Quakes Increase, Then Subside Near Mammoth Lakes

There was a slight step-up in the ongoing swarm of volcanic earthquakes near Mammoth Lakes late Thursday night and early Friday after a two-week slowdown, the U.S. Geological Survey reported.

The agency said there were 146 quakes in the area in the 24 hours ending at 10 a.m. Friday. The strongest was magnitude 3.8, centered six miles southeast of the Eastern Sierra town, at 4:56 a.m. Friday.

A 3.2 earthquake was centered in the same vicinity, close to the Mammoth Lakes Airport, at 11:06 p.m. Thursday.

Later Friday, the rate of quakes slowed to as few as one an hour, and the number of earthquakes over the last week was estimated at 530. This compared with a weekly rate of 2,300 when the swarm was at its peak in late November.

During the swarm that began at the end of June near Mammoth, the pattern has been that each quake approaching magnitude 4 or higher sets off a spasm of lesser quakes for several hours.

Scientists believe that magma, or molten rock, several miles beneath the surface is causing pressure that is bringing on the quakes.

The latest 3.8 and 3.2 quakes were centered about three miles below the surface, a little shallower than most of the swarm.

In seismic activity elsewhere in California, Caltech scientists said, a magnitude 3.6 earthquake was centered in the Salton Sea near the southern end of the San Andreas fault Friday at 7:05 a.m.

None of the latest quakes caused any damage or injuries, according to authorities.

Graphs released by the Geological Survey’s volcanic monitors at Mammoth Lakes show that the rate of ground deformation this month first slowed and then leveled off near Mammoth Lakes.

Ground deformation is regarded as a precursor of volcanic activity. But the chief scientific monitor in the Mammoth area, David P. Hill, has cautioned that volcanic areas differ, and no specific level is the critical point for triggering an eruption.

There has been no eruption in about 250 years in the active area from Mono Lake south to the Long Valley caldera surrounding Mammoth Lakes. And the area of the swarm--in the so-called south moat next to an uplifting “resurgent dome” in the caldera--has had no eruption for about 100,000 years.