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More than 80 aftershocks have rumbled through Napa Valley, California, since Sunday's earthquake, and a USGS scientist said gradually diminished shaking would likely continue for up to two weeks.
The U.S. Geological Survey has monitored at least 80 aftershocks since Sunday morning's 6.0-magnitude quake. Most have been too small to feel, but a handful have been strong enough to shake the surface and rattle nerves.
Annemarie Baltay of USGS released a list of significant aftershocks measuring 3.0 in magnitude or greater.
"We've had about 80 aftershocks that we've recorded, but we've had 4 large aftershocks," Baltay said.
The first major aftershock struck moments after the initial quake at 3:24 a.m. PDT Sunday and measured 3.5 in magnitude. The next struck that same morning at 5:47 a.m. and measured 3.6.
The largest aftershock so far measured 3.9 in magnitude and rumbled the Napa Valley on at 5:33 a.m. Tuesday. The latest significant aftershock rumbled through just over an hour later at 6:45 a.m. and measured magnitude 3.0.
"After the earthquake occurs, there's still some afterslip of that stress continuing to be released," Baltay said. "And so the aftershocks are just little tiny areas that are continuing to feel that same sense of motion."
Baltay said the aftershocks should continue for about two weeks after the initial quake, with diminishing intensity.
Scientists are studying the quake to see how much pressure it relieved along the fault line, and whether it makes a major quake on another Bay Area fault more or less likely.
"It does release some of that strain, but that's still a question that we're investigating to understand the details of," Baltay said.