Mysterious tremors detected on San Andreas Fault

Mysterious tremors detected on San Andreas FaultLOS ANGELES — Scientists have detected a spike in underground rumblings on asection of California's San Andreas Fault that produced a magnitude-7.8earthquake in 1857.What these mysterious vibrations say about future earthquakes is far fromcertain. But some think the deep tremors suggest underground stress may bebuilding up faster than expected and may indicate an increased risk of a majortemblor.Researchers at the University of California, Berkeley, monitored seismicactivity on the fault's central section between July 2001 and February 2009 andrecorded more than 2,000 tremors. The tremors lasted mere minutes to nearly halfan hour.Unlike earthquakes, tremors occur deeper below the surface and the shaking lastslonger.During the study period, two strong earthquakes hit — a magnitude-6.5 in 2003and a magnitude-6.0 a year later. Scientists noticed the frequency of thetremors doubled after the 2003 quake and jumped six-fold after 2004.Tremor episodes persist today. Though the frequency of tremors have declinedsince 2004, scientists are still concerned because they are still at a levelthat is twice as high as before the 2003 quake.The team also recorded unusually strong rumblings days before the 2004 temblor.Results of the research appear in Friday's issue of the journal Science. Thework was funded by the U.S. Geological Survey and National Science Foundation."The fact that the tremors haven't gone down means the time to the nextearthquake may come sooner," said Berkeley seismologist and lead researcherRobert Nadeau.Nadeau first discovered tremors deep in the San Andreas Fault in 2005. Beforethat, the phenomenon was thought only to occur in Earth's subduction zones,where one tectonic plate dives beneath another.USGS seismologist Susan Hough found the latest observations intriguing, but saidit's too soon to know what they mean."We don't have enough data to know what the fault is doing in the long term,"said Hough, who had no part in the research.———On the Net:Science: http://www.sciencemag.orgU.S. Geological Survey: http://www.usgs.gov