Mammoth Area Rocked by 450 Quakes

A volcanic earthquake swarm that began in June intensified Thursday just east of the town of Mammoth Lakes, with more than 450 quakes recorded for the day, including a magnitude 4.1 and six others of either 3.0 or 3.1.

The U.S. Geological Survey reported that at times quakes were occurring at the rate of one a minute. The overwhelming majority were centered four miles east of Mammoth Lakes close to the intersection of U.S. 395 and California 203. Their depth was usually about four miles.

Malcolm Johnston, a Geological Survey scientist who has been monitoring the situation, said, “There is unrest in the Mammoth area, but it’s not into superactivity yet.”

Johnston disclosed that ground deformation in the area involved, the so-called South Moat of the Long Valley Caldera, has more than doubled in the last month, and, if it continues at the present rate, would lead to an uplift of eight inches in the course of a year.

While this arouses some concern and scientists have increased their monitoring, none of those contacted Thursday said they thought an eruption--which would be the first in the Eastern Sierra in 250 years--was in the offing.

But they described Thursday’s events as the high point, thus far, of the five-month quake swarm. Dan Miller, chief of a volcanic assistance team at the Cascade Volcano Observatory in Vancouver, Wash., said it constituted “fairly intense volcanic unrest.”

Meanwhile, a Cascade Observatory specialist in gas emissions, Terry Gerlach, reported an increase of several percent in carbon dioxide emissions in recent weeks from under Mammoth Mountain, several miles west of the present seismic activity.

Gerlach said he thought the quake swarm may have opened shallow fault channels under the mountain, allowing more underground gas to escape.

A Mammoth Lakes spokeswoman, Dana Hartshorn, said the 4.1 quake was felt by residents at 11:42 a.m., but did no damage.

“It was a normal 4.0 quake that we feel from time to time,” Hartshorn said. “It wasn’t anything unusual to us. . . . Everybody’s working, it’s business as usual.”

The Mammoth region has been closely monitored since 1980, when four magnitude 6 earthquakes in two days signaled a potential volcanic danger in an area which has seen many eruptions over hundreds of thousands of years.

The chief monitor of the activity for the Geological Survey, David Hill, gave a comprehensive report on present prospects at a meeting Monday in Pasadena of the California Earthquake Prediction Evaluation Council.

Hill said the total uplift in the area east of Mammoth Lakes since 1980 has been about 28 inches. Uplift can signal magmatic pressure from molten rock below the surface.

The 28 inches is less than some other places, such as Rabaul in the South Pacific, experienced before eruptions, Hill said.

But he warned, “We can’t tell what the failure rate might be. You can have a little uplift and have an eruption.”

Hill said it would require a magnitude 5 earthquake and 500 quakes in an eight-hour period for scientists to consider declaring a yellow volcanic alert at Mammoth, indicating intense unrest, and mandating the establishment of a round-the-clock monitoring station.

(BEGIN TEXT OF INFOBOX / INFOGRAPHIC)

Quake Sites

A quake swarm near Mammoth Lakes that began in June intensified Thursday.