Southern California earthquake: More than 100 aftershocks

Monday morning's magnitude 4.7 earthquake in Riverside County was followed by more than 100 small aftershocks that radiated northeast, indicating that the quake occurred on a secondary fault of the San Jacinto fault, according to the U.S. Geological Survey.

The largest aftershock, a 3.2 quake, struck less than a minute after the first quake. The second, a 2.8 magnitude quake, occurred at 11:25 a.m. Another 2.8 quake occurred at 12:50 p.m. The vast majority of the aftershocks were largely imperceptible, with magnitudes smaller than 2.5. Valleys and other low elevation areas feel the effects most strongly, said Susan Hough, a USGS seismologist.

"Deeper earthquakes tend to have wimpy aftershocks," Hough said.

The 4.7 quake was initially recorded as three separate quakes because a foreshock tricked seismograms into recording multiple quakes of multple sizes, Hough said.

Earthquakes of a 4.7 magnitude are typically only felt about 120 miles away from the epicenter, but Monday morning's quake traveled farther, shaking coffee cups as far as Los Angeles.

That's because the quake occured in the San Jacinto Mountains, which are composed of hard granite rock that transmits energy more efficiently, Hough said.

http://latimesblogs.latimes.com/lanow/2013/03/more-than-100-small-aftershocks-follow-47-quake.html