deep-sea submersible

Pisces V video tapes. In a typical OBO deployment process, Pisces V is launched from

the University of Hawaii's R/V K'iia with its launch, recovery, and transport vehicle

LRT!. Upon arrival at Loihi's summit, the pilot maneuvers the sub's manipulator arms to

firmly emplace the OBS among the rocks at a site in the region known as Pele's Vents, thus

ensuring good coupling between the seismometer and the edifice, which is essential for

recording. The OBS is retrieved by Pisces V and the R/V Ki ia roughly six weeks later for

removal and replacement of the magnetic recording tapes and for general servicing, and it is

then redeployed for another six weeks.

The OBS package consists of a 4.S-Hz vertical geophone, set upon a free-spinning

gimbal assembly and encased within an aluminum cylinder, a hydrophone, and a two

recording decks. The hydrophone and recorders are mounted on the OBO frame. The

recording decks are regular cassette recorders called UNIMAK and ZAGREB!, heavily

modified to handle extreme temperature changes and powered with alkaline D-cell batteries.

Each deck holds up to three magnetic analog cassette tapes. They continuously record on

four different tracks: horizontal motion, vertical motion, time code, and hydrophone. The

12

recorders are adjusted prior to deployment to record at a speed of approximately 76.2 cm

hr, thus recording approximately eight days' worth of data on each cassette, with one day

of overlap between consecutive cassettes. The recorders have a gain range of 48 dB and a

recording range of 40 dB,

Data

The first deployment returned four usable tapes out of the system's six. Tapes 5

and 6 had unreadable time codes, and nmes were not identifiable from tape overlap. The

second deployment returned two usable tapes � and 2! as tapes 3 and 5 were blank due to

battery problems, and tapes 4 and 6 had unreadable time codes. Once recovered, a

visicorder paper record, a hard copy of the continuous analog data, was produced for each

tape, The analog cassettes were then played on a Yamaha. MT100 II multitrack cassette

recorder and viewed in digitized form via the Macintosh. program SoundEdit. This

allowed for audiovisual analysis of the recorded digitized signal, Whether or not an event

was saved for analysis was based on the strength of the signal, both audibly and visually.

For example, many events were heard while audibly scanning through the cassette, but a

closer look at the visual signal revealed insufficient amplitude for analysis less than

roughly twice the background noise amplitude!. Earthquakes which were identifiable,

visually on the visicorder paper as well as both audibly and visually using SoundEdit, were

recorded and saved on 3-1/2 " diskettes for analysis; others were neglected. Those saved

were given identification numbers corresponding to their location on the cassette shown by

the recorder's counter.

13

Plate l. Image of the, ocean-bottom observatory. The OBO is the largepackage in the upper right corner. Two temperature probes are seen on the,left. The two large cylinders on the underside of the OBO are the recordingdecks, UNIMAK and ZAGREB. The seismometer is seen buried withinthe rocks and nontronite deposits in the bottom right corner Image takenfrom HURL video tapes!.

15

Data Set Selection

The first week of usable 1991 OBS data September 1-7! was chosen for analysis,

This set was selected because the times of the events were deterzninable: as mentioned,

time codes were not recovered on several of the analog tapes. The set also provided

quantitative and qualitative representation of the seismic activity recorded during the twoOBS deployment periods, during which over 3,000 earthquakes were recorded. There was

an average of about 380 events per week, with the chosen week providing a total of 286

individual earthquakes �5% of the average! for analysis from the approximately 550 heard

by strictly listening to the au5.o tape. This 75% of the average was considered to be areasonable number of events for the study. The other data tapes were also heard and found

to contain the same type of activity as the first week's tape.

After selection of events, their arrival times needed to be determined. The tapes

were played on a time code reader, which reads the tape's time code in Greenwich Mean

Time GMT!. Thus, once the original starting time was resolved, subsequent hours and

days could be counted and labeled on the visicorder record using its minute inarks. The

UMIVIAK recording deck's internal clock showed 9 seconds of time drift for its 3 weeks of

recording time, according to HURL technician Brian Daniel. It is assumed that drift was at

a linear rate, therefore the tape used in this study was 3 seconds fast by its end, so all

events were seen comiiig in earlier than they actually should have. The tape was then

divided into six equal-length parts; arrival times f'rom the visicorder were corrected by

adding an appropriate increment of time to each event. For example, an event occumng

within the first sixth of the tape had 0.5 seconds added to its arrival time. An event within

the second sixth had 1.0 seconds added, the third 1.5 seconds, and so on.

Verification of the times resulted from comparison of the OBS record with an HVO

catalog list of earthquakes from the same period. Time correlation of several large events

l6

confirmed that the data set was timed correctly, at least to the minute since the lack of

second marks on the visicorder paper and its small scale prohibit resolving times more

accurately without looking at the digitized events. One event in particular, a magnitude 4.5event located about 27 km south of the summit of Mauna Loa at a depth of about 11 km,

was the primary quake used to check times. The OBS's tecording of the quake showed itsarrival within the same minute as the origin time reported by HVO. Since the time was

only good to a minute, closer scrutinization of its arrival time at the OBS was performedusing the program CANARY. This time-series analysis program, developed by the CornellLaboratory of Ornithology's Bioacoustics Research Program, displays the digitized event atany scale chosen by the user and displays time in CANARY's mS, which are converted bythe user to real-time seconds. The erst amval of the magnitude 4.5 event was clearly

visible on the horizontal component and was found to be at 17:05:25.70. Origin time

determined by HVO was 17:05:16.53, thus travel time was roughly 9.2 seconds, plus 2.0seconds to account for the tape's time drift, giving a corrected travel time of 11.2 seconds.

The event occurred nearly 48 km away from the OBS epicentral distance!, and if one

assumes an average P-wave velocity of 5 km/sec, this would give an expected travel timeof about 9.6 seconds. Therefore, the quake's arrival was close enough to the expected time

to confirm that the timing of the data was correct and that the event recorded by the OBS

was indeed the same event on the HVO catalog list.

Because this study was initially performed to describe Loihi's seismicity, it was

necessary to remove from the data set any events which occurred off the seamount. Onceevent times were determined, a careful examination of both the HVO catalog list and the

OBS event list was done in order to pick out all correlating earthquakes. Initial comparison

of the two lists resulted in the possible correlation of 33 non-Loihi events, including two T-

phases from large distant earthquakes. The magnitude 4.5 earthquake, just mentioned isone of the correlating events.

17

At this point it was necessary to further examine the HVO and OBS recordings inorder to verify correlation of the events. The first one examined was the one Loihiearthquake recorded by HVO during the study week, which they ca11 a "1.4 PPL"; 1.4 isthe magnitude and PPL the epicentral region. Figure 6 shows a map of the Island ofHawaii and it offshore areas and the various delineated regions used in earthquake

identification. The HVO seismic network is seen in Figure 7, with each station pertinent to

the study labeled with a three-letter code, HVO's records show a trave1 time of 6.87seconds to station PPL 33 km away epicentral distance!. Determination of P-

compressional! and S- shear! wave amval times at station PPL gives a P-S separationtime of 4.51 seconds. Parameters such as these were determined for all 6 of the stations

which recorded both P and S arrival times of the Loihi quake. This process was repeated

for all of the non-Loihi events, except for the two T-phases. Stations at similar distances

from an event should show comparable travel and P-S separation times. Therefore,

comparisons were made of OBS arrival times for non-Loihi events with HVO arrival timesfor the same events at similar distances. Travel times for the OBS recordings of the Hawaii

events were also compared with travel times of the 1.4 PPL event to various stations at

similar distances. For example, if the travel time to the OBS of a Kilauea south flank event

was greater than 12 seconds, assuming accurate pick of the first P amvaI, then it was likelythat the two events did not correlate, since travel time should be less than 10 seconds,

based upon the travel times of the Loihi event to stations in the south flank region. Eventswhich correlated were placed into a set called Hawaii events, except the 1.4 PPL

earthquake, which was classified as a Loihi events. Events that did not corre1ate were also

placed into the set of Loihi events for later analysis.

Waveform, or seismogram, analysis also aided in verification of event

classification. If an event was questionable for some reason for example, if the P-wave

arrival was difficult to pick!, then looking at the OBS and HVO seismograms of events of

18

20O l56~155~

Figure 6. HVO earthquake identification regions on Hawaii. Heavy lineboxes contain geographic regions with three-letter codes, All areas locatedoutside the regions are considered distant modified from Koyanagi, 1982!.

19

7P0

19'

156'

Figure 7. HVO seismic network on Hawaii. Triangles mark locations ofseismic stations operational during 1991 on Hawaii. Stations most pertinentto this study are labeled with their three-letter codes modified from Nakataet af., 1994!.

