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. Online Material: Relative timing of NSAF and Cascadia events and estimated Cascadia slip models.

Ocean loading effects on stress at near shore plate boundary fault systems

“…The shear stress perturbation along the CSZ is larger than along the SAF, mostly because the sense of shear on the CSZ is thrust motion as opposed to strike slip, and tends to inhibit failure along the coastline. A smaller coefficient of friction in this region, m f = 0.4 [Goldfinger et al, 2008], makes the Coulomb stress perturbation at 10 km depth near the shoreline about −0.4 MPa inhibiting failure, smaller than at either the SAF or Alpine fault and corresponding to a stress perturbation rate of −0.04 kPa/yr, much smaller than the tectonic loading rate. Because the fault zone straddles the coastline, the relative strengthening and weakening of different sections of the fault could possibly lead to a shift in the type of rupture on the CSZ.…”

Rapid detection and characterization of large earthquakes using quasi-finite-source Green's functions in continuous moment tensor inversion

“…Its seismicity is mostly located offshore: along the Mendocino Transform Fault (MTF), within the highly deformed Gorda plate, and along the Cascadia Subduction Zone (CSZ) (Figure 1). Paleoseismic evidence of great earthquakes along the CSZ [Atwater, 1987], including radiocarbon dating and several offshore turbidite episodes of deformation in the last 2000 years [Clarke and Carver, 1992], indicate that the CSZ is capable of multiple M8+ earthquakes, as large as M9 [Goldfinger et al, 2008;Satake et al, 1996;Heaton and Hartzell, 1987].…”

“…[6] In northern California, current realtime earthquake monitoring is performed sequentially by the U.S. Geological Survey at Menlo Park and the Berkeley Seismological Laboratory [Gee et al, 2003]. Because seismic stations are located onshore, poor azimuthal coverage of the seismic wave radiation patterns often leads to difficulties in detecting and locating events, and thus in determining their full source parameters.…”

“…It becomes possible to monitor all M > 3.5 earthquakes with a direct procedure that autonomously detects, locates, and computes source parameters. This is performed using a single step procedure in contrast to the standard cascade-type analysis currently employed in northern California [Gee et al, 2003]. …”

Bridge Seismic Retrofit Measures Considering Subduction Zone Earthquakes

“…The Cascadia subduction zone is the over 1000km long boundary between the Juan de Fuca and North American plates. Geological evidence has shown that 13 significant earthquakes have occurred in the past 3000 years (Goldfinger et al 2008). The most notable of which, the M9.0 earthquake of 1700, produced a tsunami large enough to reach Japan (Atwater et al 2005).…”