Title Investigation of Microearthquakes in Kinki District ... · St .ASep.Oct.Nov.Dec.Feb. 1964...

Title Investigation of Microearthquakes in Kinki District -Seismicityand mechanism of their occurrence-

Author(s) HASHIZUME, Michio; OIKE, Kazuo; KISHIMOTO,Yoshimichi

Citation Bulletin of the Disaster Prevention Research Institute (1966),15(3): 35-47

Issue Date 1966-03-19

URL http://hdl.handle.net/2433/124708

Right

Type Departmental Bulletin Paper

Textversion publisher

Kyoto University

Bulletin of the Disaster Prevention Research Institute, Vol. 15, Part 3, No. 99, March , 1966

Investigation of Microearthquakes in Kinki District

—Seismicity and mechanism of their occurrence—

By Michio HASHIZUME, Kazuo DIKE and Yoshimichi KISHIMOTO

(Manuscript received January 13, 1966)

Abstract

Observation of microearthquakes in the western part of Kinki District has been carried out at 5 stations attached to the Tottori Microearthquake Observatory. Epicenters of

about 200 earthquakes for ten months since Angust, 1964 were determined, giving a particular pattern of distribution. The areas in which the earthquakes occur frequently

are separated in a complicated but nevertheless sharp manner from the areas where earthquakes seldom occur.

Magnitude of these earthquakes was distributed from -1 up to more than 4. The distribution of direction of the first P motion shows a definite pattern for almost

all earthquakes. This pattern coincides well with that of earthquakes with larger magnitude in this district.

Introduction :

We have been advancing since 1963, the construction of the microearthquake net-work to conduct research into their natures and seismicity. Now we have five stations and the data recorded at those stations has now amounted to fairly large amount.

We intended to make a report in a little more detail after the preliminary report." The outlines of observation stations attached to Tottori Microearth-

quake Observatory are shown in Table 1. The net-work covers the northern Kinki district with nearly 30 km to 40 km span. The observation system is

TABLE 1. Outline of observation stations attached to Tottori

microearthquake observatory.

Mikazuki Funaoka Oya Hikami Izumi

Abrevation Mz Fo Oy Hm Iz

Latitude (E) 34°59'01.3" 35°19'59.3" 35°20'02.0" 35°13'35.5" 34°58'20.0" Longitude (N) 134 26 44.6 134 16 18.4 134 39 52.2 135 02 36.6 134 53 15.5

Height (m) 200 160 260 250 230

Lat. (km) 0 38.768 38.831 26.940 — 1.272 Long. (km) 0 —15.851 19.932 54. 507 40. 351

Components 1V 2H 1V 1V 2H 11/ 1V Sensitivity 400pkine/cm 400pkine/cm 400pkine/cm 400pkine/cm 4001 kine/cm

36 M. HASHIZUME, K. OIKE and Y. KISHIMOTO

N S E W V pt 100 VAC Pick-Up P. U. P. U. 51

1

:i 7 _....„ Voltage Amplifier Amp. Amp.Wr 7 Re•ulator1111111 .5

9

111^W^11•1Er^IIIJ w 01 Cystal Golvanome Ga I v. Galt'.

Clock •-• Recorder Rec. Rec.

Battery 100V 60 c/g 60 c4- Amp. 001 I 0.1 10 _,./ I%) 100

Fig. 1. Blockdiagram of observation Fig. 2. Overall frequency response. system.

illustrated as a block diagram in Fig 1. The figure shows the recording system from the output of seismometer with 1 sec natural period, through electronic amplifier, to drum recorder with ink writing pen galvanometer. A X'tal clock for time mark is corrected once a day by Japanease Standard Time (J. S. T.) and it is always kept in the accuracy of 0.01 sec. Sensitivity is kept at 400

pkine/cm for each station and component. Paper speed is 4 mm/s. Frequency response curve for the allover system is as shown in Fig. 2. In this paper we shall discuss the seismicity, earthquake mechanism and some problems related to them.

