This is an archived article and the information in the article may be outdated. Please look at the time stamp on the story to see when it was last updated.

SAN DIEGO – After a series of small earthquakes rattled the eastern San Diego region last weekend, many residents might be asking themselves why there were so many shocks all of a sudden or if something bigger is on the horizon.

Experts say that the back-to-back shocks are something that residents should be aware of, but not all too concerned about for forecasting the future likelihood of a major earthquake.

The earthquakes that hit the region last week are part of what scientists call a “swarm,” which is a series of dozens of smaller earthquakes within a span of a couple days up to several weeks.

In the last week, there were a few different areas nearby San Diego that had a swarm event, according to the U.S. Geological Survey: Ocotillo Wells in San Diego County and the two Imperial County towns, Niland and Heber. 

Niland is located just south of the Salton Sea and the tail of the San Andreas fault, while Heber is near El Centro at the edge of the Imperial Fault Zone that runs into Mexico. Ocotillo Wells is situated in the middle of the two areas, closer to the San Jacinto Fault System.

Screenshot of the USGS earthquake map showing parts of the San Diego and Imperial County regions, as of 12:30 p.m. on May 4, 2023. Labels have been added by (Screenshot by KSWB/FOX 5)
Screenshot of the USGS earthquake map showing parts of the San Diego and Imperial County regions, as of 12:30 p.m. on May 4, 2023. Labels have been added by (Screenshot by KSWB/FOX 5)

These events in the Southern California region are not necessarily rare, according to professor of geology emeritus at San Diego State University Dr. Pat Abbott, but they’re not a frequent occurrence. 

As Abbott explained to, the eastern San Diego area is particularly susceptible to these one-after-another earthquake events, but they occur only every couple of years.

“This area is very famous throughout the world for having earthquake swarms,” Abbott said of the land south of the Salton Sea. “We’ve had 85 of them in the last 100 years.”

That frequency is not from a fault running directly through the region – the south Salton Sea is in between the Imperial fault and the end of the San Andreas fault. As Abbott explained, it’s due to geothermal energy in that 35 mile stretch of land.

In the area, there is a lot of magma and warm volcanic rocks under the surface at a low depth. That thermal energy under the surface pumps hot water that creates energy and movement, thus causing earthquake activity.

“It’s a very active, volcanic, heated area,” Abbott said. “Once one little thing moves there, it commonly makes others move and that’s the swarm.”

Looking historically, Abbott said, the vast majority of these swarm events do not lead to anything else, like a larger earthquake. Generally, if a swarm were to serve as a foreshock, the big one would happen about 72 hours after the event.

However, it doesn’t mean that it’s not a possibility that this kind of movement could trigger something that leads to a bigger earthquake.

Most earthquakes are caused by a shifting of stress: when one plate moves at a fault, it pushes up against another and applies more pressure to that area. The process of movement is why there are foreshocks, mainshocks and aftershocks with larger earthquake events.

“You don’t just have one earthquake,” Abbott explained, “an earthquake is a series of events that take place over days, weeks, months, even years for the really huge ones.”

These kinds of swarm events could move the pressure enough to set-off the kind of destructive “big one” earthquake that seismologists are concerned about.

For Southern California, seismologists say that this could look like any large-scale quake along the southern San Andreas that causes widespread damage in urban areas close to the fault.

“Everytime we get a swarm of earthquakes, we take them seriously, but we also know from history that usually it’s not going to lead to anything bigger,” Abbott said.

That’s why he says that residents should not increase their worry about a big one after the swarm last weekend, but it’s instead something to be cognizant of.

“It’s just sort of like one of the little features about living in California – we get lots of little earthquakes,” he said. “I prefer to label them as reminders (that) we live in earthquake country.”

Abbott stressed that Californians should take these events as a sign to prepare for the eventuality that a major earthquake occurs near where they live.

An expert with the Governor’s Office of Emergency Services Seismic Hazards Branch told earlier this year that the best way people can do that is by:

  • Identifying and securing things in one’s home or work that could fall during a major quake, like china cabinets or ceiling fans.
  • Making a household preparedness plan.
  • Securing an emergency kit that has items – like flashlights, cash or food – in case damage to critical infrastructure occurs.
  • Download the MyShake app for California earthquake warnings.

“We’ve had it pretty lucky here for some time,” Abbott said. “Mother Nature does not work on a timescale for fault movements. So, as always, we kind of have to wait and live it day by day and know how to react on those days that a big earthquake does occur.”