Possible link found between earthquakes along the Cascadia and San Andreas faults

San Francisco (April 3, 2008) -- Seismic activity on the southern Cascadia Subduction fault may have triggered major earthquakes along the northern San Andreas Fault, according to new research published by the Bulletin of Seismological Society of America (BSSA). The research refines the recurrence rate for the southern portion of the Cascadia fault to approximately every 220 years for the last 3000 years.

Chris Goldfinger, associate professor of marine geology and geophysics at Oregon State University, and colleagues published their results in the April issue of BSSA as part of a special section on the 1906 San Francisco earthquake. BSSA is published by the Seismological Society of America (SSA), which was created in response to the 1906 earthquake.

Using marine sediment cores collected along the northern California seabed, researchers identified 15 turbidites, which are sediment deposits generated by submarine landslides and commonly triggered by earthquakes. The 15 turbidites, including one associated with the great 1906 earthquake, and the corresponding land paleoseismic record establish an average recurrence rate of approximately 200 – 240 years for the San Andreas Fault.

In a parallel study, they found that during the same period, 13 of these 15 San Andreas earthquakes occurred at almost the same time as earthquakes along the southern Cascadia Subduction Zone, which stretches from northern Vancouver Island to northern California. The marine and land paleoseismic record suggest a recurrence rate of approximately 220 years for the southern Cascadia fault, which is substantially shorter than the 600-year cycle suggested by previous research for full ruptures in Cascadia.

The Cascadia earthquakes also preceded the San Andreas earthquakes by an average of 25 to 45 years. “It’s either an amazing coincidence or one fault triggered the other,” said Goldfinger. The generally larger size of the Cascadia earthquakes, and the timing evidence suggests Cascadia may trigger the San Andreas Two seismic events on the San Andreas were apparently not associated with Cascadia, including the 1906 earthquake which followed the previous Cascadia earthquake by approximately 200 years.