Quakes can weaken fault zones worldwide

HOUSTON, Oct. 5 (UPI) -- U.S. scientists studying the giant 2004 earthquake that caused Indian Ocean tsunamis say they've determined the quake weakened part of the San Andreas Fault.

The seismologists said their findings suggest the Earth's largest earthquakes can weaken fault zones worldwide and might trigger periods of increased global seismic activity.

"An unusually high number of magnitude 8 earthquakes occurred worldwide in 2005 and 2006," said Rice University Associate Professor Fenglin Niu, one of the study's authors. "There has been speculation that these were somehow triggered by the Sumatran-Andaman earthquake that occurred Dec. 26, 2004, but this is the first direct evidence that the quake could change fault strength of a fault remotely."

The magnitude 9 earthquake in 2004 -- the second-largest ever measured by seismograph -- spawned tsunamis as large as 100 feet that killed an estimated 230,000 people, mostly in Indonesia, Sri Lanka, India and Thailand.

In the new study, Niu and co-authors Taka'aki Taira and Paul Silver, both of the Carnegie Institution of Science in Washington, and Robert Nadeau of the University of California-Berkeley, examined more than 20 years of seismic records from Parkfield, Calif., which sits atop the San Andreas Fault.

The team found fault strength at Parkfield changed markedly at three times during the 20-year period -- all immediately after major quakes occurred elsewhere.

The study is detailed in last week's issue of the journal Nature.