Magnitude 7.1 Earthquake Follows July 4th Ridgecrest Temblor, Becoming the ‘Mainshock’ in a Massive Swarm

After a swarm of more than 1,400 earthquakes hit the Searles Valley region over the past two days, a magnitude 7.1 temblor — the biggest yet — struck Friday evening.

The quake hit roughly 10 and a half miles from Ridgecrest at about 8:19 p.m., the U.S. Geological Survey reports. After previously downgrading the magnitude to 6.9, seismologists pushed it back up to the preliminary figure of 7.1.

The epicenter was about 7 miles northwest of the 6.4 magnitude Fourth of July foreshock that was previously thought to be what seismologists call the "mainshock," or largest event in a seismic series.

Shaking was felt across Southern California and as far as Mexico and Las Vegas, where an NBA Summer League game was halted when the stadium began rattling. Videos aired by Sacramento station KOVR showed pools sloshing in Elk Grove, just outside the state’s capital city, and in Modesto in the north San Joaquin Valley.

Friday's was 10 times stronger than Thursday's quake, according to Caltech seismologist Lucy Jones.

It was also about 4.7 miles deeper. Preliminary information had listed it shallower.

It’s the most powerful temblor to hit Southern California in at least 20 years, tying the magnitude 7.1 quake that struck the Hector Mine area, nearly 50 miles east-southeast of Barstow, in October 1999. The Northridge earthquake in 1994 was magnitude 6.7.

While Friday's event may be the largest in this week's series, it won't be the last — and there's still a chance a bigger one will come.

Hundreds of quakes will likely exceed magnitude 3 in the area over the next week, and the aftershocks for years will continue for year, albeit at a diminishing rate, experts say.

Earlier Friday afternoon, seismologists had said there was a 6% chance of a magnitude 6 or larger earthquake striking.

Every earthquake has a one in 20 chance of being followed by an even bigger one, according to Jones.

“This is that one in 20 time,” she said.

Ridgecrest resident April Hamlin told KTLA she and her children were watching TV in her bedroom when the ground began to shake.

“We just felt the rumble coming and we said, ‘Here comes another one,’ ” she said. “It just kept rolling and got harder, and we grabbed on to one another. The TV flew off the stand and the lights went out … we couldn’t see but we could hear the glass breaking.”

Hamlin said there are now cracks all around her two-bedroom house, rendering it unlivable.

“Everything just piled into the center,” she said. “TVs flying, speakers flying. A lot of breakage. There were leaks outside a trailer park behind us.”

The mother said she had “no choice” but to go to the Red Cross shelter in Ridgecrest because her home was becoming too warm for her special needs daughter.

“She can’t be in heat above 75 degrees and the house is already getting hot,” Hamlin said.

The 7.1 magnitude quake was soon followed by two magnitude 5 aftershocks and 16 above magnitude 4 — and the shaking will continue.

“The ground is moving enough that we’re not seeing anything below magnitude 3,” Jones said, of experts’ seismic recording systems.

The chance for another magnitude 5 is “approaching certainty” and there will probably be “quite a few more,” Jones said.

The chance of one greater than a magnitude 6 is about 50%, and the preliminary estimate of another in the 7 range is about 10%, she added.

“Those names — mainshock, foreshock, aftershock — are semantic,” she said. “They’re our tools for trying to describe what’s going on. But when you have a sequence going on, every earthquake makes another earthquake more likely. That’s what we’re seeing right here.”

USGS systems were overloaded by the swarm of seismic activity. There was still “a ton of data” coming in, and it would take some time to sort through, USGS seismologist Robert Graves said.

The tremor sparked two fires in Ridgecrest, ruptured several gas lines and cracked Kern County roads. Shifted homes and cracked foundations were reported in northwest San Bernardino County.

Earlier assessments showed damage was more significant than Thursday's quake.

Caltrans crews were surveying roadways in the Ridgecrest area, spanning Kern and San Bernardino counties. Highway 178 was shut down for hours due to large transverse cracks, falling boulders and rockslides before reopening around 1 a.m.

Gov. Gavin Newsom declared a state of emergency for the affected area following Thursday's temblor.

The Los Angeles city and county fire departments went into earthquake mode, but both agencies reported no major infrastructure damage or serious injury.

Read KTLA’s full coverage of damage from the 7.1 magnitude quake here

The L.A. County ShakeAlert app and USGS early earthquake warning system both worked as designed, according to Graves.

Graves said the 7.1 didn’t trigger a notification from the L.A. County app because the threshold is above what the estimated shaking was on the Modified Mercalli intensity scale. The shaking intensity around L.A. was at level 3, while the app sends an alert at level 4.

The USGS system, meanwhile, is still in pilot mode and only available to a limited group of people.

Near the epicenter, shaking intensity Friday night was at level 9 — one level higher than the intensity level 8 quake that hit Thursday morning and one level below the maximum 10, Jones said.

The quakes are occurring in the Little Lake fault zone and are believed to involve two faults that are perpendicular, according to Caltech seismologist Zachary Ross.

“It’s looking more and more like many earthquakes have multiple fault ruptures,” Ross said. “Years ago, that wasn’t necessarily assumed to be the case, so why that might be is something that we need to understand better.”

However, USGS seismologist Susan Hough says the Mojave Desert quake swarm doesn’t affect the probability of earthquakes elsewhere in California. The fault lines they're on are roughly 150 miles from the massive San Andreas Fault.

The ruptures are largely occurring on the more vertical of the two faults involved, causing it to grow further northwest — “so away from the metropolitan area, as far as we can tell,” Jones said.

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