Volcano Hazard Alert Issued in Mammoth Lakes Area

The U.S. Geological Survey on Thursday declared a moderate-level volcano hazards alert in the Mammoth Lakes area after a swarm of more than 100 earthquakes during the day.

Research geophysicist Malcolm Johnson of the USGS said the alert is “fairly uncommon.” It reflects “a moderate level of increased activity and unrest in the vicinity of the Mammoth Mountain volcano,” he said.

However, he emphasized, this does not mean that scientists believe an eruption near the Eastern Sierra community is imminent. There has not been such an eruption for at least 200 years.

Johnson said the “Level D” alert is the second of five volcanic warning stages under a system revised in 1990.

According to the warning system, a Level A alert would indicate that an eruption was likely within hours to days. Level B would indicate intense unrest, with the volcano deforming rapidly and many earthquakes. Level C would reflect strong unrest, Level D moderate unrest and Level E weak unrest or possible instrument problems.

Alerts have been called before in the geologically active Mammoth Lakes area, the last about a year ago, Johnson said. No volcanic activity occurred at that time.

The strongest of Thursday’s earthquakes, at 12:16 p.m., was only a mild temblor of magnitude 3.5, and most of the quakes were in the magnitude 1 and 2 range.

Later in the day, the earthquakes were continuing, often in the barely noticeable magnitude 2 range, according to Lucy Jones at the Pasadena office of the USGS.

All the quakes were centered near the intersection of U.S. 395 and California 203, three miles east of Mammoth Lakes, which is almost exactly at the epicenter of four magnitude 6 earthquakes that struck May 25-27, 1980.

Since those strong quakes, scientists have been closely monitoring the Mammoth area. Their interest was further aroused two years ago when comparatively high carbon dioxide emissions were confirmed around Mammoth Mountain, west of the town, killing numerous trees.

Such emissions have not been reported in the immediate vicinity of Thursday’s earthquakes.

Johnson said that as of Thursday afternoon, recording instruments had not shown any sign of deformation or tilting of the mountain, nor any change in gas emissions, which would be disturbing signs.

He said the chief consequence of the alert would be that scientists would check all their instruments to be sure they are in working order.