San Andreas Weakened by Quake Half a World Away

A new study shows massive quakes can weaken faults half a world away

By Lori Preuitt
|  Wednesday, Sep 30, 2009  |  Updated 4:37 PM PDT
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It's one of those moments that causes those who live in earthquake country to catch their breath.  Experts say they have the first ever evidence that earthquakes that happen half a world away can weaken faults here in California.

U.S. seismologists right now they think this happens only with massive quakes.  And before you think those don't often happen,  it comes just one day after a 8.3 earthquake hit deep in the Pacific Ocean killing at least 99 people in the region of American Samoa. 

The biggest effect hearing about that death and destruction for most in California is compassion and possible a drive to get out the check book and make a donation to the Red Cross.

But the study, may change all that.

Specifically, researchers say the 9.0 earthquake that hit Sumatra Christmas 2004 weakened the San Andreas fault. The Sumatra quake is the one that triggered killer tsunamis that left nearly a quarter of million people dead, mostly in Indonesia, Sri Lanka, India and Thailand.  It was the second largest quake ever measured.

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

The studies authors focuses their attention on Parkfield, Calif., which sits right on the San Andreas Fault and is one of the most seismically active areas in the country.

Over the past two decades tiny earthquakes have repeatedly hit the area. By closely comparing seismic readings from these micro-quakes, the team was able to determine the stress level required to cause the fault to slip.

They just recently were able to determine one of the slips came during a five day window in late December 2004, when the Samtran quake hit.

"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 Taira. "

The results of the study will appear this week in the journal Nature.

Here's a look at earthquakes in the past century that have been big enough to trigger tsunami's:

  • September 2007: A 7.8-magnitude earthquake rattles Sumatra island, triggering regional tsunami alerts and damaging scores of buildings.
  • September 2007: An earthquake measured at a magnitude of 8.4 near Sumatra triggers a wave in the coastal city of Padang.
  • April 2007: At least 28 people in the Solomon Islands die in a tsunami and earthquake measured at a magnitude of 8.1.
  • July 2006: A magnitude-6.1 earthquake triggers a tsunami off Java island's southern coast, killing at least 600 people.
  • December 2004: An Indian Ocean tsunami, triggered by a magnitude-9.0 earthquake, kills 230,000 in a dozen countries
  • August 1976: A magnitude-8.0 earthquake hits near the islands of Mindanao and Sulu in the Philippines, generating a tsunami and leaving at least 5,000 dead.
  • March 1964: An 9.2-magnitude earthquake in Prince William Sound, Alaska, kills 131 people, including 119 from a tsunami.
  • April 1946: An earthquake measured at a magnitude of 8.1 near Unimak Islands, Alaska, triggers a tsunami, killing 165 people, mostly in Hawaii.

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