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.
The San Andreas fault’s southernmost stretch has not ruptured since 1680 to 1690. Big earthquakes on this section of the southern San Andreas fault rupture on average every 250 years — although there can be wide variations as to how often they actually do occur.
The last time a similar swarm happened was nearly four years ago. That series of moderate quakes — the worst of which were two at magnitude 4.3 and one at magnitude 4.1 — increased the chance of a magnitude 7 or larger quake from 1 in 10,000 to 1 in 100 and prompted a rare warning by the U.S. Geological Survey.
The warning convinced San Bernardino officials to close City Hallfor two days in 2016; the building was months away from being vacated over longstanding worries about its seismic safety. No bigger quake occurred.
Monday's swarm "does increase the chance of a big earthquake on the San Andreas somewhat," said USGS research geophysicist Morgan Page. "So it's definitely something to watch."
Still, "it's not necessarily doom and gloom," given that the last three similar swarms — in 2001, 2009 and 2016 — did not result in larger, catastrophic quakes, Page said. "But every time it happens, we do worry that this could be the time that it triggers something."
Computer simulations were underway Monday to calculate what the increased risk of a major quake on the San Andreas fault might be.
Monday's swarm is farther from the San Andreas fault than the 2016 swarm was, said seismologist Lucy Jones. Scientists have not seen a foreshock that has triggered a larger earthquake more than 6.2 miles away, and Monday's quake sequence so far is about 7.5 miles from the San Andreas fault, Jones said.
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