20

similar depth and location generally helped, because their signals either looked similar

indicating occurrence off Loihi! or distinctly different indicating Loihi origin!,

At the conclusion of this stage of analysis, it appeared that the OBS recorded 26

non-Loihi events: 17 events located on Hawaii, 4 located offshore, and 5 T-phases from

distant large earthquakes. Two of the T-phases were recorded by HVO and the OBS,

while three others, only recorded by the OBS, were discovered after consulting an

information list on distant earthquakes which occurred during the week of September 1-7,

1991. All T-phases were omitted from the data at this point, and the remaining 21 events

became the set of Hawaii events. As mentioned, this project was intended to characterize

Loihi seismicity, but it was later decided that a study should be included that looked at what

type of activity was recorded by the OBS, whether it was local activity or activity

originating elsewhere. Thus, the thesis also includes a section describing and discussing

the Hawaii events.

It is important to point out that not all of the events in this study are determined

definitively. Because of the difficulty in precisely picking the first arrivals on many of the

OBS-recorded signals, which is due to the lack of timing marks other than just minute

marks, high background noise, and the location of the OBS in relation to the events' ray

paths, it is possible that a few of the events were categorized incorrectly. Events cited to

have originated on Hawaii could actually be local Loihi events, Similarly, it is possible that

the OBS recorded signals from small offshore events that were not picked up by enough

HVO stations to be included in the original catalog list. It would have been assumed that

those events were local Loihi ones. These possibilities cannot be overlooked; however, the

probability that these situations occurred is small, and if they did occur, their frequency of

occurrence is also likely to be small. Furthermore, because we are dealing with only one

seismometer stationed on Loihi, there is nothing else that can be done to further resolve the

problems with this data set. The possible errors, however few, are therefore mentioned

21

here, but the assumption from this point an is that all events were correctly categorized and

separated into Loihi events and Hawaii events.

CHAPTER 3: LOIHI ACTIVITY

OBS Recording of Loihi Activity

The 1991 data set contains 280 events, 21 of which occurred off Loihi, leaving 259

Loihi events for analysis. Of these 259, there ate three major event families consisting of

95 short-period events, 48 long-period events, and 116 spindle events. Given in the

appendix is a table of all the events saved in this study, which includes all Hawaii and Lojhi

events; T-phases were not included. The list contains event file numbers, durations, and

relative amplitudes,

Also heard on the recording tape are two other types of distinct sounds: "Paper

Rustling" PR! and "Pele's Canaries" PCs!, each referring to the acoustic signal made

when the recording tape is played back at a speed of approximately 235 times over actual

recording time. Representative examples including seismograms of the vertical and

horizontal components and the power spectra from the vertical components from these five

signal families are included in this section, following a brief description of typical

background noise recorded by the QBS.

For analysis comparison of power spectra between different families of events, a

spectrum was produced for a one-minute window of typical background noise Figure 7!.

The vertical top! and horizontal bottom! seismograms are also included. Generally, there

is one broad dominant frequency peak at about 1.5 Hz, but a continuous, lower-energy

peak is also present up to a frequency of about 10.2 Hz. The majority of the background

noise on the data tapes result from the electronic geophone preamplifiers. A smaller portion

of the noise comes from inherent noise within the circuitry and the geophone itself.

1000

-I 000

1000

Vl

0

- 'I 000

10 sec

0

-5

~ -10g -15+o -20~ -25

~ -30

C g! -3514 166 8 10 12

Frequency Hz!

Figure 8. Seismograms and power spectrum for typical background noise.The signals are 60-second vertical top! and horizontal middle!seismograms. The high-frequency �0 Hz! signal on the horizontalcomponent is a minute mark MM!. The power spectrum bottom!represents the 60-second vertical seismogram.

Ev

Short-period SP! events, also considered "normal earthquakes" Koyanagi er at.,

1987!, are local tectonic events, analogous to those occurring on Kilauea and Mauna Loa.

As magma exexts pressure and generates or alteis stress on host rock during migration and

intrusion, SP earthquakes occur as tectonic responses Koyanagi, 1982; Koyanagi et al.,

1987!. Shallow ones directly resulting from magmatic intrusions have been referred to as

volcano-tectonic; those occurxing at a distance from volcanic activity are tectonic and

generally result from the volcano's long-term growth processes Latter, 1981!,The family of 95 short-period events has unique waveform and spectral

characteristics. P-wave arrivals are usually clear and fairly easy to identify, while S-wave

axxivals are often difficult to discern. Signal durations range from 4.1 to 19,0 seconds,

with the average &om the data set being 7.1 seconds. As is done with Kilauea

earthquakes, durations are measured from P-wave onset to the point in the coda where ithas decayed to almost noise level Nakata et al., 1991!. The xnajor characteristics of short-

period events can be seen in the summary table following the discussions of the three major

event families Table 1!

The time difference between the arrivals of P- and S-waves is a primary

characteristic used in earthquake analysis. P-S separation times are not identifiable in most

of the SP events but appear to be on 25 of them. Times range from roughly 0.5 to 1.3

seconds; the average is 0.9 seconds. The separation times indicate that the SP earthquakes

were shallow, occurring at distances between 0.5 km and 1.3 km, as every second of P-S

separation time coxresponds to oxie kilometer of distance. This was calculated using anaverage F-wave velocity of 5.00 km s- and S-wave velocity of 3.33 km s . One feature

seen in many of the events which makes picking the S-wave difficult is the arrival of the

water wave reflected off the space of the ocean. This reflection should arrive at least 1,3

25

seconds after the P-wave's first arrival, since the wave must travel roughly 2 km or more

through sea water at a velocity of roughly 1.5 km s->.

Horizontal and vertical components show comparable amplitudes, like most of the

other event families. The power spectra for these events are f'airly complex and rarely

show a dominant frequency peak. Instead, they are generally multipeaked or have a broad

peak ranging from 3.4 to 8.5 Hz. On Kilauea, these events have initial frequencies of about

10 Hz, decreasing to less than 1 Hz as the earthquake signal travels and its energy is

attenuated Koyanagi et al., 1987!.

A typical intermediate-amplitude short-period event ¹507! is represented in Figure

9 by 15-second-long vertical and horizontal seismograms. The signal amplitudes are

comparable, although the horizontal signal seems to have a slightly higher level of noise.

Duration of the signal is roughly 7.4 seconds, and its P-S separation time is approximately

0.82 seconds. The reflected wave arrival is seen approximately 1.7 seconds after the P-

wave arrival. The dominant frequency range depicted in ¹507's spectrum is approximately

6 to 7 Hz, typical of these events. The onset portion of this earthquake has a frequency of

about 8.7 Hz, the reflected wave portion is 6.3 Hz, and the end of the coda is 6.6 Hz.

Manual calculations of frequencies for all examples resulted from taking 3-second signal

portions, counting the number of cycles, and dividing the numbers.

An SP event of low amplitude ¹446�!! is shown in Figure 10 as a 15-second

window. 4n this example, the horizontal component shows a slightly weaker signal than

the vertical, which is common in most of the lower-amplitude signals. Duration of the

event is approximately 4.2 seconds. As with many of the smaller short-period events, the

P-wave arrival is fairly easy to discern. The S-wave is not as discernible, but a possible S-

wave arrival is marked in the figure. Assuming a correct pick, a P-S separation time of

about 0.54 seconds is calculated. The signal's onset has a frequency of about 8.8 Hz, 5.4

Hz for the reflected wave portion, and 5.9 Hz near the end of the coda.

26

I000

-I000

Reflected Wave

1000

-1000

5 sec

0

-10

~ ~-15o ~20> -254P~ -30

-35CIt

40

W j'I/

vx 2

-45 ~

0 10 12 14

Frequency Hz!

27

Figure 9. Seismograms and power spectrum of a typical short-periodearthquake event ¹507!. Vertical top! and horizontal middle! l5-secondseismograms are shown. The spectrum bottom! represents a 60-secondvertical record segment of the event.

1000

-1000

C-C

1000

' III 0

-1000

5 sec

0

-5CCI

-10

-15

20

o -25

~ -30pl-

~ -35CO

45126 8 10

Frequency Hz!

14

Figure 10. Seismograms and power spectrum of a typical short-periodearthquake 8446�!!. Vertical top! and horizontal middle! 15-secondseismograms are shown. The spectrum bottom! represents a 60-secondvertical record segment of the event.

It should be mentioned that another type of event was also heard and seen in the

data: microshocks. These are local events resulting from a variety of processes including

degassing explosions and continual thermal and structural adjustment of the edifice,

particularly around the active vents Koyanagi et al, 1987!. In this data, they typically have

very short durations and sometimes have waveforms similar to the SP events. For

categorization purposes, any SP event with a duration less than 4.0 seconds was

considered a microshock. Due to their relatively infrequent occurrence in the data, the

microshocks were omitted from the family counts, but still included in the overall tape

count of 550 events, assuming they were strong enough to be heard.