Outline of seismicity :

First of all we sum up in Table 2 the number of earthquakes observed at those stations for each month from 1964 Aug. to 1965 June. Here the data are confined to earthquakes of which the maximum velocity amplitude is greater than 40 pkine and the P-S time is less than 15 sec. This figure does not neces-sarily represent the true seismicity because of a little difference or change of sensitivity, the difference of observation components, interuption of observa-tion for a short time (interuption for a long time is corrected proportionally) and so on. But at Mikazuki the earthquakes observed were comparatively many and at Funaoka less earthquakes were observed. As for variation of seismicity,

TABLE 2. Number of earthquakes observed at five stations for each month

from 1964 Aug. to 1965 June.

5196 1964 St.ASep.Oct.Nov.Dec.Feb. Mar. Apr. May. Jun. ugJan.

Mz 80 225 154 108 64 101 80 92 80 193 222

Fo 41 51 33 44 47 57 35 34 43 42 78

Hm 119 76 187 65 64 87 / / 30 57 265

Oy / / / 61 50 67 28 88 72 47 I 107

Investigation of Micro-Earthquakes 37

N M2z.00FoOy"M 2.03 2.011.95

100 -

10 ID •

•

1 1 11111) 11 I1111 I 1 1 111I1 I I 1 1 11111

1 5 1 5 1 5 1 5 Av (mm)

Fig. 3. Relations between the number of earthquakes and muximum amplitude.

however, we must continue the observations for a longer period. In June, 1965, a comparatively large number of earthquakes were observed but they were mainly due to a earthquake swarm occurring at the northwestern part of Hyogo Pref. Then we investigated the so called Ishimoto-lida's coefficient in equation

(1) and show the result in Fig. 3.

N (A) --kA-n5 (1)

A : maximum trace amplitude, N: number of earthquakes,

k: constant. The value of 'm' is said to be about 1.9 to the tectonically caused earthquakes.2) In this district the value is nearly 2.0 and a signficant difference between respec-tive stations could not be seen so far as this stage is concerned.

As for the same kind of statistics, Miyamura, for example, studied the so called Gutenberg-Richter's coefficient 'b' in equation (2) using the data obtained by the net-work of the Japan Meteorological Agency (J.M.A.).

log N=a+b (8—M) (2)

M: magnitude, N: number of earthquakes,

a: constant. He said that 'b' differs for the various stages of tectonic movement, and in our district it is about 0.7.3)

To know the difference between the stations in datail, we must deal with separately the earthquakes in each small region over a longer period.


Distribution of hypocenters :

We determined epicenters of about 200 examples of which the P-S times were within 15 sec and both initial P and S phases were recorded clearly at more than three stations. These earthquakes make up about 20% of the total earth-

quakes observed in our net-work. As for focal depth only about 60 earthquakes could be determined because the data was selected more strictly than in the case of epicenter. Hypocenters were determined as follows.

Adopting the suitable crustal structure for this district and assuming Poisson's ratio to be a =1/4, we determined graphycally the most fitting situations of

hypocenters using P-S time. The crustal structure was determined by the Research Group for Explosion Seismology (R. G. E. S.) by Kurayoshi-Hanabusa explosions, who presented two models, Model I and Model 11,4' and we tentatively used Model II. The results would not be affected greatly by the difference of the models in determing hypocenters because in this case deep earthquakes did not be found out where the difference is great between two models.

The accuracy is about 2-3 km for epicenters and about 5 km for focal depth in average epicentral distance and magnitude. Concerning the method of determining hypocenters an improved one is now being investigated by the authors. The result is shown in Fig. 4 and Fig. 5 for the distribution of epicenter and focal depth respectively. In Fig. 5 the frequency distribution is weighted by reciprocal accuracy for each earthquakes.

One can find the following features in these figures, although the earthquakes were not picked up equally over the long epicentral distance due to the attenua-tion of seismic energy. Some aseismic zones seem to exist, namely,

• •

• 20-

2 . e .

