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Multi-fault ruptures can trigger stronger earthquakes

Staff writer |
A study has lent historic evidence to support the idea that the rupturing of multiple faults can trigger more powerful earthquakes.

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As the talk about "a big one" has been around for decades in California, a researcher, during his work at Stanford University in Northern California, looked into a 7.5-magnitude temblor more than 200 years ago.

The quake on December 8, 1812, one of the biggest in the history of the Golden State, took place along the San Andreas Fault, a continental transform fault that forms the tectonic boundary between the Pacific Plate and the North American Plate.

The fault, which extends some 1,300 km through California, has previously been blamed entirely for the 1812 event. However, the new study suggested that the nearby San Jacinto Fault, considered to be the most seismically active, slipped first somewhere in nowadays Southern California.

As the rupture along the San Jacinto Fault propagated north, it crossed over to the San Andreas Fault in a region where the two fault lines run as close as 1.5 kilometers, triggering the deadly quake in Southern California.

"This study shows that the San Jacinto Fault is an important player that can influence what the San Andreas Fault is doing and lead to major earthquakes," said Julian Lozos, the author of the study, who conducted the work as a postdoctoral fellow at Stanford.

"It's important that we learn more about how activity on any single fault can affect other faults."

Lozos, now an assistant professor of geological sciences at California State University, Northridge, used physics-based computer simulations known as dynamic rupture models and then compared the results to geologic records of slip in prehistoric earthquakes on both faults.

"People shouldn't just be thinking about the San Andreas Fault. There are lots of other faults, so it's important for everyone in the regions at risk to have their earthquake kits ready."

Taking possible multi-fault ruptures into considerations, the most recent Uniform California Earthquake Rupture Forecast (UCERF) report, which was published a year ago, projected that the estimated chance for a magnitude 8 or larger earthquake to jolt California by 2045 rose from about 4.7 percent in a previous UCERF report to about 7 percent in the latest forecast.

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