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.

Source: Los Angeles Times – Southern California earthquake: More than 100 aftershocks