Story highlights Three dozen earthquakes in last 2 days near Fresno, California

Swarms of earthquakes not uncommon in the region

Last swarm of earthquakes produced over 2500 between 1997-98

A flurry of small earthquakes rumbling near the Mammoth Mountain Volcano, have been categorized as "volcanic unrest" by the United States Geological Survey.

Nearly three dozen earthquakes ranging from magnitude 2.6 to 3.8 have swarmed the area, northeast of Fresno, California, over the last two days.

Earthquake swarms in this region are not uncommon. David Shelly, a seismologist with California Volcano Observatory, said this swarm is "bigger than we've seen recently, but normal in the area." He adds they are keeping a close eye on the movement, "but in the larger scheme of things, it's within in the range of activity over the last several decades."

Mammoth Mountain is in an area called the Long Valley Caldera. The center of the caldera has been uplifting slowly over the last several decades and seismologists continuously monitor it. "We think there is fluid coming up from the crust triggering the earthquakes," said Shelly.

The last swarm of quakes from 1997 to 1998 produced over 2,500 earthquakes per week at its height, according to the USGS. Since then, the region has been relatively quiet.

In July, USGS scientists installed a volcanic-gas monitoring station on the mountain. It tracks gases that fluctuate with earthquake activity to better track the changes in real-time.

The awareness level on the USGS website remains at green/normal. Mammoth Mountain, standing at 11,053 feet, last erupted about 57,000 years ago, according to the USGS.