Long-period LP! earthquakes have been observed on Kilauea as well as on Loihi

Koyanagi et al., 1987!. They are related to magma movement within the edifice's dike

and conduit system. Qn Kilauea, the predominantly shallow quakes are induced during

magma intrusion episodes primarily in the summit and rift zone areas and may or may not

be followed by eruption Koyanagi et al�1987!, It cannot be confirmed at this time

whether or not the LP events recorded on Loihi were accompanied by eruptions, as none

have as yet been observed nor visually recorded However, the presence of fre,sh pillow

lavas and lobate and sheet flows exposed on the summit and upper rift zones indicate recent

volcanic eruptions Malahoff, 1987!. Pressure fluctuation data also supports this, as there

is evidence of a major inQation/deflation event in December of 1991 Malahoff et al,, in

prep., 1995!.

It has been suggested that LP events result from the sudden extension of a crack tip

or the opening of a channel between cracks Aki and Koyanagi, 1981; Chouet et aI.,

1987!. Evidence also suggests that LP events are the fundamental processes of volcanic

tremor, based upon their frequent simultaneous occurrence with one another, spatial

attenuation of amplitude, and spectral similarities Fehler, 1983; Chouet et al., 1987;

Koyanagi et al., 1987!. Although tremor generation is attributed to unsteady fluid flow,

the exact mechanism is an unanswered question Chouet et al., 1987!. These authors point

out that the suggestions of an extending crack and an opening channel remain speculative;

other triggers are highly possible, including chemical and thermal processes and rock

failure. Tremor has been detected on various types of active volcanoes in different

locations such as Mount St. Helens, Washington Fehler, 1983!, Mount Erebus, Antarctica

Shibuya et al,, 1989!, and Kilauea, Hawaii Aki and Koyanagi, 1981; Koyanagi et al.,

1987!, to name a few. It is presumed to be indicative of magma movement and/or eruptive

activity. Deep tremor was detected on Loihi near the same period as the October swarm in

1993. However, association of deep tremor on Loihi with its swarms is still an open

question P. Okubo, pers. comm., 1993!.

Long-period events have waveforms which are relatively constant in frequency and

show a gradual build-up and decay of energy. However, similar to what often occurs with

Kilauea LP events, some of the events have wider frequency bands and more sudden

onsets Koyanagi et al., 1987!. The vertical and horizontal seismograms typically have

comparable relative amplitudes. P-wave and S-wave arrivals ate difficult to locate on the

seismograms, thus P-S separation times are nearly impossible to determine. Durations

from the data set range from 5 to 57 seconds, with an average duration of 25.6 seconds.

Power spectra for these events are distinctive. Energy arrives between about 0.85 and 6.38

Hz, but peak frequencies are typically around 1.3 to 1.5 Hz; very little energy arrives at

frequencies above 8.5 Hz.

An example of a relatively large amplitude LP event is shown in Figure 11. The

signals are 60-second windows of both the vertical top! and horizontal middle!

components. Unlike with SP events, it is difficult to determine the first arrivals in LP

waveforms; however, it is presumed to be located at the 'P' mark on the vertical

30

component. The event has a duration of about 37 seconds, just over the average duration

for this earthquake family. The power spectrum bottom! shows the broad, low-frequency

peak common only to these events. Although frequency is fairly constant overall, there is

small-scale variability, which is negligible relative that seen in other event families. In this

example, the onset portion has a frequency of about 2.2 Hz, the middle 2.9 Hz, and the

end of the coda 1.4 Hz.

An interesting type of LP event has waveforms with more than one pulse, as seen

with ¹798�!, primarily in the vertical seismogram Figure 12!. A 'P' marks the suggested

arrival of the signal; however, it is nearly impossible to be certain. This typical example is

shown again in a window of 60 seconds and has a duration of about 25 seconds. The

dominant frequencies aa: seen in a narrower spectral band than that in Figure 11. The

majority of the energy arrives with frequencies between 1.3 and 3.3 Hz, and a minor

amount arrives with a, frequency of about 5.7 Hz. The variation in frequency with time

follows the pattern seen in the previous example: onset frequency is roughly 2.2 Hz,

increasing to 2.8 Hz in the middle portion of the signal, and decreasing to a frequency of

1.9 Hz, again lower than onset frequency.

Qin~lg

Spindles, so called due to their spindle-shaped waveforms, most likely represent a

phase of what HVO caHs "gas piston activity", a pattern of magmatic cyclic oscillation due

to magma transport Koyanagi et aL, 1987!. During these periods, magma within Kilauea

migrates and ascends the vent column, creating low amplitude tremor. The higher

amplitude cigar-shaped signals lasting up to one minute are presumably associated with the

collapse of the vent column and have dominant frequencies of 2-5 Hz at their peak

amplitudes; this is at a distance of about one kilometer from the eruptive vent. Harmonic

volcanic tremor is also associated with these events Koyanagi et al., 1987!.

31

I 000

- IQOO'a

O.

4I

CC4J

1000

-1000

10 sec

-5PQW -10

>~ -15~o -20o -25

-30C

gQQ~iQ

45

0148 10 12

Frequency Hz!

Figure 11. Seismograms and power spectrum of a typical long-period event�1 82�!!. The signals are 60-second vertical top! and horizontal middle!seismograms. The power spectrum bottom! represents the same 60-secondvertical seismogram.

1000

-'1 000

1000

~ 1000

j0 sec

0

-5

1 -10y -15>o 20~ -25

-30CC~ -35< -40

0 6 8 10

Frequency Hz!

12 14

33

Figure 12. Seismograms and power spectrum of a typical long-period event A'798 l!!. The signals are 60-second vertical top! and horizontal middle!seismograms. The power spectrum bottom! represents the same 60-secondvertical seismogram.

Loihi's OBS-recorded spindles, suggested to be analogous to Kilauea's cigar-

shaped events, have similar characteristics: average signal durations of roughly one

minute, gradual build-up and decay of energy at comparable rates, fairly constant frequency

pattern, and variable amplitude between different events. The variability is dependent on

the intensity of the magma movement and location of the seismometer relative to the activity

Koyanagi et a/., 1987!. The spindles axe of higher frequency than the long-period events

presumably associated with magma transport within Loihi. Because higher frequencies are

attenuated more rapidly than low frequencies, it is possible that the higher frequency

signals observed with the OBS on Loihi are due simply to shallower magma movement

beneath Pele's Vent field, as opposed to intermediate or deep movement on Kilauea.

Spindles always produce a distinct sound during playback of the recording tape,

making them easily distinguishable from other families by audio analysis alone. Because

of their gradual onset of energy, P- and S-waves are again difficult if not impossible to

discern in these events. They range in amplitude from very low to very high; many were

too low to include in the data set, while some were high enough for the signal to be

automatically clipped off. Durations are also variable, ranging Rom roughly 12 to l 80

seconds with an average of about 47 seconds. Dominant frequencies of these events

generally arrive as a broad peak or several peaks ranging from 3.5 to S.5 Hz and

commonly centered around 6.3 Hz Malahoff et al., in prep., 1995!.

Figure 13 shows event ¹6, a relatively low-amplitude spindle. Comparable vertical

and horizontal waveform amplitudes are seen once again. In addition to the gradual

decrease of energy, the automatic gain change at the minute mark makes it dif6cult to

determine the end of the signal. The windo~ displayed is 60 seconds long; the estimated

signal duration is about 40 seconds, just under the average for this type of event. A 'P'

marks the suggested onset of the event, although the pick is not definitive. Dominant

frequencies in the spectrum are seen as a broad peak centered around 8.0 Hz. A manual

34

calculation directly from an expanded waveform results in frequencies of 12.0 Hz at, the

onset of the signal, 11.5 Hz about 20 seconds later, and 8.2 Hz at the end of the coda.

Spindle ¹185 is shown in a 60-second window Figure 14!. The vertical and

horizontal waveforms again show comparably high amplitudes. Signal duration is above

the average at about 60 seconds, although the exact time of P-wave amval is not certain

indicated on the figure by 'P?'!. A power spectrum was produced for both a 60- and 90-

second segment; both produced roughly the same spectrum, thus the 60-second one is

shown in the figure to be as consistent as possible. Peak frequencies seen in the power

spectrum range from about 5 Hz to 9 Hz, and another minor peak is at roughly 12 Hz. In

this event, manual calculation results in frequencies of 7.1 Hz at the onset of the event, 7.6

Hz at the peak amplitude, and 7.0 Hz at the end.

