IntroductionIntroduction

Sediments are transported Sediments are transported

and deposited byand deposited by density density flowflow, not by tractional or , not by tractional or frictional flow.frictional flow.

Bouma sequence:Bouma sequence: from conglomerates at the from conglomerates at the bottom to shales on the topbottom to shales on the top

Turbidites:Turbidites: geological formations that have their origins geological formations that have their origins in turbidity currents deposits, that deposit from a form of in turbidity currents deposits, that deposit from a form of underwater avalanche that are responsible for distributing vast underwater avalanche that are responsible for distributing vast amounts of clastic sediment into the deep ocean. amounts of clastic sediment into the deep ocean.

Idealised sequence of sedimentary textures and structures in a classical turbidite, or Bouma sequence (Bouma, 1962).

IntroductionIntroduction Interest of the off-fault paleoseismologyInterest of the off-fault paleoseismology

– GPS GPS → high degree of certainty, in few years, of the crustal strain → high degree of certainty, in few years, of the crustal strain accumulation.. But just for a portion of a cycle..accumulation.. But just for a portion of a cycle..

– Earthquake records Earthquake records → not long enough→ not long enough– Onshore paleoseismology Onshore paleoseismology → erosion, urban area..→ erosion, urban area..– Off-fault paleoseismologyOff-fault paleoseismology

Interest of marine turbidite recordsInterest of marine turbidite records– Have to prove they are earthquake-triggeredHave to prove they are earthquake-triggered– Marine records: more continuous, extend further back in time, more Marine records: more continuous, extend further back in time, more

precise in time (datable foraminifera)precise in time (datable foraminifera) Method usedMethod used

– 74 piston, gravity cores from channel/canyon systems draining 74 piston, gravity cores from channel/canyon systems draining Northern CaliforniaNorthern California

– Mapping channels with multibeam sonar (bathymetry, channel Mapping channels with multibeam sonar (bathymetry, channel morphology, sedimentation patternsmorphology, sedimentation patterns

– Sampled all major channel systems between Mendocino and north of Sampled all major channel systems between Mendocino and north of Monterey BayMonterey Bay

ResultsResults– Good agreement with shorter land recordGood agreement with shorter land record– Opportunity to investigate long tem earthquake behaviour of North Opportunity to investigate long tem earthquake behaviour of North

San Andreas FaultSan Andreas Fault

Piston corerPiston corer

Piston core Piston core removed removed from corerfrom corer

Split piston Split piston core being core being subsampled. subsampled.

http://oceanworld.tamu.edu/students/forams/forams_piston_coring.htm

4 segments of 4 segments of SAF:SAF:– Santa Cruz Santa Cruz

MountainsMountains– PeninsulaPeninsula– North CoastNorth Coast– OffshoreOffshore

Several onshore Several onshore paleoseismic paleoseismic sites:sites:– Vedanta:Vedanta: max max

slip rate in late slip rate in late Holocene 24 +/- Holocene 24 +/- 3mm/yr and 210 3mm/yr and 210 +/- 40 years+/- 40 years

– Fort Ross:Fort Ross: ~230 ~230 yryr

– South of the South of the Golden Gate:Golden Gate: 17 17 mm/yrmm/yr

How to identify earthquake-triggered turbiditesHow to identify earthquake-triggered turbidites Possible causesPossible causes of turbidites: of turbidites:

Storm or tsunami wave loadingStorm or tsunami wave loading Sediment loadingSediment loading Storm dischargesStorm discharges EarthquakesEarthquakes

Seismically triggered turbidites are Seismically triggered turbidites are differentdifferent::

Wide area extentWide area extent Multiple coarse fraction pulsesMultiple coarse fraction pulses Variable provenanceVariable provenance Greater depositional volumeGreater depositional volume

Use a Use a temporal and spatial patterntemporal and spatial pattern of of event correlation event correlation over 320 kmover 320 km of coastline of coastline

Synchronous triggering and Synchronous triggering and correlative deposition of turbiditescorrelative deposition of turbidites

Regional stratigraphic datum Regional stratigraphic datum missingmissing

Correlations depend on Correlations depend on stratigraphic correlations of stratigraphic correlations of other datums and other datums and radiocarbon agesradiocarbon ages

The The Confluence TestConfluence Test::– If one canyon contains If one canyon contains nn

turbidites and a second turbidites and a second canyon also shows canyon also shows nn turbidites, and if these turbidites, and if these nn events have been events have been independently triggered, the independently triggered, the channel below the confluence channel below the confluence should contain at least 2should contain at least 2nn instead of only instead of only nn..

