Multiple earthquakes struck the North Bay, the strongest registering a magnitude of 4.0.

A series of moderate to small earthquakes struck the Geysers, a geologically active area in Northern California south of Clear Lake, on Thursday, according to reports from the U.S. Geological Survey (USGS). The quakes, the strongest of which registered a magnitude of 4.0, occurred near Sonoma County.

The USGS said the strongest earthquake occurred at 5:48 a.m., with its epicenter approximately 13.5 miles northeast of Healdsburg.

The 4.0 magnitude quake was followed by three smaller 3.0 magnitude quakes in the same area at 5:51 a.m., within seconds of each other.

The epicenter of the earthquakes was approximately three miles northwest of Geysers Volcano, a volcanic region prone to minor earthquakes. Several steam power plants have been built in the area due to ongoing geothermal activity.