To summarize the seismic activity seen on Loihi by the OBS, Table 1 displays the

various characteristics demibed in the discussion of each family. Of the three major

families of events, spindles have the widest duration range and longest average duration.

All three families have comparable frequency ranges; spindles and short-period events

show similar values as well. Short-period events show the highest average frequency,

most likely due to their hi@-frequency onsets. Quantitatively, spindles represent the

largest proportion of the Loihi events observed with 44.8%, while SP and LP events make

up the remainder with 36.7% and 18.5%, respectively.

The family of earthquakes called "Paper Rustling" in this study refers to the signal

segments which sound as if one were rustling papers or walking through leaves, when

listening to the acoustic signal produced as a result of the data tape being sped up 235 times

over recording time Malahoff et al., in prep., 1995!. The PR segments consist mainly of

35

1000

-1 000

3000

-1000

t0 sec

5

~ -104

15

g -20~~ -25

~ sele -30

C4 -35

-4014126 8 10

Frequency Hz!

Figure 13. Seismograms and power spectrum of a typical spindle event P6!. The signals are 90-second vertical top! and horizontal middle!seismograms. The power spectrum bottom! represents the same 90-secondvertical seismogram.

36

1 000

a -1000

1000

0

-1000

IO sec

5

~ -l0

~ -15

- 0

e0 -30Y!gg -33 ~

0 2 14IZ10

Frequettey Hz!

Figure 14. Seismograms and power spectrum of a typical spindle event ¹185!. The signals are 90-second vertical top! and horizontal middle!seismograms. The power spectrum bottom! represents the same 90-secondvertical seismogram.

37

Table 1. Laihi event families: numbers, durations,frequencies, and percentages. bT and f! are duration andfrequency, respectively. The percentages of total Loihi eventssaved are out of 259 events.

95 4,1 � 19.0

48 5- 57

116 12 - 140

Short-period

Long-period

Spindle

7.1 3.4 - 8.5 7.0 36.7

25.6 0.8 - 6.4 1.4 18.5

43 3.5 - 8.5 6.3 44.8

background noise with intermittent, incoherent signals of higher amplitude, each ranging in

duration from 2 to 20 seconds.

Durations of PR episodes were observed to range from about 6 minutes to over 8

hours in this study. Segment durations were only computed- when it was certain that the

noise was being heard consistently. There appears to be no regularity in their occurrence.

They are often heard simultaneously with Pele's Canaries PC!, which will be discussed inthe following section, but they ate also heard exclusively. SP events, LP events, andmicroshocks sometimes occur during periods of PR, but not always. Periods of active PR

totaled just over two hours of the week's 168, therefore only affecting about 1.2% of the

data. Any events which were clearly distinguishable amidst the PR were tallied into their

appropriate family count. It's likely that only a few events were missed, because theacoustic signals of the other families are very distinct from that of PR. PR appears to cover

a large range of frequencies, and relatively few prominent frequency peaks are shown on

the spectra.

An example of PR is seen in Figure 15 as a 90-second segment. The vertical and

horizontal components show comparable amplitudes. Its power spectrum was produced

for a 60-second portion of the signal; one was also produced for the entire 90 seconds, but

38

Event Family ¹ Saved hT Range hT Average f Range f Average % of TotalH z d

it resulted in a nearly identical spectrum. At first glance it appears that there is significant

variability in the power, but the power axis only has a 15-dB range, whereas other spectra

have up to 45-dB ranges. Due to the sampling rate, the frequency axis shows no higher

than nearly 16 Hz, although the jump in power at 15 Hz suggests that there are even higher

frequencies within the signal.

The most interesting and unusual sound recorded by the OBS at the Pele's Vents

field is called "Pele's Canaries", so named for the canary-like whistling noise produced

when the cassette is again played back at a speed of about 235 times over recording time

Malahoff et aL, in prep., 1995!. Previous studies describe the effects of water currents on

QBSs deployed in various regions such as off Lopez Island, Washington Sutton et at.,

1980!, offshore south of Hawaii Duennebier et aL, 1981!, and on Loihi seamount Bryan,

1992!. According to Sutton and others �980!, current noise may be induced by direct

shaking of the geophone or by shaking of another instrument in such a way that the energy

is coupled to the seafloor and radiated to the geophone as surface waves. The Lopez Island

experiment produced no current noise; maximum cuzrents during their deployment were

only 6 cm s i. Another noise generation mechanism is Karman vortex shedding

Duennebier et al., 1981!. In this phenomenon, beyond a current speed threshold of about

10 cm s i, a value probably dependent upon the system's configuration,

an object in the cutrent's path such as the OBS! causes vortices of water to spin away

downstream of the object. The regular time intervals between vmtices create a harmonic

force on the object which in turn can create the so-called "current-generated noise".

The problem of current apoise drove scientists at the Hawaii Institute of Geophysics

to design an OBS with less susceptibility to the noise. What resulted is an OBS package

with an isolated geophone that is deployed away from the rest of the OBS array. The

39

1 000

0

! E-1000

E

1000

CC

0

-1000

10 sec

-10 l-

Cl

~4 ->5 I

~ 'j 'IJ-20 f

-25

6 8 10

Frequency Hz!

12

4O

Figure 15. Seismograms and power spectrum of Paper Rustling. Thesignals are 60-second vertical top! and horizontal middle! seismograms.The power spectrum bottom! represents a 60-second segment of thevertical seismogram.

redesigned OBS was also developed to improve coupling to the ocean bottom and to

remove possible effects of tape recorder noise Duenne bier er al., 1981!.

This study utilizes data obtained from an isolated-geophone OBS; however, signals

were still recorded with characteristics resembling those of current-generated noise. In this

case, it is suggested that currents, which generally have velocities of less than 25 cm s <

�.5 knots! and on rare occasion up to l00 cm s > Terry Kerby, pers. comm., 1994!,

shake the ocean-bottom observatory as described by Sutton et al. �980!. Direct shaking of

the geophone itself is not likely since it is solidly emplaced among the rocks at the vent site.

One of the most interesting characteristics of PCs is the varying amplitudes of the

signals. Relative amplitudes range from low to high between episodes on the vertical

component, yet are consistently low in the horizontal, sometimes not even visible or

audible. Presence of the signal on the hydrophone record supports Sutton er al.'s �980!

explanation for currents indirectly producing the noise. A hydrothermal source would have

to be fairly strong in order for the signal to be picked up by the hydrophone.

Durations of these whistling episodes are also highly variable, lasting from 15

minutes to over 12 hours. The duration time was determined by hstening to and watching

the tapes with SoundEdit and noting the times of the periods when the whistling noise was

heard and seen constantly. The stronger signals were also present on the visicorder paper,

so times could be verified in that manner. Individual PCs lasting from 10 to 18 seconds in

duration vere also observed as single 'canary' events; these were not considered when

totaling the number of hours of PCs. A total of approximately 19.5 hours of canaries was

heard on the data tape; thus, currents affected approximately 11.7% of the data. Although

Duennebier er al. �981! found a strong correlation between OBS noise level with bottom

current speed and theoretical ocean tide, no pattern or regularity in PC occurrence was

found, regardless of duration. Relatively narrow frequency peaks dotninate the power

41

spectra and are fairly consistent, ranging from about 7 Hz to 14 Hz, the most commonbeing at about 9.8 Hz. Rarely are PCs heard with frequencies outside the dominant range.

The following example of Pele's Canaries is a 60-second segment taken from a

111-minute intense episode Figure 16!. The same vertical segment produced the

accompanying power spectrum. The dominant frequency is 9.5-10 Hz, with the peaktapering off around 11 Hz. The low-frequency peak around 1.5 Hz results frombackground noise. This spectrum, with its few and relatively narrow peaks, characterizes

the majority of Pele's Canaries.

The predominant frequencies in Pele's Canaries are higher than those 2-Hz

Duennebier et al., 1981! and 2 to 4-Hz Sutton et al., 1980! "whistling. noises" recordedby previous studies. They are also higher than those recorded during the 1986 Loihi OBSstudy; dominant frequencies from it range from 2.3 to 7.S Hz. In nearly every PC episodefrom the 1991 data, the vertical component shows a much stronger signal than both the

horizontal component and the hydrophone. A possible explanation involves current

direction and interaction with the OBS packay;, if currents are the source of the noise. If

ocean currents are not responsible, then another possibility is that vertical currents from

within the vents are causing the acoustic signals.

