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Big earthquakes can weaken other faults

By James G. Robertson, National Geographic Digital Media

U.S. researchers announced Wednesday, the same day earthquakes and tsunamis rocked the South Pacific, that the 2004 earthquake that caused tsunamis in the Indian Ocean also weakened the San Andreas Fault in California (See pictures of the 2004 tsunami).

Photo: The San Andreas Fault. NG Photo by James P. Blair

The researchers say this is the first evidence that an earthquake can change the fault strength, or the stress level required for the fault to slip, in a different location.

“The long-range influence of the 2004 Sumatran-Andaman earthquake on this patch of the San Andreas suggests that the quake may have affected other faults, bringing a significant fraction of them closer to failure,” said Taka’aki Taira, one of the co-authors of the study, in a statement.  “This hypothesis appears to be borne out by the unusually high number of large earthquakes that occurred in the three years after the Sumatran-Andaman quake.”

The study used two decades of seismic data from Parkfield, California, which sits near the San Andreas Fault.  Researchers used the data to measure the fault strength, and found it significantly changed three times: the first after a 1992 magnitude 7 earthquake in Landers, California, the second after a 2004 magnitude 6 quake in Parkfield and the third after the 2004 magnitude 9 earthquake in the Indian Ocean.

The Indian Ocean quake was the second-largest recorded, causing up to 100-foot (30.4 meter) tsunamis and killed more than 230,000 people, according to the statement.