A swarm of small earthquakes has struck the area around Mammoth Lakes in California, where one of North America's worst prehistoric eruptions has once occurred, but federal environmental officials said the phenomenon poses no immediate risks and should not cause an alarm.

Hundreds of quakes that range between faint to magnitude 3.8, or large enough to be felt by those near the epicenter, struck the region geologists call the Long Valley Caldera, which was formed about 760,000 years ago as a result of one of the Earth's most powerful volcanic eruptions.

The US Geological Survey (USGS) recorded over 500 earthquakes since early Thursday, including eight with magnitudes between 3 and 3.8, but no damage has so far been reported because of the shaking.

David Shelly, a USGS seismologist, who has been studying the volcanic system in the area, said that earthquakes swarms are not something new in the region because of volcanic and tectonic stress. He acknowledged that the phenomenon last week is among the largest earthquake swarms that occurred in the region in the past ten years but it was within the range of seismic activities that were observed over the last several decades. The earthquake swarm that rocked the region between 1997 and 1998, for instance, generated over 2,500 earthquakes in a week at its peak.

The intensity and number of quakes have already dwindled during the weekend but it isn't yet clear if the quake will continue to die down. USGS said that it is closely monitoring the situation.

Does the earthquake swarm indicate an impending volcanic explosion? Although earthquakes are often associated with a looming eruption, scientists believe magmatic activities have nothing to do with the earthquakes and that the swarm does not indicate a looming danger.

Shelly said that the seismic activities do not mean that the volcano has become more active. Although there is magma deep down the Earth, it is neither moving nor behind the earthquakes.

"It doesn't mean that the volcano is any more active," he said. "It's an ongoing process in a volcanic system."

As to what is behind the ground shaking, Shelly said that the quakes may have been caused by the underground movement of water, carbon dioxide and other gases stressing the tectonic plates.

"This fluid moves episodically into cracks or faults in the crust," Shelly said. "We think these quakes were triggered by this movement but driven by existing tectonics."

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