April 1, 2008 - ACORVALLIS, Ore. A new comparison of earthquakes that have taken place along the West Coast during the past 10,000 years suggests that seismic activity in the Cascadia Subduction Zone off Oregon and Washington may actually have triggered earthquake events in the San Andreas Fault in the San Francisco Bay area.
The analysis also concludes that major earthquakes occur much more frequently in the southern part of Cascadia in a range of 270 to 525 years depending on location rather than every 500-600 years as is known for northern Cascadia.
The study is being published in the April issue of the Bulletin of Seismological Society of America.
A research team led by Chris Goldfinger, an associate professor of marine geology at Oregon State University, sampled marine sediments along the northern California coast to look for evidence of historic seismic activity along the San Andreas Fault. The researchers were looking for “turbidites,” which are coarse sediments that accumulate in the abyssal plain during major earthquakes.
“The turbidites stand out from the finer particles that accumulate on a regular basis between major tectonic events and provide a nice timeline for seismic activity,” Goldfinger said.
The core sampling revealed 15 separate turbidite layers that were deposited over the last 3,000 years and correspond to evidence from the terrestrial paleoseismic record along the northern San Andreas Fault. But what surprised scientists was the discovery that 13 of those earthquakes occurred in close conjunction with major earthquakes in the southern Cascadia Subduction Zone.
“It’s either an amazing coincidence,” Goldfinger said, “or one fault triggered the other. It looks like when Cascadia is hit by a major earthquake, another will occur in the San Andreas region on average, within several decades, but possibly less.
“They could be separated by decades or years,” he added, “but it is possible that it could be days or hours.”
Cascadia earthquakes are generally larger, Goldfinger pointed out, and the timing suggests that earthquakes in Cascadia would be more likely to be the triggering mechanism to San Andreas activity than vice versa. This conclusion is supported by stress modeling, including work outlined in a paper by Roland Burgmann and Kelly Grijalva of the University of California at Berkeley.
In previous research, Goldfinger has documented 34 major earthquakes in the Cascadia Subduction Zone during the past 10,000 years, including at least 19 quakes that ruptured along the entire length of the zone. Such a major event would have required an earthquake of magnitude 8.5 or larger, he says.
Going back farther than 10,000 years into the geologic record has been difficult because the sea level used to be lower and West Coast rivers emptied directly into offshore canyons, making it difficult to isolate the turbidites from storm debris.