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May 15 / Dave Schumaker

Sichuan Quake Ruptured in 2 Stages

According to Yuji Yagi, a seismologist at Tsukuba University in Japan, Monday’s Sichuan earthquake in China ruptured in 2 stages.

Yuji Yagi, a seismologist at Tsukuba University, said data show the 155-mile Longmenshan Fault tore in two sections, the first one ripping about seven yards, followed by a second one that sheared four yards.

Despite the two-stage quake, which he estimated lasted for about two minutes, it was the shallowness of the epicenter – only 6 miles – that contributed most to the temblor’s destructive power, he said.

“The damage was very severe because the quake’s epicenter was shallow, and the quake occurred in densely populated areas,” said Yagi.

The AP article doesn’t have many details, but it sounds like they are talking about a phenomenon that is similar to a “cascading earthquake,” which happens when rupture on one fault starts a chain reaction that causes a simultaneous rupture on a nearby fault, in essence, creating two earthquakes.

I thought we’ve talked about cascading earthquakes before on Geology News, but looking through the archives, it appears I have never posted about them here. I did briefly mention them on Metafilter back in 2006 (and referenced the same article above).

James Dolan of the University of Southern California in Los Angeles says that the work “reinforces our sense that earthquakes on one fault can and have triggered earthquakes on another.” He has dug trenches on the Cucamonga fault, showing that the fault breaks every 500 to 1,000 years, evidence that the USGS team used. The Sierra Madre ruptures every 5,000 to 10,000 and the San Jacinto every 100 to 300 years. An event cascading across all three would be the region’s “doomsday quake,” Dolan says. “If this ever happens, it’s incredibly infrequent.”

The doomsday event Ralph Archuletta, a seismologist at the University of California at Santa Barbara, imagines is the San Andreas Fault failing along with a big thrust fault, which might be hidden in the valley. But he agrees that the three faults acting alone, which are closer to metropolitan areas than the San Andreas, would be worrisome. “A 7.5-magnitude earthquake near a populated area is not insignificant — whether or not it implies a Denali-style event,” he says.

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