8 major confluences8 major confluences 3 heavy minerals3 heavy minerals

Event “fingerprinting”Event “fingerprinting” All cores are scanned, collecting P-wave velocity, gamma-ray density, All cores are scanned, collecting P-wave velocity, gamma-ray density,

magnetic susceptibility data, imaged with X-radio and grain size analyzedmagnetic susceptibility data, imaged with X-radio and grain size analyzed

Event “fingerprint”Event “fingerprint”

First, these data were used to correlate First, these data were used to correlate stratigraphy between coresstratigraphy between cores at a single siteat a single site

Found that it was possible to correlate unique Found that it was possible to correlate unique physical property signatures ofphysical property signatures of individual individual turbiditesturbidites from different sites within the from different sites within the same channelsame channel

Even possible to correlate turbiditesEven possible to correlate turbidites between between different channelsdifferent channels (some of which never met)(some of which never met)

The turbidite “fingerprint” = basis of long-The turbidite “fingerprint” = basis of long-distance correlationsdistance correlations

Event “fingerprint”Event “fingerprint”

Evolution of a single event down channel over a distance of 74 km

Radiocarbon analysisRadiocarbon analysis

Extraction ofExtraction of planktic foraminiferaplanktic foraminifera from the from the hemipelagic sediment below each turbiditehemipelagic sediment below each turbidite

Bioturbation and basal erosion do not biase Bioturbation and basal erosion do not biase 1414C C agesages

Method:Method:– Determine hemipelagic thicknessDetermine hemipelagic thickness– Estimate the degree of basal erosionEstimate the degree of basal erosion– Observe that differential erosion is most likely Observe that differential erosion is most likely

source of variability at any sitesource of variability at any site– Conversion of hemipelagic thickness to time Conversion of hemipelagic thickness to time

(using average of sedimentation rate)(using average of sedimentation rate)

ResultsResultsU

pper

sect

ion

poorl

y p

rese

rved

Less dated turbiditesLow foram abundance

Both have 22 events

Results: confluence and mineralogyResults: confluence and mineralogy

Good correlationGood correlation between these cores between these cores suggests that input mixing at each confluence suggests that input mixing at each confluence has little effect on the stratigraphy of the has little effect on the stratigraphy of the turbidites turbidites

Synchronous triggering is theSynchronous triggering is the only viable only viable explanationexplanation

Non-synchronous triggering should produce an Non-synchronous triggering should produce an amalgamated record that increases in complexity amalgamated record that increases in complexity below each confluence, with only partial below each confluence, with only partial correlations for the synchronous eventscorrelations for the synchronous events

Strict test of synchroneityStrict test of synchroneity

Results: stratigraphic correlationResults: stratigraphic correlationRegional correlation of turbidite stratigraphy spanning the Holocene

Results: stratigraphic correlationResults: stratigraphic correlationNoyo canyon is cut by the NSAF and as an epicentral distance of zero → explains thicker turbidite records→ explains thicker turbidite records

Time seriesTime series

-The youngest 15 events have a mean repeat time of ~200 yr +/ 60 yr-~95 yr: minimum interval-~270 yr: maximum value-Values consistent with previous paleoseismic data onshore-Same total number of events onshore and offshore = land and marine record the same events

DiscussionDiscussion Good correspondenceGood correspondence with land paleoseismic dates with land paleoseismic dates

(individual matching, total number of events)(individual matching, total number of events) Offshore turbidites as paleoseismic indicators for Offshore turbidites as paleoseismic indicators for

the NSAFthe NSAF Mean recurrence interval coherent with onshoreMean recurrence interval coherent with onshore Epicentral distanceEpicentral distance is the controlling factor for turbidite is the controlling factor for turbidite

sizesize Turbidites correlate across channels where the mineralogy is Turbidites correlate across channels where the mineralogy is

different, the physiography is different the sediment sources different, the physiography is different the sediment sources are different and the underlying geology is different tooare different and the underlying geology is different too

Minimum magnitude and triggering distanceMinimum magnitude and triggering distance from from the earthquake hypocenter : at least M7.4the earthquake hypocenter : at least M7.4

But observations of turbidites of small events may also be a But observations of turbidites of small events may also be a function of the resolutions of the observationsfunction of the resolutions of the observations

Majority of repeat time intervalsMajority of repeat time intervals between 150 and 250 yrbetween 150 and 250 yr