More than 500 small earthquakes struck a volcanic region in central California on Thursday and Friday, posing no immediate risk, federal environmental officials said.

The “earthquake swarm” shook the Long Valley Caldera area north of Fresno, an area that has seen an increased amount of quakes, volcanic gas emissions and other disturbances in recent decades, the U.S. Geological Survey said.

“These are just kind of ordinary, everyday earthquakes,” USGS spokeswoman Leslie Gordon said.

Eight of the quakes were recorded between 3.0 and 3.8 magnitude and the rest were less serious, the agency added.

The caldera, or circular volcanic depression, was formed following a massive eruption about 760,000 years ago and last erupted about 50,000 years ago, the USGS said.