• h P

• 1

N Oy r• i.•1 • ••1.%.• • ...qt. 1 • r. • •11/

•

• Mz a•

• • • A. • • 5

.• • • ° • % Ilk • 6

•

4

. 0 50km

0 10 20 H (km) 30

Fig. 4. Distribution of epicenters. Fig. 5. Distribution of focal depth . x : ofservation stations, H: focal depth in km ,

• : epicenters, N: frequency in number.

O: earthquake swarm at June 1964.


1. The region of the radius of 25 km around Oya, 2. Most western part of Hyogo Pref.,

3. Around the region of Tamba-Sasayama, Hyogo Pref..

As for the regions of high seismicity they are tentatively divided into six

regions referring to geological knowledges and for the convenience of study.

1. Tamba belt,

2. San'in district,

3. Around Oya,

4. Seto inland sea, 5. Rokko mountains,

6. Yodo river zone.

As a whole, northern part of Kinki is thought to be a transient zone from

the highly seismic region of the eastern part to the low-seismic region of the western part. The focal depth is very shallow, concentrating mainly at 10 km

and no earthquakes deeper than 30 km were found.

In Fig. 4 OO mark is a earthquake swarm occuring in June, 1964. The hypo-

centers of 63 earthquakes were determined and they were considered to occur in a very small domain at the depth of 15 km with nearly the same mechanism.

Earthquake swarms of a similar kind MZ but on a much smaller scale have been

N

AUG ••••• APR. observed here and there in this district.

150 - It is an interesting problem to know how

those small earthquake swarms occur

and whether they systematically develop 100 -

to the great earthquake swarms or not.

Now some questions are left as to 50 -pAIV whether or not the above illustrated distribution which was obtained from earth-

0quakes of a limited number represents ••• •

F 0 the actual earthquake activity.

AUG MAY However, reflecting the P-S time dis-

50 -

• •••tribution for each stations shown in Fig. 6, in which the earthquake swarm in 0June, 1964 was omitted as it may not

H

AUG MAY represent the stationary tendency, this

50figure seems to correspond well to Fig. 4. For instance, P-S time of less than

2 sec can rarely be found at Oya, which

° coincides very well with an aseismic zone

OY around Oya, and some frequency peaks

DEC.--M AY

50 are explained well by the previous seismicity map in Fig. 4.

At Mikazuki, a peak of P-S time of

010 20 30 sec 13 sec corresponds to the earthquakes

Fig. 6. P-S time distribution for each near Wakayama City, occurring fre- station. quently and at other stations also this


NIXATIJK•

N 0E5 JAN, 10

°O • 10 20 30

FEL

10 -

0

1 IAINAZUKI MAR.

111114 JAN. 10+

0 i .....--..- ARR.

30 10 ,..Al2._ 20 0

MAY 10

10 O 10 10 i0

MI.

i°-7unE-s--__....

0

0

yak 00 10 20 30 s",,,

0 (a)

O AIM

APR

10

O FUNAOKA

10-kaki- MAY N 1964 MAY

10

...... —411. 10- JUNE

JUNE "

0 10 JUL Y

0 As 0 JULY 10- AUG

.3 0 10-SEP

O4.A •UO0

10- NOV. 10

,,,,,Ct.--, O ...MN, 10- OEC

SEP

10

Aguiiikin_i°c>..1105J:,,,,e,.., • O

1 OCT

100 0

.... ,'MAR O JINA,- NOV 10 APR.

10 I 0 .1..-

O .-, IC. MAY

OEC I • __&_..L ^ 10 ICIALNE 0 10 20 30.22 0 —....A.

10 20 30 um

(a) (b) Fig. 7. P-s time distribution for each month.


6160 MI N 1964 AUG.

10

O - 10- SEP

10- OCT

0 0- NOV

Y A 0N 1964 DEC.

10- DEC0

0 -A

_

0 0 .65 JAN.

10 65 JAN

Abilill&AAa.A0 - A

0

10- FEB. FEB.

I 0 AmAAm... A

o

MAR. MAR.AR 10

• . .AAmA.A 0 10 I APR 10 APR.

