Magnitude 4.3 earthquake near Rialto shakes Southern California

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 aftershock about four minutes later in Rialto, according to USGS.

The Rialto Fire Department reported that a series of small earthquakes shook the northwestern 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 has been no reportable damage or any calls related to the cluster of earthquakes,” said Chris Jensen, the department’s interim fire chief. Additionally, there weren’t any medical calls related to the quakes, he said.

There had been at least five aftershocks as of 11 a.m. Thursday, said Gabrielle Tepp, a staff seismologist for Caltech.

“The largest was a magnitude 3.1 a few minutes after the mainshock,” Tepp said. “These are expected.”

Data show the quake was felt over a wide swath of Southern California, from Los Angeles to San Diego and as far north 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 last 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.

Susan Hough, a seismologist with USGS in Pasadena, said for people wondering whether the rumble was associated with the powerful earthquake that hit Russia on Tuesday, the answer is no.

“That, as enormous as it was, was on the far side of the Pacific,” Hough said. “And it’s not expected to influence California faults. Faults in our area are just doing their own thing on their own time.”

Hough said she and her colleagues felt shaking from the quake at their office in Pasadena.

“It is unsettling, even for a seismologist, when things start to come unglued around you,” she said.

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, seismologist Lucy Jones’ most important advice and more at latimes.com/Unshaken.