U.S. Geological Survey (USGS)
Southern California experienced a bit more than fireworks yesterday morning when, at 10:34 a.m. PDT, a magnitude 6.4 earthquake occurred 7.5 miles southwest of Searles Valley. The July 4th quake shook the city of Ridgecrest hardest, but shaking was felt by millions of people as far away as Los Angeles and Las Vegas.
Peak shaking near the epicenter was a very strong Modified Mercalli Intensity (MMI) level 7. The USGS PAGER system has projected damages to be $10-$100 million. The earthquake was large enough that the fault break did reach the earth's surface. Surface ruptures will be mapped by geologists in the coming days, with results released in the next few weeks.
This M6.4 earthquake triggered many aftershocks, including a M5.4 early this morning at 4:07 a.m. PDT. As of 9:30 a.m. PDT, 83 M3.0+ aftershocks, 17 M4.0+, and one M5.0+ have occurred. Over 1,000 aftershocks have been recorded by the seismic network, most too small to be felt by people. Aftershocks will decrease in frequency over time.
The ShakeAlert earthquake early warning system successfully issued its first alert 6.9 seconds after the quake started, with a final alert issued after 14.9 seconds. The initial ShakeAlert estimated a M5.7 and final estimate of M6.0. Due to the proximity to the epicenter, the city of Ridgecrest could not have received advance warning. However, areas farther away, like California City, did receive 15.1 seconds of warning.
For those in LA, the ShakeAlertLA app delivers alerts to residents for M5+ quakes with potentially damaging shaking. LA was far enough away from the epicenter that shaking was expected to be non-damaging. The ShakeAlertLA app performed as designed for the earthquake.
The Searles Valley has had numerous moderate-sized quakes over the past 40 years. Eight other M5+ earthquakes have occurred within 30 miles of the July 4th epicenter. The largest was a M5.8 event on Sept. 20, 1995, about 8 miles from yesterday’s event.
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