Earthquake in Reno, Nevada, 4.7, and swarms beforehand with over 100 aftershocks.

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Seismologists said the recent activity is unusual because the quakes started out small and continue to build in strength. The normal pattern is for a main quake followed by smaller aftershocks.

"A magnitude 6 quake wouldn't be a scientific surprise," John Anderson, director of the seismological lab, said Saturday. "We certainly hope residents are taking the threat seriously after last night."

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Associated Press