A magnitude 4.3 earthquake rumbled through Muscoy in San Bernardino County on Thursday morning, triggering shaking across the Inland Empire, Los Angeles and Orange counties, according to the U.S. Geological Survey.
The earthquake occurred at 9:32 a.m. at a depth of about 3.3 miles and was followed by a magnitude 3.1 quake about four minutes later in Rialto, according to USGS.
The Rialto Fire Department reported that a series of small earthquakes shook the northwest portion of the city Thursday morning. The first foreshock in the region, a magnitude 3.0, struck about 8:34 a.m., followed by a magnitude 2.8 about eight minutes later. Two smaller temblors occurred over about 13 minutes before the magnitude 4.3 hit, according to USGS.
There were no immediate reports of damage.
“There’s no reportable damage but it’s still pretty early,” said Chris Jensen, the department’s interim fire chief. There hadn’t been any medical calls related to the earthquake but Jensen said the agency will continue to monitor the situation.
Data shows the quake was felt over a wide swath of Southern California, from Los Angeles to San Diego and as far east as Ventura County. Residents who felt the earthquake closest to the epicenter and sent information to the USGS’ Did You Feel It? website reported experiencing the equivalent of Intensity 4 shaking on the Modified Mercalli Intensity Scale, which is light shaking too weak to cause any damage.
Areas farther away from the epicenter, from Simi Valley to San Diego, reported Intensity 2 to Intensity 3 shaking, or weak shaking that wouldn’t cause damage.
Seismologist Lucy Jones said the earthquakes were a small swarm in the Fontana Trend, a series of earthquakes that run along a northeast geographic line under the sediments of the San Bernardino Valley. Scientists suspect there’s a fault in that area, but they can’t map it because it’s covered by sediment, she said.
“It’s a place that has lots of little earthquakes and they do often come in little clusters, like this one,” she said.
Since 1990 there have been roughly 130 magnitude 3.0 earthquakes and eight magnitude 4.0 along the Fontana Trend, Jones said.
In the past 10 days, there have been three earthquakes of magnitude 3.0 or greater centered nearby. An average of 25 earthquakes with magnitudes between 4.0 and 5.0 occur per year in California and Nevada, according to a recent three-year data sample.
The temblor comes days after a powerful earthquake struck off Russia’s sparsely populated east coast, sending waves slamming into buildings in Siberia and northeastern Japan. At magnitude 8.8, the earthquake ranks as the sixth-most powerful ever recorded.
Did you feel this earthquake? Consider reporting what you felt to the USGS.
Find out what to do before, and during, an earthquake near you by signing up for our Unshaken newsletter, which breaks down emergency preparedness into bite-sized steps over six weeks. Learn more about earthquake kits, which apps you need, Lucy Jones’ most important advice and more at latimes.com/Unshaken.
Quakebot contributed to this report.
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