About three dozen earthquakes between magnitudes 2.5 and 3.8 that shook the Mammoth Lakes region in California in less than 24 hours are considered "volcanic unrest," the U.S. Geological Survey said.

The quakes struck the region between 9 a.m. Thursday and 1:49 a.m. Friday.

The USGS said it's because Mammoth Mountain, a lava-dome complex, lies along the rim of the Long Valley Caldera.

"Recent volcanic unrest, including seismicity, gas emission, and tree kill, is thought to be related to a dike intrusion beneath Mammoth Mountain in 1989," the USGS website said. "Both Long Valley Caldera and Mammoth Mountain have experienced episodes of heightened unrest over the last few decades (earthquakes, ground uplift, and/or volcanic gas emissions). As a result, the USGS manages a dense array of field sensors providing the real-time data needed to track unrest and assess hazards."