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Southern California earthquake: More than 100 aftershocks

March 11, 2013 |  2:38 pm

Earthquake index map

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.

No damage or injuries were reported from the temblor. But it did rattle some nerves in the desert.

Minnesota transplant Shannon Haber said that even though she's lived here since 1996 she definitely has not gotten used to earthquakes.

“I was just a little frightened,” Haber said. “There was small shaking and it made me nervous because I’m 23 floors up.”

Haber was working in Los Angeles Unified School District headquarters in Westlake when the earthquake shook at 9:56 a.m. more than 100 miles away in Anza in Riverside County. The shaking was the biggest and longest-lasting she could remember.

“It was a slow, swaying motion,” she said. “It sort of felt like I was on a boat, a sort of wavy feeling that lasted 10 to 20 seconds. … No one else reacted around me. They’re all veterans of earthquakes.”

In Anza, about 10 customers had sat down to a late breakfast at a Diner 371 when the quake struck about 20 miles away.

Nothing was broken and no one was hurt, said Diner 371 waitress Michelle Padaron, 30, who was stocking tables with napkins when the quake struck.

After four seconds of shaking, customers quickly returned to their meals of burgers, burritos and eggs, Padaron said.

 "Everyone just kind of looked up, then looked at each other, and that was it," Padaron said.

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-- Frank Shyong

twitter.com/frankshyong

Map: United States Geographical Survey

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