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.

Changing Planet


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More than forty years in U.S., UK, and South African media gives David Max Braun global perspective and experience across multiple storytelling platforms. His coverage of science, nature, politics, and technology has been published/broadcast by the BBC, CNN, NPR, AP, UPI, National Geographic, TechWeb, De Telegraaf, Travel World, and Argus South African Newspapers. He has published two books and won several journalism awards. In his 22-year career at National Geographic he was VP and editor in chief of National Geographic Digital Media, and the founding editor of the National Geographic Society blog, hosting a global discussion on issues resonating with the Society's mission and initiatives. He also directed the Society side of the Fulbright-National Geographic Digital Storytelling Fellowship, awarded to Americans seeking the opportunity to spend nine months abroad, engaging local communities and sharing stories from the field with a global audience. A regular expert on National Geographic Expeditions, David also lectures on storytelling for impact. He has 120,000 followers on social media: Facebook  Twitter  LinkedIn