Introduction

2. Introduction • Turbidites:geological formations that have their origins in turbidity currents deposits, that deposit from a form of underwater avalanche that are responsible for distributing vast amounts of clastic sediment into the deep ocean. • Sediments are transported and deposited bydensity flow, not by tractional or frictional flow. • Bouma sequence:from conglomerates at the bottom to shales on the top Idealised sequence of sedimentary textures and structures in a classical turbidite, or Bouma sequence (Bouma, 1962).

3. Introduction • Interest of the off-fault paleoseismology • GPS → high degree of certainty, in few years, of the crustal strain accumulation.. But just for a portion of a cycle.. • Earthquake records → not long enough • Onshore paleoseismology → erosion, urban area.. • Off-fault paleoseismology • Interest of marine turbidite records • Have to prove they are earthquake-triggered • Marine records: more continuous, extend further back in time, more precise in time (datable foraminifera) • Method used • 74 piston, gravity cores from channel/canyon systems draining Northern California • Mapping channels with multibeam sonar (bathymetry, channel morphology, sedimentation patterns • Sampled all major channel systems between Mendocino and north of Monterey Bay • Results • Good agreement with shorter land record • Opportunity to investigate long tem earthquake behaviour of North San Andreas Fault

4. Piston core removed from corer Piston corer Split piston core being subsampled. http://oceanworld.tamu.edu/students/forams/forams_piston_coring.htm

5. 4 segments of SAF: • Santa Cruz Mountains • Peninsula • North Coast • Offshore • Several onshore paleoseismic sites: • Vedanta: max slip rate in late Holocene 24 +/- 3mm/yr and 210 +/- 40 years • Fort Ross: ~230 yr • South of the Golden Gate: 17 mm/yr

6. How to identify earthquake-triggered turbidites • Possible causes of turbidites: • Storm or tsunami wave loading • Sediment loading • Storm discharges • Earthquakes • Seismically triggered turbidites are different: • Wide area extent • Multiple coarse fraction pulses • Variable provenance • Greater depositional volume • Use a temporal and spatial pattern of event correlation over 320 km of coastline

7. Synchronous triggering and correlative deposition of turbidites • Regional stratigraphic datum missing • Correlations depend on stratigraphic correlations of other datums and radiocarbon ages • The Confluence Test: • If one canyon contains n turbidites and a second canyon also shows n turbidites, and if these n events have been independently triggered, the channel below the confluence should contain at least 2n instead of only n. • 8 major confluences • 3 heavy minerals

8. Event “fingerprinting” • All cores are scanned, collecting P-wave velocity, gamma-ray density, magnetic susceptibility data, imaged with X-radio and grain size analyzed

9. Event “fingerprint” • First, these data were used to correlate stratigraphy between coresat a single site • Found that it was possible to correlate unique physical property signatures ofindividual turbiditesfrom different sites within the same channel • Even possible to correlate turbiditesbetween different channels(some of which never met) • The turbidite “fingerprint” = basis of long-distance correlations

11. Event “fingerprint” Evolution of a single event down channel over a distance of 74 km

12. Radiocarbon analysis • Extraction ofplanktic foraminiferafrom the hemipelagic sediment below each turbidite • Bioturbation and basal erosion do not biase 14C ages • Method: • Determine hemipelagic thickness • Estimate the degree of basal erosion • Observe that differential erosion is most likely source of variability at any site • Conversion of hemipelagic thickness to time (using average of sedimentation rate)

14. Both have 22 events Less dated turbidites Low foram abundance Results Upper section poorly preserved

15. Results: confluence and mineralogy • Good correlationbetween these cores suggests that input mixing at each confluence has little effect on the stratigraphy of the turbidites • Synchronous triggering is theonly viable explanation • Non-synchronous triggering should produce an amalgamated record that increases in complexity below each confluence, with only partial correlations for the synchronous events • Strict test of synchroneity

16. Results: stratigraphic correlation Regional correlation of turbidite stratigraphy spanning the Holocene

17. Results: stratigraphic correlation Noyo canyon is cut by the NSAF and as an epicentral distance of zero → explains thicker turbidite records

18. Time 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

19. Discussion • Good correspondencewith land paleoseismic dates (individual matching, total number of events) • Offshore turbidites as paleoseismic indicators for the NSAF • Mean recurrence interval coherent with onshore • Epicentral distanceis the controlling factor for turbidite size • Turbidites correlate across channels where the mineralogy is different, the physiography is different the sediment sources are different and the underlying geology is different too • Minimum magnitude and triggering distancefrom the earthquake hypocenter : at least M7.4 • But observations of turbidites of small events may also be a function of the resolutions of the observations • Majority of repeat time intervalsbetween 150 and 250 yr