A wave of small earthquakes hit an area near the border between California and Mexico on Monday.
A 2.8-magnitude earthquake struck south of El Centro in Imperial County around 6:07 p.m., followed by a magnitude 4 six minutes later—making it the largest earthquake so far to have hit the area, reported the Los Angeles Times.
According to the United States Geological Survey (USGS), 73 earthquakes and counting have hit the area since Monday, including 18 aftershocks, Caltech seismologist Egill Hauksson said, adding that one of them was a 4.0 magnitude quake.
By Tuesday morning, there had been dozens more.
According to the LA Times, there were no reports of damage in El Centro.
Aftershocks follow larger quakes and are usually small in comparison to its stronger counterpart, according to USGS.
“Aftershocks can continue over a period of weeks, months, or years. In general, the larger the mainshock, the larger and more numerous the aftershocks, and the longer they will continue,” the agency’s website states.
Earthquakes aren’t uncommon in the area because it’s near a fault line, Hauksson said. Seismologists used to gather in the region, because of the many earthquakes that took place there in the ’70s and ’80s, according to the LA Times. In 2010, a 7.2-magnitude earthquake also hit El Centro.
“Swarms in this region are often prolific and can last for weeks. There’s no lack of faults there,” Hauksson told the paper. “These have happened numerous times before without being followed by a major earthquake.”