Quake Could Awaken Sleeping Volcano : U.S. Geological Survey Researchers Point to Mammoth Lakes Area

A major earthquake eventually could trigger an eruption of a volcano slumbering beneath a popular California ski resort, although there is no sign of any imminent threat, a new government report says.

Eruptions usually occur when molten rock, or magma, rises from deep underground to fill a chamber beneath a volcano, creating tremendous pressure that forces the magma to escape above ground.

But a report by researchers at the U.S. Geological Survey and the University of Nevada at Reno said a major quake on the Hilton Creek Fault could crack the ground to allow an eruption even if the magma chamber beneath Long Valley were not filled.

Possibility of Quake

Long Valley is a 10- by 20-mile, oval-shaped volcanic caldera--a very large volcanic crater--on the eastern fringe of the Sierra Nevada, about 250 miles north of Los Angeles.

The popular ski resort town of Mammoth Lakes sits within the caldera.

“We must recognize the possibility that an earthquake (measuring at least 7.0 on the Richter scale) could cause an eruption sooner than it might otherwise occur,” said geophysicist David P. Hill, chief Geological Survey scientist for Long Valley volcanic studies.

The Hilton Creek Fault starts at the south edge of Long Valley and runs 12 miles south into the Sierra.

The significance of a premature eruption at Long Valley is that it could occur with few advance warning signs--such as increased ground swelling--that scientists believe they would see in the days before a more conventional eruption, Hill said from his office in Menlo Park.

A major temblor probably would have to rupture the ground in Long Valley itself to spur an eruption, Hill said, adding that many major quakes have shaken the region during the last 11,000 years without setting off an eruption.

But, he said, earthquakes have triggered eruptions elsewhere. Puyehue volcano in Chile erupted 48 hours after the great Chilean earthquake of 1960, and a quake on May 18, 1980, caused a massive landslide off the bulging north face of Mount St. Helens in Washington state, allowing a tremendous release of pressure during the explosive eruption there, he said.

Hill emphasized that earthquake activity and swelling of the ground at Long Valley have diminished during the last two years, sharply reducing scientists’ initial concerns that an eruption might occur in several years. “The activity is low now, so our concern is low,” he said.

Prompted by periodic earthquakes that rattled Mammoth Lakes starting in the late 1970s--quakes linked to the injection of magma beneath the caldera--the Geological Survey issued a “notice of potential volcanic hazard” for the area in May, 1982. But quake activity abated after January, 1983, and the agency issued a statement in mid-1984 indicating that the volcanic threat had diminished.

The report by Hill, Geological Survey geologist Roy Bailey and former University of Nevada seismologist Alan S. Ryall, was published in the November issue of the Journal of Geophysical Research.

Catastrophic Eruption

In another of a dozen Long Valley studies in the same issue, Stanford University geologists J. M. Metz and G. A. Mahood suggested that an earthquake along the Hilton Creek Fault was the most likely cause of the catastrophic eruption that created the Long Valley caldera 730,000 years ago.

Such an eruption today would cause “severe to total” devastation up to 75 miles away and dump volcanic ash, 6 inches thick, 300 miles away, the Geological Survey said in a 1982 report.

But Hill’s report re-emphasizes that another catastrophic eruption is less likely than a small eruption, such as the one that created the Inyo-Mono craters near Mammoth Lakes 500 to 600 years ago.

Hill said calderas such as Long Valley are noted for having repeated periods of increased quake activity and ground swelling for many decades before an eruption occurs.