Orange County Register

San Andreas fault. Image courtesy of Rice University.

The magnitude 9.0 earthquake that occurred off Sumatra, Indonesia in 2004, killing about 230,000 people, might have weakened California’s San Andreas fault, scientists from Rice University say in a new paper published in the journal Nature.

There’s evidence that the Sumatra event, combined with a 6.0 near the central California town of Parkfield in 2004, changed stress levels on the San Andreas, which has shaken Orange County many times. But it’s unknown whether the suspected change will soon lead to a major quake in central San Andreas, about 200 miles northwest of Santa Ana. And at least one prominent scientist — Susan Hough of the U.S. Geological Survey in Pasadena — says the paper’s logic is not “air tight.”

Researcher Fenglin Niu. Image courtesy of Rice University

Rice researcher Fenglin Niu and his collaborators studied the strength of the central San Andreas by examining microquakes that occurred in the Parkfield area between 1987-2008. Parkfield is located along the fault. The new study concludes that the fault significantly changed three times during that period, including a change tied to the 1992 Landers quake near Palm Springs. (Press advisory).

“It has been found that earthquakes can be triggered by passing seismic waves generated by a remote large earthquake,” Niu told me by email. “But our results indicate that such waves can also change the strength of the fault by redistributing the fluids within the fault zone. We speculate that this effect becomes more prominent when the fault is weak, such as the case in Parkfield, where the M6 Earthquake created many fractures. So you are right that we believe that at least the Parkfield portion of the San Andreas fault were  weakened collectively by the M6 Parkfield earthquake and the M9 Sumatra earthquake.”

Map by David K. Lynch

The USGS’ Hough said,  “I read the paper…it presents some interesting ideas and data analysis, but when you look closely at the observations, the argument is not exactly air-tight.  For example, the changes that were supposedly set into motion by Sumatra did not coincide precisely with the supposed anomalies observed at Parkfield.  Also, it isn’t clear the anomalies are significant when you compare them to the fluctuations at other times.

“Finally, neither the big/close Landers quake nor the huge/distant Sumatra quake triggered any earthquakes at Parkfield.  The authors — the senior one of whom died very tragically in a car accident a month or two ago — are asking interesting questions, and doing careful analysis of the data, but we’ll need to look at data from a bunch of quakes like Landers and Sumatra to draw any definitive conclusions.”

A dozen small waves generated by the Samoan tsunami enter the  mouth of the Santa Ana River

Still time to register for the Oct. 15 Great California Shakeout earthquake drill

• Live Twitter feed on earthquakes
• MULTI-MEDIA: What a big tsunami might do to O.C.
• More than a dozen small tsunami waves enter Santa Ana River
• What if a big tsunami hit Orange County?

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