A swarm of small earthquakes in California, close to the Mexican border, is being closely monitored as to whether it might raise the chance of a much larger event on the San Andreas fault.

The largest earthquake on Monday was a magnitude 4.6, reported at 8:56 a.m. under the southeastern part of the Salton Sea, according to the U.S. Geological Survey. It was among a series that began at 6:33 a.m. with a magnitude 3.2 earthquake. Magnitude 4 quakes struck at 9:03 a.m. and 12:29 p.m.

It’s only the fourth time in the 88 years of modern records that such a swarm has occurred in this part of California — a region that raises concern among seismologists for the chance that it can trigger a significant earthquake on the San Andreas fault.

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