The suggestion that relatively intense hythothermal activity could produce a signal

similar to PCs was posed recently Malahoff et at., in prep., 1995!. It has been observed

that Pele's Vents emit water at velocities of up to 10 cm s-> Maximilian Crerner, pers.

comm., 1995!. Chemical evidence for subsurface boiling within Loihi was shown by

Sedwick et aL �992!. According to the classic two-phase boiling curve for a seawater

analog, at a depth of about 1000 m, temperatures of just over 300'C would be needed forboiling to occur within Loihi. They suggest that a high-temperature endmember up to

35PC is responsible for the evidence. Since the geophone sits directly at a vent site, it is

plausible that hydrothermal acoustics within the edifice could be detected.

42

1000

CJ

g

-1000

1000

-1000

10 sec

5

~ -10

-15

>o -20~~ -25ca -30

Ce -35

-40

to 12

Frequency Hz!

Figure 16. Seismograms and power spectrum for a typical segment ofPele's Canaries. The signals are 60-second vertical top! and horizontal middle! seismograms. The power spectrum bottom! represents the entirevertical seismogram and shows the typical dominant frequency of 10 Hz.

43

Supporting evidence for this hypothesis is found in the subaerial realm. A few

decades ago, when research of geothermal reservoirs as potential energy sources was

active, seismic survey was one of several methods used for locating these systems. These

sites were also surveyed in order to determine the noise levels and types of seismic signals

produced by geothermal activity gyes' and Hitchcock, 1974; Iyer, 1975; Douze and Laster,

1979!. One such system was Yellowstone National Park. Iyer and Hitchcock's study

�974! suggests that noise in the 2-8 Hz range results from deeper geothermal convection,

while higher frequency 8-16 Hz noise is produced by surface and subsurface activity such

as waterfalls, boiling, and steam bursts. Similar frequencies are seen in the Loihi OBS data

set, as PCs consistently show peaks between 8 and 14 Hz; infreq~M:ntly they are seen are

single bursts of 6 and 7 Hz. It should be pointed out that the Yellowstone Park results

were obtained using an 84-station survey, whereas the, OBS was solitary; logically, the

OBS would have to sit directly over a conduit in order to pick up the signals above

background noise.

Further evidence supporting a hydrothermal source for the whistling sound comes

from another subaerial geologic feature: Mt. Semeru volcano in Java, Indonesia. In a

1992 study, Hellweg and others �994! monitored the volcano's seismic activity and

correlated its recordings with visual and acoustic observations from the same period, taken

at the summit and crater rim In their seismic records, along with steam explosions, they

observednvo types of activity: low-frequeiicy �-5 Hz! cyclic tremor and high-frequency

�5-20 Hz! banded tremor. The latter accompanied a "puffing or pumping noise", emitted

from the crater at regular intervals of 0.6 to 1.5 seconds. Seismogram segments of both

types of "tremor", particularly the high-frequency type, show signals similar to those of

Loihi's PCs: monochromatic, gradual increase and decrease in amplitude, variable

intensity, and variable episode durations. According to Hellwig and others, the signals are

most likely associated with crater activity rather than activity deeper within the volcano.

44

Despite the OBS's placement directly in the vent field with good coupling to the

edifice, currents are still considered the source of the noise, at least until further evidence is

found either from another OBS study at Pele's Vents or from laboratory experiments.

Without concrete knowledge concerning the complexities of Loihi's hydrothermal system,

one can only speculate. However, the existence of similar OBS -recorded signals in

regions known to be inactive hydrothermally Sutton et al,, 1980; Duennebier et al., 1981;Bryan, 1992! strongly suggests that currents are the most likely explanation for the

phenomenon.

Discussion

Several interesting observations can be made from the data obtained by the OBS

during the week of September 1-7, 1991. First, it appears that Loihi's seismicity during

the study period was relatively low, compared to activity on IGlauea. Figure 17 shows the

total number of Loihi earthquakes per hour for the week's 168 hours. The numbers only

range from 0 to 9 earthquakes per hour. Activity was particularly low during the first 60

hours, averaging 1.0 earthquake per hour. Activity then increased to 1,8 per hour for the

remainder of the week. The average for the entire week was only 1.5, thus indicating a low

level of seismic activity, because swarms have much higher numbers. Cooper and

Duennebier �987! reported that during the first 3 days of the 1986 Loihi swarm, they

recorded roughly 60-180 events per hour in the summit region and 10-20 per hour in the

flank regions. The month of data obtained just following the 1991 study week also shows

a similar level of seismicity. It appears that the!ow level characterizes the seismic activity

on Loihi between earthquake swarms. In other words, it is likely that the data represents

Loihi's background level of seismicity during non-swarm periods.

Hour

Figure 17. Total number of Loihi earthquakes per hour. The average is 1.5events per hour, indicating a relatively low level of seismic activity for thestudy period,

In an overview of the three major event families LP events, SP events, and

spindles!, it appears that there was a shift in the type of activity during the study week.

Figure 18 displays the numbers of earthquakes of the three families per day. The week

apparently began with predominantly spindle events, but towards the middle of the week

JD 247!, SP events sharply rose in number while spindle activity decreased, After this

shift, LP events steadily increased in number. Although the final day shows a drop in the

number of SP events, it seems clear that within this one-week period, seismicity generally

shifted from spindle-dominated activity to that more dominated. by LP and SP events.

A plausible explanation for the shift in activity is magma movement, as seen by the

spindle activity, and then a phase where new conduits and cracks are opening or extending,

as evidenced by the long-period events, Heating and pressure exerted on the host rocks by

the moving magma could be causing the short-period rock-fracturing events.

Recalling that Kilauea's cigar-shaped events are supposedly a result of magma

collapse within a vent, it is likely that spindle events result from processes somewhat

46

30

c 20

10 244 245 246 247 248 249 250Julian Day

Figure 18. Total number of Loihi earthquakes per day by family. Note theapparent shift on Julian Day 247 from spindle-dominated activity to onewhich seems dominated by SP and LP events.

different from those which produce the cigars. There has been no evidence of eruptive

vents on Loihi. However, the processes operating on Loihi which produce spindles are

likely to be a variety of processes also involving magma transport. Assuming that Loihi's

magma reservoir is shallow �-3 km!, as is described in the model for typical Hawaiian

volcanoes Malahoff, 1987!, it is logical that the depths of the spindle events are also

relatively shallow less than 15 km!. Once these events axe locatable, after a network of

OBSs is established, it will be possible to further define the processes producing not only

the spindle events, but the SP and LP events as well.

47

In addition to thermal and pressure changes, SP events could also result from

hydrothermal activity in the nearby area. Heating and cooling within the system as hot

water escapes and cold water's sucked in through the plumbing system may cause thermal

cracking. Such events would occur at very short distances from the OBS, and it is possible

that if the events were very close, their P-S separation times may not be determinable.

Pez'haps some of the events in the data set are such events. If hydrothermal activity does

generate SP earthquakes which are locatable, then the ability to detect and locate

hydrothermal systems would be valuable for correlating the seismic signals produced by

the venting and the SP events caused by the thermal changes. It would also be a

breakthrough to be able to locate hydrothermal systems acoustically in addition to having to

search and discover them via visual and photographic observations and chemical

measurements.

Although Paper Rustling episodes affect only a small percentage of the data, Pele's

Canaries appear to af'feet a fairly large percentage. Figure 19 displays the total number of

minutes of PR and PCs audible on a daily basis during the study week. PR appears to be

unrelated to the PCs and is clearly a relatively rare phenomenon. On a daily scale, PCs do

not appear to have any occurrence pattern, yet the amjor increase on the last day of the

week is interesting. The question of their origin is an important and intriguing one which

should be addressed and solved with further research. However, speculation is all that can

be presented in this study; further work has been planned for the near future. The subject

of Pele's Canaries is very relevant to the study and warrants discussion since the noise

affects almost 12% of the data tape. If the noise is indeed current-generated, then the

significance lies in the immediate need to create seismometers which are less susceptible to

the noise. On the other hand, if it's due to hydrothermal activity, this opens a whole new

door for discovering these systems and their dynamic processes.

48

700

600

500

400

300

200 0 244 245 246 247 248 249 250Julian Day

Figure 19. Total number of minutes of PR and PCs per day. Note that PRand PCs appear to be unrelated.

Conclusions

Based on the analyses of the data set obtained by an ocean-bottom seismometer

during the week of September 1-7, 1991, several major conclusions were drawn:

�! The seismic data obtained by an OBS from September 1-7, 1991, suggests a

relatively low level of seismic activity on Loihi seamount during that particular week.

Based upon the similarity between the data and that obtained during the subsequent month

by the same OBS, it appears that the low level of activity characterizes the seismic activity

on Loihi between earthquake swazms. In other words, the data probably represents Loihi's

background level of seismicity duzing non-swazm periods.

