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Swarm of Quakes


Last updated 10/14/2020 at 1:40pm

Hundreds of earthquakes, the largest of which had a magnitude-4.9, rattled south of the Salton Sea on Sept. 30, sending light tremors into Riverside and San Diego County.

In a two-and-a-half-hour span, 45 earthquakes of at least a magnitude-3.0 struck the Westmorland area, according to seismologist Lucy Jones, formerly of the U.S. Geological Survey. By 8 p.m., the United States Geological Survey had recorded at least 240 quakes, the largest a magnitude-4.9 at 5:31 p.m.

The swarm began around 4 p.m., when a 3.0 magnitude tremblor struck. Several more quakes occurred later into the evening in the same area. The swarm remained active till 10 p.m.

The quakes happened in the Brawley Seismic Zone, a network of small faults that connect the San Andreas and Imperial faults. This is one of the largest swarms in the Imperial Valley – and it is historically the most active swarm in Southern California.

Historically, the largest quake in that zone measured 5.9 in 1981.

The Brawley Seismic Zone is too far from the San Andreas fault to change the probability of a quake occurring there, Jones added.

It’s likely that the rate of earthquakes in the area will decrease over the next week or so, with some additional events with a magnitude between 4.5 and 5.4. It’s less likely, but possible, that a larger quake with a magnitude up to 6.9 will strike, according to USGS.

Probability for an earthquake of magnitude 7.0 or larger occurring is very low, seismologists say.

“Definitely, at some point in the future, we’re going to have much bigger ones,” Jones said. “Those will do a lot more damage.”

Determining when another series of earthquakes will happen or when the big one will hit California is not realistic. We often think that the weather also helps us predict when an earthquake will strike.

“Quakes are so far down in the earth’s surface that weather doesn’t change what’s happening,” Jones said. “There’s a tradition of earthquake weather in every culture with earthquakes, because we want to have a pattern.”

But what you can predict is your own preparedness.

“We need that muscle memory of getting under the table to remember that’s the right thing to do,” Jones said. “I wouldn’t worry about it particularly today, these are just too far away, but in the long run you’re in one of the most active places of California.”

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