UPDATE: A preliminary 2.8 magnitude earthquake struck in the same area as this morning's 5.1 magnitude quake in the South Bay, according to the USGS.
Annemarie Baltay, a seismologist with the USGS, said the location of the epicenter suggests the quake occurred within the Calaveras Fault zone.
There is a one-in-100 chance of an aftershock greater than a magnitude 5.0 happening in the next day, according to Baltay. There could be 10 to 15 aftershocks of magnitude 3.0 or greater over the next week. "Aftershocks of this magnitude and duration are totally normal for an event of this type," Baltay said.
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