5.6 Quake Jolts Eastern Sierra Near Mammoth Lakes

A magnitude 5.6 earthquake, followed by several hundred aftershocks, shook an Eastern Sierra area 12 miles southeast of Mammoth Lakes early Saturday, causing minor damage at stores and homes but no injuries.

The U.S. Geological Survey said the 6:22 a.m. temblor was outside the area that has been monitored since 1980 for possible volcanic activity, and it did not alter the green, or normal, volcanic status of the region.

Scientists said the quake was the most powerful in the vicinity since the 6.1 Chalfant Valley earthquake on July 21, 1986.

Geological Survey scientists said in an advisory:

“It is probably prudent to avoid venturing into the back country for the next week or so because of the possibility of additional aftershocks and possible rock fall activity.”

High Sierra regions are still covered with snow, and the prime backpacking season is several weeks away.

The epicentral area of Saturday’s quakes was in the unpopulated mountains overlooking Mammoth Lakes from the south, but hotel keepers reported that the quake was distinctly felt in the resort town.

A few items fell from shelves and broke at Crowley Lake, along U.S. Highway 395 southeast of Mammoth Lakes.

Quakes are common in the Eastern Sierra, and there were two 5.1 jolts about five weeks apart near Mammoth last year.

In the first 10 hours after Saturday’s quake, the Geological Survey reported 418 aftershocks, including two of magnitude 4.3 within seconds of each other, at 6:38 a.m., and a 4.7 at 10:54 a.m. There were 10 aftershocks of between 3.0 and 4.0.

A high number of aftershocks is customary when sizable temblors occur.

The last important spasm of volcanic quakes took place within the Long Valley Caldera east and south of Mammoth Lakes in the summer and fall of 1997. In one day, on Nov. 22, 1997, more than 1,000 quakes were detected, the strongest a magnitude 4.8.

The Long Valley Caldera, measuring about 18 miles by 12 miles, was the site 760,000 years ago of one of the biggest eruptions the world has experienced. A caldera is formed when so much magmatic material is expelled that the overlying terrain collapses into the subterranean emptiness created.

If such an eruption were to reoccur today, it could cause catastrophic ash falls through much of California and even in the Midwest. But scientists say such an event is extremely unlikely.

There have been recurrent eruptions, often every few hundred years, near the present-day Mammoth Lakes, but none have been nearly as large as that early cataclysmic event.

The last eruption in the area is believed to have been about 250 years ago, and it was fairly minor. Scientific interest rose in the possibility of a resumption of activity in May 1980, when four magnitude 6 quakes occurred within two days.

Saturday’s quake was several miles south of the Long Valley Caldera, in an area where scientists believe the seismic activity is tectonic in nature, not volcanic.

There was no sign that Saturday’s quake was related to Friday’s 5.0 aftershock of the 1992 Landers quake near Yucca Valley.

David Hill of the Geological Survey said there is a small chance of another quake similar to Saturday’s in the next two weeks.