49

�! Loihi seismicity appears to be dominated by 3 major event families: short-

period earthquakes, long-period events, and spindles. Family percentages for the study

week are as follows: SP earthquakes - 36.7%, LP events - 18.5%, and Spindles � 44.8%.

If the spindle events are indeed analogous to Kilauea's cigar-shaped events, then I-oihi's

earthquakes appear to be predominantly related to magma movement within Loihi, as both

long-period and spindle events are presumably volcanic in nature. Tectonic and volcano-

tectonic events appear to be less frequent.

�! A look at the type of events occurring with time shows that the number of

events by type is not constant. The shift in event type Rom spindle-dominated to that

dominated by long- and short-period events demonstrates the short-term variability of

activity on the seamount.

�! Seismic activity on Loihi resembles that seen on Kilauea. For example, short-

period, or normal,.earthquakes have relatively high onset frequencies which change with

time to lower frequencies, as do Kilauea's short-period earthquakes. Long-period events

on Loihi and Kilauea have similar amplitude envelopes, with gradual energy increases and

decreases and relatively constant frequencies throughout their coda. Spindles on Loihi and

cigar-shaped events on Kilauea have similar signal durations, amplitude envelopes, and

frequency patterns.

Differences, such as in frequency or duration, between event types on Loihi and

Kilauea could simply result from the differences between the two volcanic systems, It

would be expected that two different systems would produce seismic signals with

somewhat dissimilar characteristics. However, discrepancies could also be due to the fact

that the Loihi events characterized in this study were done so based on waveform

appearance and not location, since only one seismometer collected the data. Therefore,

locations of the Loihi events were indeterminable. Because signal characteristics are partly

affected by epicentral locations, it is understandable that certain characteristics would show

50

variability between events. Due to the uncertainty inherent in truly characterizing Loihi's

seismicity with knowing the distribution of events, one may only speculate at this point.

�! There is still an unanswered question as to the sources of Paper Rustling and

Pele's Canaries episodes. In particular, are the latter due to ocean curreiits, or could they

results from hydrothermal activity? Further research will undoubtedly soon provide an

answer.

�! Finally, it can be stated that a single ocean-bottom seismometer on Loihi

provides a significant amount of information on the seismicity of the local region and a

relatively distant region Hawaii Island!. However, the installation of a seismic network on

Loihi would undoubtedly provide us with information which would give us a much clearer

understanding of the dynamics and growth processes of a Hawaiian volcano in its early

stages of development Until then, there are stiU existing data sets which, when used in

conjunction with OBS data, can provide further insight to Loihi's seismicity. For example,

it would be worthwhile to analyze a week's worth of events recorded at Kilauea's summit,

classify and count the events, and compare the results to those obtained on Loihi's sumtrut.

This would have to be done after distant earthquakes were removed, like it was done with

this study's events. Perhaps one would Gnd the percentages of event families similar, or

perhaps one would find that the families' event counts are completely different between the

two volcanoes. There is still much room for speculation.

51

CHAPTER 4: HAWAII EVENTS

The initial goal of this study was to characterize the seismicity of Loihi based on the

OBS data; however, since a substantial set of 21 Hawaii events was also obtained, it was

decided that a look should be taken at those events as well to see what type information

could be seen in the OBS recordings, Each event was analyzed during a visit to HVO by

viewing the signals as recorded by various seismic stations. Picks of both P- and S-waveswere made wherever possible; many stations recorded signals which were too noisy or

simply did not have an arrival clear enough to pick accurately. Table 2 is a list of the 21Hawaii events recorded during the study week by both HVO and the OBS. Displayed for

each event are the location, magnitude, depth, and travel time data.

Seen in Figure 20 is an elevation map of Hawaii Island and the bathymetry of its

offshore regions. The map shows a fairly scattered group of events; however, most seem

to cluster in four particular regions. The first three regions are recognized by ovals

enclosing the cluster. Region l contains 6ve events and is referred to by HVO as the Hilea

region HEA!. Region 2 has four events and is the Upper Kaoiki Fracture zone UKF!.The cluster of six is in region 3, the Upper East Rift zone and Poliokeawe Pali UER/POL!

areas. Finally, region 4 no oval in the figure! contains two events in the Lower Southwest

Rift zone LSW!. The remaining four events are the distant DIS! quakes and an event to

the east of Loihi which HVO the network would classify as a Loihi event. Anything in this

offshore region PPL! is considered to be a Loihi quake by HVO; however, this particularevent is considered a Hawaii event in this study. All of the Hawaii events are listed in

Table 2 with their magnitudes, depths, origin times, and travel time data. The next sectio~

of the study will focus an examples from each of the first three regions described above,

the HEA, UKF, and UER/POL regions. Recall that the regions and HVO seismic stations

are shown in Figures 6 and 7, respectively.

52

EARTHQUAKES RECORDED BY OBS AND HVO

20' 20'N

20' 00'N

19 40'N

19' 20'N

19' 00'N

156 19'W 155 59'W 155' 39'W 155' 19'W 154' 59'W

Figure 20. Map of Hawaii and Loihi with earthquakes recorded by OBSand HVO. Triangles represent individual seismic stations of HVO's seismicnetwork. The three, main regions of clustered earthquakes are shown byovals and each is given a region number: �! HEA, �! UKF, and�! UER/POL. The star marks the epicenter of the only Loihi eventsrecorded by HVO during the study week. Elevation data was compiled byJohn R. Smith and Terri Duennebier SOEST! and obtained by the USGS,NOAA, NOS, NURP, and the U.S. Navy.

53

QO QO C! ~ C! QOI hl; 4 0 eD

CD , 'e QO eh O e O QO OCh

QO r4V1

CD t M t W M W M QOIA M W QQ M W ~ M CO v1

QO t e eeI e

QQ Ch

A R

54

0

QO M

5 4

ch

aO C- aOV!

ChCD & & Y! K IA O

Ch ~ QO

ch 4> A % ~ eD et m

C C> QO

QO ~ CD

O & & O

m C OO QO Ch O O t QQ C W QO

O '4 QO

QO ~ W W QOcn Ch Ch O W t R O C

en 4! QO O

CD m QO Ch N t uD O t Oln

Ch uD QO

Ch O O

Ch QO ~ OQO W CD Ch

O QO QO X O A QO

t W M m C Ch

Ch ~ W QQ OO O

55

Examples of Hawaii Events

The HEA, or Hilea, region is the area located about 15 km south of Mauna Loa

refer back to Figure 6!. It is a flank region primarily characterized by a tectonic stress

regime, thus producing tectonic earthquakes. The five HEA earthquakes from the studyperiod range in magnitude from 1.3 to 4.5 and range in depth from 1.6 to 11,3 km.

The example given in Figure 21 is a large event with a magnitude of 4.5, the

highest magnitude recorded during the week and also the deepest event from the region.

The upper portion of the figure shows 83-second seismograms segments from eight

network stations; the SPT station displays both horizontal components and the vertical

component of motion. The lowermost s tation, KHU, is the station closest to the

earthquake's origin and thus received the seismic waves first. Note the strong first-motion

in all of the vertical signals, particularly at station AIN 25 km away from the event's origin.

The strong arrival is rather expected considering the magnitude of the event.

Also shown by the seismograms is a fair amount of attenuation relative to KHU's

signal in several other stations' signals PPL and SPT! and very little in the remaining

stations WOO, HTC, TRA, and AIN!. Signals from all seven stations for the five HEA

events were analyzed in order to eliminate the possibility of station response as a cause for

weaker signals at several stations. Aside from station PPL generally having a high level of

background noise due to its location near the ocean!, there appears to be no consistency in

the strength or quality of the responses, suggesting heterogeneity within the system being

the main cause of the weaker signals. This was also done for the events in the following

two sections discussing the UKF and UER/POL events, and it resu1ted in the same

conclusion; only one or two stations seemed to be consistently weak, apparently due to the

56

tendency of their locations to be fairly noisy. In addition to system heterogeneity, travel

distance also plays an obvious role in the amount of signal attenuation

Seen below the network signals is a roughly seven-minute segment of OBS record.

Again, there is a very strong first-motion, indicating low first-motion attenuation between

the origin and the OBS. However, the smaller-magnitude events from the HEA region

show more attenuated signals arriving at the OBS, suggesting that perhaps there is a

threshold range of magnitudes above which attenuation appears to be relatively low,

The UKF region, again seen in Figure 6, is the Kaoiki seismic zone located

between Mauna Loa and Kilauea volcanoes. It generally produces tectonic earthquakes at

depths between 5 and 13 km Klein et al., 1987!. The four UKF earthquakes recorded in

this study range in magnitude from 1.5 to 1.9 and range in depth from 7.5 to 10.8 km.