.41111,. AL.-A _ o •—^ 100 ill MAY 10 MAY oJUNE0

10JUNE •

O 10 20 30 sec 0 10 20 30sse

(c) (d) Fig. 7. P-S time distribution for each month.

peak is observable at respective P-S time. In Fig. 7, P-S time distribution for each month is shown. The peaks vary

from month to month but as a whole great change of seismic activity can not

be seen.

Magnitude :

Then we determined the magnitude of which epicenters were known. The

method is after Muramatsu who extended and improved the formula of Magni-tude given by Tsuboi for the microearthquakes, studying the microearthquakes

in Gifu Pref.')

In this study he said the maximum velocity amplitude decayed exponentially to the epicentral distance. In Fig. 8 some of the attenuation curves are shown

for each region above-mentioned. In this figure the maximum velocity ampli-tude is estimated from the vertical component.

Sensitivity was regulated fairly well and the irregularities of attenuation curves would be largely owing to other causes such as the difference of phase,

crustal structure, local effects and earthquake mechanism. Irregularity is

great especially in the Rokko mountains. Earthquake magnitude distribution is shown in Fig. 9 except for the earthquakes for which we were not able to

determine the magnitude by this method. Trying to get the coefficient of Gutemberg-Richter's 'b', the coefficient became rather small compared to that

of large earthquakes. This fact would only show the difficulty of determina-tion of microearthquake magnitude, althought it is very neccessary to deter-


' .

•

74

'"'

10, \‘t \ s \ \, ' ' N A\

...._

ioo -'• ioo Ns.• \I• ;„ ,,, 2

, .„._, .. .

6--.. 10

0 10 SEC10e \ ^,''' SEC

(a) (b)

. _

! • '4

y )04//2::. _ CL \.,................

MOO — ssI 2 3.. t , •

•\\•,,,

•

‘\,V111\\ •Ct\\ 0 s....... ,0

'

.1/4XX'..100 _-'S<3 .....A-4 100 :X IOks

•, _ \ x

A) 0x.9, .

, 'cl0.. , I is. 1 1--- 1 1 1 n'

oJo. 10SEC 10SEC

(c) (d)

----------): 5 N 2

1 \ \ , • .----------:it-----X...._ 6

_

P‘, sss, \ --,\\ —x ' s,,•-7---'----, 1%\, r--'

a;1,>.-s• .00 sss„.....„...,...,.........„,

; \ \x ,(x ,...... ‘,....„.........c.....m1..... ..„...x. i "---1•--- - ,...,0\ ssiN x ,..1.._- 1 I I- 01II 10 SEC100 10 SEC

(e) (f) Fig. 8. Attenuation curve for each region.


mine the magnitude correctly enough to

•

get the relation between the microearth- O.

quakes and the large earthquakes ac- .°0

)0 X 0companied by great damage.

x-• 0 If° The mechanism of earthquakes :

t...0000 0 0 XemX_

0`";0'po0The mechanism of earthquakes has ° been studied by many seismologists,

8,00 4 0 theoretically and observationally. And

cog various models of earthquake mechanism o are generally proposed.6)

0 One of them is called "type I", which is represented by a single couple, namely o 2>ma1 two equal and opposite forces with a

• 1>M moment which act at a point. The other,

" type II " , is a force system of double Fig. 9. Earthquake distribution with couples

, perpendicular to each other, magnitude. which also act at a point . The source

of slipping fault plane is explained by the integration of the force system "type IP''" . On the other hand, another model called "cone type" is proposed

in some cases.

The sources of both "type I" and "type II" give the same quadrant distribu-tion of first P motion. Those two types of force system can be distinguished

by investigating the distribution of the first S waves. From the observational standpoint these models of mechanism were given mainly by earthquakes of

comparatively large magnitude. It is also well known that the mechanisms of earthquakes in the same region are frequently similar to each other, which is supposed to suggest that these earthquakes might be caused by the same

stress acting around the region. But little is known about the mechanism of

microearthquakes because of the lack of sufficient data over a wide area,

although it is considered to be very

0 o important to clarify the mechanism of earthquakes.

