Hundreds of small earthquakes have been detected across Southern California over the last three weeks, mainly in Riverside and San Bernardino counties. The series of minor earthquakes, also known as an earthquake swarm, has some residents calling the event “swarmageddon” out of concern that a larger earthquake is developing.
However, seismologist Lucy Jones told the Los Angeles Times that the small quakes do not necessarily represent a higher risk for a major earthquake. She said that there is only a five percent chance that a small seismic event will be followed by a bigger one.
From May 27th to June 3rd, 432 quakes hit Southern California’s Jurupa Valley in an area of a few square miles. At that time, USGS seismologist Robert Graves reported that the quakes measured 0.8 to 3.2 in magnitude, and that only a few were strong enough to be felt.
Graves explained that since there are no major faults nearby, he believed the swarm was likely due to small cracks or a weakness in the Earth’s crust.”That zone is a place of weakness and that’s what’s causing these small earthquakes to occur,” he said. “Large earthquakes occur on large faults.”
According to Graves, a magnitude 7 event would have a fault length of about 30 miles, while a magnitude 8 event would have a fault length of more than 250 miles. He pointed out that the recent activity occurred in a much smaller region of only a few square miles.
In the couple weeks that followed, the tremors continued in Jurupa Valley, which is about 40 miles east of downtown Los Angeles. Various media outlets are reporting that a total of more than 1,000 small earthquakes occurred in a matter of three weeks.
Across the region, more seismic sensors are being installed as part of the state’s new early warning system. After years of development, the app ShakeAlertLA has been launched to relay important alerts based on data from the USGS. The app aims to send out information when a 5.0 or greater earthquake happens in Los Angeles County, even before the shaking can be felt.
Many experts believe that small quakes felt near major fault lines, such as the San Andreas fault, are more likely to be a precursor to a major earthquake.
“I would redefine normal as: You should still be prepared for a large earthquake,” said USGS research geophysicist Andrea Llenos. “We do know a big earthquake is going to happen.”
By Chrissy Sexton, Earth.com Staff Writer
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