More than 400 small earthquakes have struck the Inland Empire since May 25, officials with the U.S. Geological Survey (USGS) said during a press conference on Monday.
Three of the earthquakes registered a magnitude of above 3, according to the Southern California Earthquake Data Center at the California Institute of Technology. The largest quake hit the area around 4:36 p.m. Sunday, approximately three miles north-northwest of Glen Avon. The quake registered a magnitude of 3.2, at a depth of about 2.1 miles. The other two larger quakes were 3.1 magnitude quakes reported on Saturday at 7:19 p.m., approximately three miles north-northwest of Glen Avon, and at 1:04 p.m. Sunday, approximately four miles northeast of Nuevo.
Most of the quakes fell below 2.0, meaning that many of them were likely too light to be felt.
Officials with USGS said that although they are unable to determine what is causing the swarm of quakes, it is not uncommon due to fault systems in the area. The Inland Empire contains the San Andreas fault system, which is the tectonic boundary between the Pacific and North American tectonic plates. Many other faults, such as the San Jacinto fault, create smaller, yet more frequent earthquakes, according to the California Earthquake Authority.
During Monday's press conference, USGS seismologist Robert Graves said that the swarm isn't cause for concern, adding that the earthquakes rocking the Inland Empire in recent days are, "a reminder that we live in earthquake country."
Dr. Lucy Jones, a seismologist with USGS, said on Twitter that this is not uncommon for the area. "When it has quakes, they are usually in clusters like this," Jones said. "In other words, ordinary, common California quakes."
There were no immediate reports of damage. While there are many variables involved, geologists say damage does not usually occur until the earthquake magnitude reaches somewhere above 4 or 5.
USGS provides information about earthquakes by state and preparedness information, including helpful information for those in California.