Two sets of earthquake swarms have hit California. What’s going on along the Mexico border?

Another earthquake swarm has been rumbling along the California-Mexico border.

More than two dozen quakes greater than magnitude 2.5 have occurred since just after midnight Saturday, with epicenters about 175 miles southeast of downtown Los Angeles and 100 miles northeast of San Diego, according to the U.S. Geological Survey.

They have occurred largely along farmland between the towns of Brawley and Imperial in Imperial County. The largest quake was a magnitude 3.9 that struck at 4:05 p.m. Saturday, bringing light shaking to the Imperial Valley as well as south of the border and rattling Mexicali.

An even larger earthquake — a magnitude 4.1 — occurred at 5:17 a.m. Monday about 28 miles northwest of the swarm that began Saturday.

The epicenter of that quake was in a remote desert area east of Anza-Borrego Desert State Park, and weak shaking was felt as far away as San Diego, parts of Orange County, Temecula, the Coachella Valley, El Centro and Holtville.

A separate swarm of earthquakes occurred a week ago, about 40 miles southeast of the most recent quake activity. Last week’s swarm occurred about 18 miles southeast of Mexicali in Baja California, with the largest a magnitude 4.2 that was felt as far away as El Centro in California and Yuma, Ariz.

Earthquake swarms are common in the Imperial Valley. Swarms have happened many times in this part of California without being followed by a major, damaging earthquake.

Nonetheless, scientists typically pay close attention to the area, where there are many faults. And there have been moments of greater concern in recent years.

In 2016, as an earthquake swarm topped out with a magnitude 4.3 quake in a different part of the region — directly beneath the Salton Sea, just south of where the mighty San Andreas fault ends — scientists issued a rare warning advising of an elevated risk for a big San Andreas fault earthquake.

That series of quakes was concerning “because that 2016 swarm was so close to the known San Andreas fault, which has been building up a lot of stress over the past 300 years,� said Clara Yoon, a seismologist with the USGS’ Pasadena office.

But this last weekend’s earthquake swarm occurred farther south, away from the San Andreas, Yoon said.

The southern San Andreas fault — which stretches from the Salton Sea through Los Angeles County to Monterey County — is capable of rupturing in a magnitude 8.2 earthquake. The last megaquake to occur on that fault was in 1857, when an estimated magnitude 7.8 earthquake hit between Monterey County and the Cajon Pass in San Bernardino County.

In 2020, another swarm of quakes close to the Mexican border attracted attention and would have been more concerning had it started moving closer to the San Andreas fault.

The Imperial Valley region was particularly active with earthquakes in the late 1970s and 1980s.

Two of the bigger quakes in historic time in the area — a magnitude 6.9 earthquake in 1940 and a magnitude 6.4 quake in the Imperial Valley in 1979 — came without any preceding swarms, seismologist Lucy Jones has said. The 1979 earthquake caused $30 million in damage, injured 91 people and damaged more than 1,500 homes and over 400 commercial buildings.

More significant earthquakes occurred in the area in 1987. The magnitude 6.6 Superstition Hills quake, which followed a magnitude 6.2 temblor on a nearby fault that struck 11 hours earlier, caused $4 million in damage and injured 94 people in Imperial County and left 3,000 people temporarily homeless in the Mexicali area.

South of the California-Mexico border, a magnitude 7.2 earthquake struck on Easter Sunday in 2010, killing two people in Mexicali and damaging buildings on both sides of the border.