Rain raises Parkfield quake alert.
The San Andreas fault near Parkfield, Calif., shifted suddenly last month, prompting officials to issue a level B earthquake alert for that section of the fault. Scientists have predicted a strong shake for Parkfield sometime in the next few years, and the recent alert, which expired after three days, represents the highest level announced since the monitoring program began there in 1985. Researchers suspect, however, that the recent surface shifting resulted from heavy rains, and not from changes indicating the predicted quake is nigh.
As part of a multimillion-dollar experiment, the U.S. Geological Survey (USGS) keeps close tabs on the San Andreas near Parkfield in hopes of issuing a short-term prediction minutes to days before the quake. The USGS has establishes five alert levels, ranging from E to A, to describe the near-term likelihood of an earthquake. Information from monitoring instruments serves as the basis for determining the alert status. When the USGS calls a level A alert, officials will issue a warning to residents of Parkfield and nearby areas.
According to the rating system, a level B alert signifies an 11 to 37 percent chance the expected temblor will strike in the next three days. The USGS issued the March 19 alert after two instruments detected a substantial acceleration in the normally slow surface creep along the fault. the opposite sides of the fault slipped 5 millimeters past each other in 16 hours.
That slip, while sufficient to trigger a level B alert, did not trully indicate a state of heightened seismic risk, scientists believe. The recently heavy rins, which dumped more than 3 inches on the area, probably weakened the soil at the surface, allowing it to creep faster, says Evelyn A. Roeloffs, head of the Parkfield experiment. Other instruments indicate that similar movement did not occur on deeper sections of the fault, where the quake is expected to start.
To prevent a repeat of last month's debatable alert, the USGS now plans to alter its rating criteria, Roeloffs says.
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|Title Annotation:||heavy rains may have caused a shifting of the San Andreas fault near Parkfield, California|
|Date:||Apr 6, 1991|
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