Then we summed up the distribu- ,

'eer 0•tion of first P motions around those 04°

• ,• g• epicenters as a origins'. Here first P •• •• 4%• .0. 4• motions are confined to only very

•dri`e .• cb••g clear phases. The summed up result ••0••

•• •• of all the earthquakes seems to have •• •

P • a definite pattern except for a few

earthquakes and can be divided into ° • ' quadrants by two nodal lines perpen-

°•

dicular to each other, as shown in

•

Fig. 10. Distribution of first P motionFig. 10. In this figure 0 and 4110 in- around epicenters. dicate the first P of push (or out-

0 : push, • : pull. ward) and pull (or inward) motions,


I

.0,%• ooo se

•••8 •

•

••,.leer

•

• ••r0 0®o..

•

•

. °• •Io 0 • 2

• a o 0

(a) (b)

/ 0 0

I

,

0 ®0 a0 • 1-0

"

0,,0

IP •^• 0'91•8419, 0 SO 1; °o 0 •C2' 0 0 0 •' • ••0 %•• 0 0.. • • •Cb

3 0 4

(c) (d)

1 t o • afb 0 •

• • • ogit 0 • *fp • gb

0 % 243 • • • do ,,•, 0 0 _ 00% CS 0 % 0 • •• •

• • o'• ID • 0 •

6

5

y \ (e) (f)

Fig. 11. Distribution of first P motion for each region.


0

000 0 • 9, o o •• o 0

• 0 .. • A 0 • % — 4,

• es 0

•

• • • • • • o o 0 IP •

• •

0

I a nit

(a)

0• •• 00

0'1° .0 43 e • . 0 . •• 0 •0 0 •

® cos •0 I•.- 80 • • • • • •• •q," • 0 ° • • • f Et% •

• • • • • • •

• • • • • 0

0 • •

2ZM>I M>2

(b) (c) Fig. 12. Distribution of first P motion for each magnitude.

respectively. The distribution is divided into 6 regions previously mentioned as Fig. 11 (a)-(f). The irregularity of distributions in Fig. 10 seems to be owing mainly to two reigions, that is region (2) and region (6). Figs. 12 (a)-(c) indicate the summed up distribution for each magnitude. From these results of micro-earthquakes, it can be said that the axis of maximum pressure lies approxi-mately along the east-west direction in this district and this tectonic pressure affects equally the mechanism of earthquakes down to magnitudes less than zero.

Comparison with the data obtained by the J. M. A. net-work :

To examine how the results obtained by our microearthquake observation are connected with the results from large earthquakes, we dealt with larger earthquakes by the data obtained by the J. M. A., the magnitude of which was mostly more than 4. Distributions of epicenters determined by J. M. A. are shown in Figs. 13, 14 and focal depth in Fig. 15. The earthquakes in Figs. 13. occured from 1926 to 1954 and Fig. 14 from 1955 to 1964. It is very difficult to compare the seismicity of larger earthquakes with that of micro-earthquakes in detail, for want of sufficient data of J. M. A. Nevertheless it may be said that, as a whole, large earthquakes and microearthquakes have


- 36, 1. 6—I I oI

.

. g.0 • e

* • ' . o •

. . • • 0 o06 • 0

, • 0 . 46 6 6•• • • • •. • o• .0• .. ,o

6 t•• ° •••

. .

° •* . .; ?OW—

• — N35•—0 4CE.V, ;

.

•• 0 3

' •

0 7!! . . • 0 6-7 Y-'. .e3":•'-- 4, cpc, • . . .04 _�.).,-_-_4,ccj. /7 \--'---)7,2 `''''- ,,35- 136.

1

c E13,1• • 135. 136• V34.

Fig. 13. Distribution of epicenters Fig. 14. Distribution of epicenters determindd by J. M. A. from 1926 to determined by T. M. A. from 1955 to

1954. 1964.

&

• ., ° t 4 : 134 135.

