Magnitude 4 earthquake rattles Southern California, the strongest to hit Ontario in a month

A magnitude 4 earthquake rattled Southern California before dawn Sunday morning — the strongest in a series of modest earthquakes to strike near the Ontario International Airport in the last month.

Sunday's 3:51 a.m. earthquake was the fifth of magnitude 3 or higher detected in Ontario, California, since early September, according to the U.S. Geological Survey.

"Moderate" shaking was felt in areas closest to the epicenter, the USGS said, as defined by the Modified Mercalli Intensity Scale. That's strong enough to awaken many people. "Weak" or "light" shaking may have been felt across a broad region, including the most populous portions of San Bernardino and Riverside counties, large swaths of Los Angeles and Orange counties, and parts of San Diego County.

People reported feeling the earthquake from Palmdale to San Diego. The USGS asked people to submit reports of what kind of shaking they may have felt — or didn't feel — at the agency's Did You Feel It? website.

Until Sunday, the strongest earthquake in the past month to hit San Bernardino County's fourth most populous city occurred on Sept. 7, when a magnitude 3.9 earthquake caused "light" shaking to be felt close to the epicenter. Light shaking is enough to disturb windows and dishes and can rock standing cars noticeably.

The epicenter of Sunday's earthquake was centered about one-third of a mile southeast of where the 60 Freeway meets Archibald Avenue. That's about 500 feet south of Mountain View Elementary School and half a mile east of the Whispering Lakes Golf Course.

The USGS said that its ShakeAlert earthquake early warning system was activated. People can download the earthquake early warning app for free at myshake.berkeley.edu .

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