California earthquake a reminder of seismic dangers

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Monday morning’s magnitude 4.7 earthquake in Riverside County hit is a seismically active area, and experts said it was a reminder of the potential dangers.

“It’s a good idea to take it to heart and make sure you’re prepared,” U.S. Geological Survey Seismologist Robert Graves said. “We live in Southern California, and we have lots of active faults; and every once in a while, it’s large enough to cause damage.”

Here are four graphics from the USGS that tell the story of Monday’s earthquake:

The USGS said the quake was felt as far away as Arizona and the Central Valley. Here’s a map showing where people responded to the USGS’s ‘Did You Feel It?’ website:

The USGS got more than 9,000 responses from the public. The chart shows when the responses were sent:
This map shows the shaking intensity of the Monday temblor:
The star shows the location of the quake with other historic quakes in the area plotted with orange circles.

The quake was initially recorded as three separate quakes because a foreshock tricked seismographs into recording multiple quakes of multiple sizes, said Susan Hough, a USGS seismologist.

Earthquakes of a 4.7 magnitude are typically only felt about 120 miles from the epicenter, but Monday morning’s quake traveled farther.

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

The quake occurred along the San Jacinto Fault Zone, which runs through San Bernardino, San Diego, Riverside and Imperial counties roughly parallel to the San Andreas Fault. It’s one of three fault zones that absorb friction from the motion of the North American Continent and the Pacific plates rubbing against each other.

“It’s capable of generating moderate to large earthquakes,” said Graves. “Today’s activity was not out of the ordinary. Actually, it’s pretty typical of the area.”

There is some evidence that the largest quake ever recorded in the fault zone, a magnitude 7, occurred in the early 1800s, Graves said.

No injuries or major damage was reported.

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--Frank Shyong and Joseph Serna

Map at left: The San Jacinto fault zone. Credit: USGS