)Ir • e I/••• 0•:477a.•• .•::.'cl'--:41/(:,-- ' • • *-::: . • .. • • •

•

• • • • •....; 8 ° A:• .0.•

“ ° . . 0 v. (kW

o • o 1 t •o

• •

o

o • •

o I00 ,m

Fig. 15. Distribution of focal depth Fig. 16. Distribution of first P motions determined by J. M. A. from 1926 to from the data of J. M. A.

1964.

a similar tendency in seismic activity. As for earthquake mechanism a similar distribution of the first P motion

previously mentioned is shown in Fig. 16. And we can find that earthquake mechanism is also nearly the same in both larger and microearthquakes. It is very interesting that an earthquake is caused by the same stress irrespec-tive of its magnitude.

Some remarks :

From the geological point of view Maizuru structural belt separates two regions which have a different type of geological structure. Namely, the north-ern part of Maizuru structural belt is called Chugoku type and the southern

part Tamba type, and these two types have different stages of the development of tectonic movement. Region 2 belongs to Chugoku type and is distinguished from other regions.9) And after the study of earthquake mechanism it seems in Fig. 11 (b) that the direction of maximum compressional stress in region 2 rotates clockwise about 30° with regard to other regions.

As for Region 5 it is well known that the so-called Rokko movement have


continued upheaval still in holocene .") And the earthquake mechanism and the attenuation curves were not so uniform as in other regions .

Region 6 has already been reported by Okano and Hirano to be a high seismic region.

Recently the direction of the principal axis of stress was calculated using the geodetically surveyed data in this district. Our seismological results do not necessarily accord to that of geodecy . This fact suggests the stress which causes earthquakes might not be the recent stress acting on the crust , but the stress confined at one geological stage to the crust and only remaining as a latent earthquake.

Ackrtowlegement :

The authors are greatly indebted to Mr. Kazuo Mino for his kind help throughout this study, and also to Mr. Sei Yabe, Miss Ritsuko Matsumura, Mrs. Mitsuko Koga, Miss Sumiko Hagura, Mr. Shigemitsu Matsuo for their assis-tance in keeping observations and analysing records. The writers' thanks are due to the staffs at the observation stations for their keeping laborious observations.

References

1) Kishimoto, Y. et al. : Some Properties of Microearthquakes in the Western Part of Kinki District (Preliminaries)., Special Contributions of the Geophys. Inst., Kyoto

Univ., No. 4, 1964, p. 51. 2) Ishimoto, M. and Iida, K. : Observations sur les Seisms Energistre par le Micro-

Seismograph Construit Dernierement (1), Bull. Earthq. Res. Inst., Vol. 17, 1939. p. 443.

3) Miyamura, S. : Seismicity and Geotectonics, Zisin, Vol. 15, 1962, No. 1. p. 23. 4) Hasizume, M. et al : Crustal Structure in the Westeron Part of Japan Derived from

Observations of the First, Second Kurayoshi and Hanabusa Explosions. Part II. Crustal Structure in the Western Part of Japan, Bull. Earthq. Res. Inst., Vol. 44. (1966) Part I. (in press).

5) Muramatsu, I. : Observation of Microearthquakes in Mino District in Gifu Prefecture, Central Japan, J. Phys. Earth, Vol. 11, 1963, No. 3. p. 35.

6) Honda, H. : Earthquake Mechanism and Seismic Waves, J. Phys. Earth, Vol. 10 (1962), No. 2.

7) Maruyama, T. : On the Force Equivalents of Dynamical Elastic Dislocations with Reference to the Earthquake Mechanism, Bull. Earthq. Res. Inst., Vol. 41, 1963. p.

467. 8) Gutenberg B.. Mechanissm of Faulting in Southern California Indicated by Seismo-

grams, Bull. Seism. Soc. Amer., 31, 1941. p. 263. 9) The Association for Geological collaboration of Japan.. The Geologic Development

of the Japanese Islands, Tsukiji shokan, 1965. 10) Kasama, T, and Huzita, K.: Geological Properties of Setouchi Geologic Province,

Cenozoic Res, Nos. 24-25, 1957.

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