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

Abstract

We relate the late Holocene northern San Andreas fault (NSAF) paleoseismic 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 Monterey Bay. Stratigraphic correlation and evidence of synchronous triggering imply earthquake origin, and both temporal records are compatible with onshore paleoseismic data. In order to make comparisons between the temporal earthquake records from the NSAF and Cascadia, we refine correlations of southern Cascadia great earthquakes, 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 postseismic deformation from great Cascadia megathrust events and compute related stress changes along the NSAF in order to test the possibility that Cascadia earthquakes triggered the penultimate, and perhaps other, NSAF events. The Coulomb failure 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) by up to about 9 bars offshore of Point Delgada, most likely enough to trigger that fault to fail in north-to-south propagating ruptures.