Julia Gutiérrez-Pastor

Late Holocene Rupture of the Northern San Andreas Fault and Possible Stress Linkage to the Cascadia Subduction Zone

We relate the late Holocene northern San Andreas fault (NSAF) paleo- seismic history developed using marine sediment cores along the northern California continental margin to a similar dataset of cores collected along the Cascadia margin, including channels from Barclay Canyon off Vancouver Island to just north of Mon- terey Bay. Stratigraphic correlation and evidence of synchronous triggering imply earthquake origin, and both temporal records are compatible with onshore paleoseis- mic data. In order to make comparisons between the temporal earthquake records from the NSAF and Cascadia, we refine correlations of southern Cascadia great earth- quakes, including the land paleoseismic record. Along the NSAF during the last ∼2800 yr, 15 turbidites, including one likely from the great 1906 earthquake, establish an average repeat time of ∼200 yr, similar to the onshore value of ∼240 yr. The combined land and marine paleoseismic record from the southern Cascadia subduction zone includes a similar number of events during the same period. While the average recurrence interval for full-margin Cascadia events is ∼520 yr, the southern Cascadia margin has a repeat time of ∼220 yr, similar to that of the NSAF. Thirteen of the 15 NSAF events were preceded by Cascadia events by ∼0-80 yr, averaging 25-45 yr (as compared to ∼80-400 yr by which Cascadia events follow the NSAF). Based on the temporal association, we model the coseismic and cumulative post- seismic deformation from great Cascadia megathrust events and compute related stress changes along the NSAF in order to test the possibility that Cascadia earth- quakes triggered the penultimate, and perhaps other, NSAF events. The Coulomb fail- ure stress (CFS) resulting from viscous deformation related to a Cascadia earthquake over ∼60 yr does not contribute significantly to the total CFS on the NSAF. However, the coseismic deformation increases CFS on the northern San Andreas fault (NSAF )b y up to about 9 bars offshore of Point Delgada, most likely enough to trigger that fault to fail in north-to-south propagating ruptures.

Earthquake Control of Holocene Turbidite Frequency Confirmed by Hemipelagic Sedimentation Chronology on the Cascadia and Northern California Active Continental Margins

This paper analyzes recurrence times of Holocene turbidites as proxies for earthquakes on the Cascadia and northern California active margins of western Northern America. We compare the age, frequency, and recurrence time intervals of turbidites using two methods: (1) radiometric dating (14C method), and (2) relative dating, using hemipelagic sediment thickness and sedimentation rates (H method). The two approaches complement each other, and when used together provide a better age framework than 14C ages alone. Comparison of hemipelagic sediment thickness in several cores from the same site is used to evaluate the erosiveness of turbidity currents and improve the correlation of turbidites and consequent paleoseismic history based only on less complete and unrefined data sets of 14C turbidite ages along the continental margin. Chronology of hemipelagic sediment thickness provides (1) the best estimate of minimum recurrence times, which are the most important for seismic hazards risk analysis, and (2) the most complete dataset of recurrence times, which shows a normal distribution pattern for paleoseismic turbidite frequencies. We observe that on these tectonically active continental margins, during the sea-level highstand of Holocene time, triggering of turbidity currents is controlled dominantly by earthquakes, and paleoseismic turbidites have an average recurrence time of ~ 550 yr in northern Cascadia Basin and ~ 200 yr along northern California margin. This difference in frequency of turbidites in a subduction zone compared to a transform-fault margin suggests significant differences in earthquake activity that compare favorably with independent paleoseismic indicators.