Figure 22 displays signals of a 1.5 UKF earthquake, again with HVO network

signals from six stations above and the OBS recording below. The network signals show

windows of 60 seconds, while the OBS recording is just over one-minute iii duration. The

boxed KFA station's signal is 48 seconds in duration and displays the event's strongest

signal of the network recordings. Relative to KFA, the other stations' signals appear

significantly attenuated, as does the OBS receding. The variable amount of attenuation

between stations on Hawaii emphasizes the effect of the volcanoes' heterogeneity on the

recorded seismic signals.

In addition, the signal pattern of the OBS-recorded signal is unlike those signals

recorded closer to the quake's origin. Given the OBS's distance of 55 km from the event

and geologic features the seismic waves traveled through, such as faults, dikes, and Loihi's

talus-dominated, a seismogram such as the OBS's is reasonably expected. An explanation

for the OBS signal is that perhaps attenuation was greater for the first portion of the signal,

57

causing the second pulse to seem large. However, background noise is relatively high,

which one should consider when looking at the second pulse's amplitude. Another

explanation for the increased amplitude towards the middle of the OBS signal is the

possible arrival of a small local event simultaneous with the 1.5 UKF earthquake.

However, many of the Hawaii events tend to show this type of 'two-pulsed' waveform, so

this is not likely to be the case, It appears that heterogeneity in the volcanic system and

associated variable attenuation rates are major influences on signals at various stations.

Region VER/POL is the upper east rift zone and the adjacent flank just south of it.

Events originating here are likely to be volcano-tectonic in nature and associated with

magma movement. The east rift is a region of known magma transport activity. As magma

intrudes into the rift, the stress regime along the rift zone and adjacent flanks is altered.

Therefore, events in this region are likely to be results of such activity. For the six OBS-

and HVO-recorded events from this region, magnitudes range from 1.3 to 2.5 and range in

depth from 5.0 to 8.6 km.

The final example of a Hawaii event, a 2.5 POL earthquake, originated in the

UER/POL region Figure 23!. Again, the figure displays seismograms from several

network stations and the OBS. HVO seismograms are just over 80 seconds in duration; the

boxed POL signal is 77 seconds long. The OBS recording of the event has a similar time

scale with a duration of roughly 70 seconds. The magnitude of the earthquake explains the

relatively strong vertical signals seen at the stations, except KAE, whose signal is slightly

weaker despite its proximity to the event. This could be due again to heterogeneity with the

waves' travel paths, or it could simply be that station KAE does not record as cleanly as

some of the other stations due to a moderately high level of background noise a result of

its location near the coastline!.

HVO Recording

11 ~ I P I'~ ~ 'I'III I

'I I I 5IJ N ~ ~QQ +1+%1 5~ !III PI j I ~ IIVA I5, 5 5N ~!I 92,VIll %

,II I I AU! I' I5I

'I 555

I ~ II JV1 H

48-second window

OBS Recording

min

Figure 22. UKF earthquake: 4.5 UKF �264!. The top portion displaysroughly 60-second-long HVO recordings from various stations in the UKFregion Kaoiki Seismic Zone!. The boxed portion of the KFA seismogramis 48 seconds. Stations MLO and AIN show both horizontal and verticalcomponents; the others show only the vertical. The bottom portion showsthe OBS recording of the same event on a similar time scale.

60

HVO Recording

-1L1> 'I

Iles 0 wdiP+PmA I~~I1

NNINW<««IWINNIW<«~+-

77-second window

OBS Recording

min

Figure 23. UER/POL Earthguake: 2.5 POL ¹605!. The top portiondisplays roughly 80-second-long HVO recordings from various stations inthe UKF region Kaoiki Seismic Zone!. The boxed portion of the POLseismogram is 77 seconds in duration. Stations AHU and PAU showhorizontal and vertical componeit ts; the others show only the vertical. Thebottom portion of the figure displays the OBS recording of the event on aslightly shorter time scale.

61

Travel Time Data

It was hoped that in analyzing travel times to the OBS of the Hawaii events, it

would be possible to further refine the existing velocity model used for locating Loihi and

Hawaii earthquakes. Essential in the process, however, is the accurate pick of the events'

first arrivals. Most of the Hawaii events in this study had determinable first arrivals, and a

fairly high amount certainty is placed with those picks. On the other hand, there were a

few events whose signals allowed for picking with only a moderate amount of certainty,

Any events with travel times greater than two seconds off the travel time curve are

addressed in the following discussion. The first arrival pick for one event in particular, the

2.0 DIS EON! event, was so uncertain that it was omitted from the curve altogether.

Seen in Figure 24 is a plot of travel times versus distance for the 16 Hawaii

earthquakes which had depths less than 12 km. Their travel time curve is the least-squares

fit line for those points, The lower line is the theoretical curve, based upon Bryan's �992!

velocity model and events which assumed an average source depth of S km. Note that

there is a fair amount of scatter among the points and that every on lies on or above the

theoretical curve. The largest amount of scatter is with two of the UKF quakes, which are

circled in the figure. The points could lie well off the observed line due to the distances of

55 and 60 km to the OBS, or because the signals were so attenuated upon reaching the

OBS that P-wave arrivals were unclear and picked incorrectly. With or without the two

main outliers, it appears that every event traveled slower than the theoretical model

predicted, suggesting that the existing velocity structure is a bit too fast and needs revision.Seen in Figure 25 is a similar travel time curve, but for the four Hawaii events with

depths greater than 30 km. There were actually five events with deep origins, but as was

previously mentioned, the 2.0 DIS KON! earthquake was otnitted from the plot; it was a

distant offshore event with an arrival which was too attenuated to pick with high certainty.

62

20

15CJ

610

0

0 30 40 50 60 702010

Distance km!

Figure 24. Theoretical and observed travel time curves for shallow �2km! Hawaii earthquakes. An average source depth of S km was assumedfor the theoretical curve.

hundredth of a second, which could not be done in this study. However, the data once

again appear to be sufficient enough to suggest that the theoretical model is slightly fast,

The major point to make from the travel time curves is that the scatter of the

observed points is expected. All points lie on or above the theoretical curve; this can be

explained by the fact that the volcanoes do not constitute a homogeneous system. Before

reaching the OBS, earthquake waves must travel through portions of up to three different

63

The lower line is the theoretical curve, based upon Bryan's �992! curve which assumed an

average source depth of 30 km. Note that all of the events, unlike those in the curve for the

shallow events, lie within 0.5 seconds of the travel time curve. For true revision of the

existing velocity model, it would be necessary to get travel times which are accurate to one-

20

15

10

0

0 120 150 18060 9030

Distance km!

Figure 25. Theoretical and observed travel time curves for deep �0 km!Hawaii earthquakes. An average source depth of 30 km was assumed forthe theoretical curve.

volcanoes. Therefore, the waves move through media with different velocities and

absorptive properties. However, the theoretical curve is based upon a layered model that

assumes lateral homogeneity, not taking into account several geological aspects which

cou1d affect travel times. First, there are numerous vertical and dipping faults and dikes

throughout the region. Second, there possibly exists a low-velocity layer at the base of the

crust irom 10 to 13 km depth Crosson & Koyanagi, 1979!. This layer could have P-wave

velocities as low as 4.0 km/s. Third, it is known that inactive flanks have lower velocities

than active rifts due to the rifts' dike systems. Fina11y, Loihi's cap is essentially covered

with a layer of talus or rubble, which would decrease velocities in the area. Therefore,

given these points, one simply cannot expect the events' travel times to follow the,

theoretical curves. If anything, it appears that the theoretical model might possibly be too

64

fast. Further studies using both OBS- and HVO-recorded Hawaii and Loihi earthquakes

are necessary in order to refine the model of Loihi's velocity structure.

Conclusions

The OBS which continuously recorded seismic activity on Loihi from September 1-

7, 1991, proved to be highly useful not only in recording Loihi seismicity, but a good

number of Hawaii earthquakes as well. There are several general conclusions which were

drawn from data involving the Hawaii events:

�! From the 284 events recorded and saved from the study week, 21 were

earthquakes from Hawaii Island. Sixteen were shallow events located on the island, and 5

were deep events, only one of which was actually located on the island � were offshore!.

�! There were 3 main regions in particular with earthquake 'clusters': the Upper

Kaoiki seismic zone UKF!, the Hilea region HEA!, and the upper section of the East Rift

Zone UER/POL!. Distinctive OBS-recorded signals from the various regions support the

fact that each area is different in structure and thus has a different earthquake producing

mechanism.

�! Scatter of the travel times about the least-squares fit line stresses existence of

lateral heterogeneity within the volcanoes and suggests future revisions are needed once a

seismic network is established on Loihi. Much of the heterogeneity within Loihi can be

identified once scientists are able to locate magma storage areas and determine times and

places of magma intrusions. Once this plumbing system is somewhat described, the

velocity structure of Loihi may be further refined. The travel time plots show all of the

Hawaii events falling above the theoretical curve, suggesting the model is too fast, and

again indicating that revisions are needed in the future.

