What gives under the Sierras? Volcano experts watch

On the eastern slopes of the Sierra Nevada, just outside Yosemite National Park, the mountains are twitching with seismic tremors.

Geologists aren't quite sure what the earthquake ''swarms'' in the Long Valley-Mono Lake area portend, but the United States Geological Survey (USGS) has issued a ''notice of potential volcanic hazard'' for the area within a 50 -mile radius of Mammoth Lakes, Calif.

The agency was careful to note that the alert ''is the lowest of three levels of formal concern that can be issued.'' While warning that ''future volcanic eruptions of moderate size'' could occur, the USGS also said that ''it is quite possible no eruption will occur.''

Since the alert was issued in May, earthquake swarms, intermittent tremors, and the appearance of new steam vents have continued in the area. A team of USGS scientists who made the assessment on which the hazard alert was based now are conducting more extensive studies of the area.

Mammoth Lakes, a popular ski resort near the center of the seismic activity, is not unfamiliar with such earth rumblings. An earthquake hazard alert has been in effect there since 1978, and several quakes of more-than-moderate magnitude have occurred in that period.

In recent years scientists discovered a magma (molten rock) chamber several miles below the surface near Mammoth Lakes. They now believe that a ''tongue'' of magma is moving toward the surface. ''It is quite possible. . .the magma will cool and solidify,'' says USGS, but if there is an eruption it could produce ''phreatic (steam) explosions, pumice and ash falls, pyroclastic (hot rock) flows, mudflows, and extrusion of a lava dome.''

In a sort of ''worst case'' projection, the geologists say that ''a catastrophic eruption like the one that formed the Long Valley caldera (basin) 700,000 years ago'' could produce ''ash accumulations of three feet or more. . .at distances up to 75 miles.'' That volcano's production of some 140 cubic miles of ash has not been equaled anywhere in the world in ''historic times,'' says USGS.

The scientists studying the Mammoth Lakes situation estimate that such an eruption is ''100 times less probable'' than one producing about one-fourth of a cubic mile of ash - the amount spewed out by Mt. St. Helens in its initial 1980 eruption.