Experts dissect Monday’s swarm of earthquakes in L.A.

The cluster included a magnitude 4.0 shock near Inglewood

Any time the earth shakes in California, residents can’t help but wonder: Is this the Big One?

A swarm of earthquakes Monday that included a magnitude 4.0 near Inglewood didn’t do much to quell those nerves.

The first rattles started at 4:15 a.m., when three foreshocks struck near Inglewood: a magnitude 2.1, a magnitude 2.4 and a magnitude 3.0, all within about three miles of one another, according to the U.S. Geological Survey.

The biggest quake, the 4.0, hit near Inglewood at 4:44 a.m. and was felt across Southern California. Residents described strong jolts, with some reporting books and picture frames falling from shelves.

Within 24 hours, more than 80 aftershocks had been recorded in the Inglewood and Lennox area, according to the USGS. Although that seems like a lot, it is not cause for alarm, said geophysicist Don Blakeman of the USGS National Earthquake Information Center in Colorado.

“It doesn’t raise a red flag,” Blakeman said. “It’s not unusual to have a swarm of quakes, and almost all of them will be small.”

The larger an earthquake is, the longer aftershocks last, Blakeman said. The magnitude 7.1 Ridgecrest earthquake of 2019 still produces the occasional aftershock.

The majority of aftershocks in the Inglewood area so far have had a magnitude less than 2.0.

“A swarm like this does not necessarily indicate something big and terrible is going to happen, but there’s always a tiny, tiny chance, since we can’t actually predict quakes,” Blakeman said. “We can’t tell you for sure what’s going to happen.”

Monday’s main shock was “pretty deep for California,” at about 12 miles down, said Paul Caruso, a geophysicist at the USGS National Earthquake Information Center.

Caruso said earthquakes in the L.A. area are typically about three miles deep. But 12 miles is far from the deepest; earthquakes in Fiji and elsewhere can be as deep at 300 miles.

“The deeper the earthquake is, the less strongly people feel it, because they’re potentially farther away from where the energy is released,” Caruso said.

Shortly after the shaking Monday, earthquake expert Lucy Jones said on Twitter that the 4.0 quake was probably not on a mapped fault because it had a thrust mechanism.

Robert Graves, a research geophysicist with the USGS’s Earthquake Science Center, based in Pasadena, explained: The Newport-Inglewood fault, the closest one to Monday’s earthquake, is a strike-slip fault, which means the two sides move laterally during an earthquake. This quake had a thrusting mechanism, which occurs when one slide of a fault slides up over another.

“Based on that, even though it was in close proximity to the Newport-Inglewood fault, it looks like it’s probably on a subsidiary fault structure, or some kind of secondary fault structure,” Graves said.

One helpful way to think of it is like a crack in a car windshield.

“There might be one big, main crack, but there could be little secondary cracks here and there,” Graves said, “and that’s the kind of fault that this is likely located on.”

Just as it might have upset you, the quake likely disturbed your pets. Animals can sometimes sense when shaking is about to happen. The first waves that arrive from an earthquake, the Pwaves, are the fastest. The Swaves that follow are the ones that bring the shaking.

This could explain why some animals — and people — seem to sense that something is amiss moments before the rattling begins.

“Sometimes what happens, particularly with dogs, is that first P-wave is very weak but can sometimes be felt,” Blakeman said. “And then what you actually think is the earthquake is the next series of waves that arrive.”

Even if you didn’t feel Monday’s small earthquake, you should be prepared for the next one, with an emergency kit at the ready.

“It sounds like a broken record, but it’s true,” Graves said. “Take this as a reminder that there are active faults, and large earthquakes will occur. Be prepared.”