65

APPENDIX

OBS-Recorded Events from September 1-7, 1991

DURATION RELATIVE sec! AMPLITUDE

KQ ¹ FAMILYFILE ¹

1 Spindle

1 Spindle

1 Short-period

1 Spindle

l Short-period

1 Spindle

1 Spindle

1 Spindle

Short-period

2 Short-period

3 HI event

2 Spindle

60 L

5016 L

25

1226

5.2 L-M34

37

2540 H

42 H

6.650

5.7 L

50 H

3361 H

Spindle

Short-period

Short-period

Spindle

Spindle

Spindle

3573

10.9 H88

7.2 H

12093 L

102

30112

Spindle

Spindle

30133

80140

66

Column 2 shows the event number within each file,as some files have more than one event. Dashed lines are used

in place of duration and amplitude for all Hawaii events.For relative amplitudes: L=low, M=medium, H=high.

T-phases have been omitted from the table.

Appendix Continued! OBS-Recorded Kvents

from September 1-7, 1991

168-169 20

170 10.4 H

173 15

175 Long-period

HI event

Spindle

HI event

Lang-period

Long-period

Short-period

Spindle

Short-period

Spindle

Short-period

45 L

177 15 H

182 30

37 H

10.6 L

183 10.4 L

185 60 H

190 L-M

L

H

198 Spindle

Spindle

42 M-H

60 M-H

221 Spindle

Spindle

Spindle

Spindle

Spindle

Spindle

Spindle

Spindle

Spindle

75

225 40

233 24 M-H

60 L

235 46 H

238 30

25

242 96

1 Spindle

2 HI event

3 Spindle

1 Short-period

1 Spindle

Appendix Continued! OBS-Recorded Events

from September 1-7, 1991

247

8.5

27258

264

54268

26271

278

30288

14.1289

62304

17315

24323

120350

18354

27

70359

60361

366

50371-374

12

5b Spindle

5c Spindle 60

5d Spindle 35

1 Spindle

1 Spindle

3 Long-period

1 Spindle

1 HI event

1 Short-period

1 Spindle

1 HI event

1 Spindle

1 Short-period

1 Spindle

2 Spindle

1 Spindle

1 Long-period

2 Long-period

3 HI event

1 Spindle

2 Spindle

2 Spindle

Spindle

1 Spindle

1 Short-period

2 Spindle

3b Long-period

I Spindle

1 Short-period

2a Short-period

2b Short-period

4 Spindle

1 Spindle

1 Spindle

60375

7,4378 L

7.5

4.2 L

24

33381 L-M

40382 H

431 Spindle

2 Long-Period

385

21

1 Spindle

Spindle

1 Spindle

1 Spindle

2 Short-period

3 HI event

21388

60 L-M389

20394

17398 L-M

9.9398 H

1 Spindle

1 Spindle

la Spindle

30

60401 H

60404 H

2 Spindle

3 HI event

4 Spindle

1 Short-period

2 Long-period

3a Spindle

3b Short-period

4 Spindle

1 Long- period

16 L

35 M-H

6.2 L-M

20

20 M-H

4.4 L

120 H

407

69

Appendix Continued! OBS-Recorded Events

from Septetnber 1-7, 1991

LLong-period

Spindle

Spindle

Spindle

Short-period

Short-period

Spindle

HI event

Spindle

Short-period

Short-period

Spindle

Long-period

Short-period

Short-period

Short-period

Spindle

Short-period

Short-period

Short-period

Short-period

Short-period

Spindle

Short-period

Short-period

Short-period

Long- period

408

30

60409-410

32

L-M

H15.2415

Hl.40

425

L30431

9.1436

4.8438

L20

L30

6,1

L7.4

4.3

L5.8451

L5.7452

4.9

453 L4.8

L5.1460

L-M60466

L-M4.8469

470

9.2471

25473

70

Appendix Continued! OBS-Recorded Events

from September 1-7, 1991

Short-period 10.8473 M-H

Spindle 30478 L

Spindle483 H

Short-period 6.6484 M-H

Short-period486 H

Short-period 6.5

10.4

493 L

Short-period494 H

Short-period 7.6

8.8

M-H

Short-period502

HI event

Short-period 6.1505 L-M

Short-period 7.3507

Long-period 33509 L-M

Short-period514-515 16.5 H

2a Short-period

Short-period2b 7.2 M

Short-period

Short-period

Long-period

Short-period

Short-period

Short-period

Short-period

Short-period

Short-period

Long-period

Spindle

4.5 L-M

10.1 H

521 20 L

527 4.1 L-M

528 4.6

7.5 M-H

10.9538 H

6.8

11.2538

34544

45547

71

Appendix Contintred! OBS-Recorded Kvents

from September 1.7, 1991

Appendix Continued! OBS-Recorded Events

from September 1-7, 1991

H30554

35558

559

26564

7.5565

L-M19568

18572

7.3574

H

60 HH576

582

19 H

4.2588

8.2 M-H592

L40

L-M48

6.4597

6.8 H599

26

602

11.3 H604

605

608

615

57617

12 L

L-M618

585-586

Spindle

Long-period

HI Event

Long-period

Short-period

Long-period

Spindle

Short-period

Short-period

Spindle

Spindle

Short-period

Short-period

Short-period

Spindle

Spindle

Short-period

Short-period

Long-period

HI event

Short-period

HI event

HI event

Short-period

Long-period

Long-period

Short-period

1 Short-period

1 Spindle

1 Short-period

1 Long-period

2 Short-period

1 Short-period

1 Short-period

1 Long-period

2 Short-period

1 HI event

1 Spindle

1 Spindle

2 Spindle

3 Spindle

1 Long-period

2 Short-period

3 Long-period

1 Short-period

1 Short-period

1 HI event

2 Spindle

2 Spindle

1 Long-period

1 Spindle

1 Short-period

1 Long-period

1 Short-period

622 L

22 L625

4.5628 L

20629 L

6.7

L634

L-M634.5

. 37635 L-M

11.8637

640

60644 L

60 M-H

60

60 M-H

37648 HH

8.5

26 H

5,5649 L-M

650

651

45 M

30

656 37 M-H

45661 L-M

6.6663

666 20 L

4.8667 L-M

73

Appendix Contintled! OBS-Recorded Kvents

from September 1-7, 1991

L301 Long-period

1 Spindle

1 Spindle

1 Spindle

2 Long-period

3 HI event

1 Short-period

2 Spindle

3 Short-period

1 Spindle

1 Spindle

2 Long-period

3 Short-period

1 Short-period

1 Short-period

2 Spindle

1 Short-period

1 Spindle

1 Spindle

2 Spindle

4 Short-period

1 HI event

2 Spindle

1 Spindle

la Short-period

1b Short-period

2 Long-period

668

60670

L-M24674

M-H50676-677

L32

H4.8683

60683

7.1683

L40692

L-M30695

25

L4.5696

5,4697

H90

H4.7699

18701

1.-M30702-703

H60

7.2

120

20705

711-712

5.2 H

37

74

Appendix Continued! OBS-Recorded Events

from September 1-7, 1991

6.93 Short-period

4 Short-period

5 Long-period

Long-period

1 Spindle

1 Short-period

1 Short-period

1 Spindle

2 Spindle

1 Short-period

2 Short-period

1 Short-period

1 Short-period

1 Spindle

2 Long-period

1 Long-period

1 Short-period

1 Long-period

2 Spindle

1 Spindle

711-712

7.1

27

20714

M60719

5.3720 L-M

7.3721 M-H

37 L-M727

53 H

5.9736

5.2736 M-H

8.2739 M-H

5.7742

743 H40

L

35749 M-H

6.9750 M-H

752-755

100 H

45758 H

760 Short-period

Short-period

HI event

Spindle

HI event

Spindle

Spindle

M-H

L-M

762

60764 H

768

30770

20776 L

75

Appendix Continued! OBS-Recorded Events

from September 1-7, 1991

1 Spindle

1 Spindle

1 Long-period

2 Long-period

3 Long-period

4 Long-period

5 Long-period

1 Spindle

1 Long-period

2 Long-period

1 Spindle

3 Long-period

4a Spindle

4b Long-period

1 Spindle

1 Spindle

2 Long-period

3 Long-period

80778

H30783

22786

31786

M-H30

L22786

18

40 L-M795

L-M26798

16 L-M

800-804

40

14

45816

27 L

14 L-M

76

Appendix Continued! OBS-Recorded Events

from September 1-7, 1991

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