Scientists keep tabs on quakes at Mammoth Lakes

Geologists are playing a seismic game of cat and mouse with a hidden magma pool on the eastern slopes of California's Sierra Nevada mountains, not far from Yosemite National Park.

In Long Valley, a series of earthquakes and tremors began on the evening of Jan. 6 with two quakes - the first registering 5.5 on the Richter scale, the second 5.6. Swarms of smaller tremors followed. United States Geological Survey (USGS) specialists and other scientists, who have been monitoring seismic developments in the Long Valley caldera since 1975, quickly moved in.

Unlike Hawaii's Kilauea volcano, which is unpredictable but hardly mysterious , the pool of magma (molten rock) which is thought to underlie Long Valley refuses to reveal its character or intent.

While the scientists ''listen'' with their seismic instruments and measure to see if an incipient dome in the caldera is swelling, resort owners, skiers, merchants, and local officials in the Mammoth Lakes area listen, too - but not with the patience of the geologists. If people get the wrong message from the quakes and tremors, the local economy could wind up in shambles - even if the earth eventually quiets down.

So far, the attraction of deep snow (which has make the geologists' task tougher) has outweighed apprehension over the possible danger posed by the earthquakes. Lodge owners said many people called before the weekend to inquire about the situation, but few canceled their plans, and 10,000 to 11,000 skiers took their pleasure on the slopes of 11,000-foot Mammoth Mountain while dozens of geologists and other scientists were digging sensors out of the snow and lugging other measuring devices into the area.

Geologist Marianne Guffanti at the USGS Western Division headquarters in Menlo Park, Calif., said that more than 3,000 shocks of less than magnitude 3 were recorded between Thursday and Monday. The frequency of the tremors began to decrease Sunday night, she added.

The 10-mile-long valley caldera - a relatively flat plain at 7,500 feet which is ringed by mountains of up to 11,000 feet - was created 700,000 years ago by huge volcanic eruptions. What the geologists call a ''resurgent dome'' began to form in the caldera 100,000 years later.

Since measurement began in 1975, the elevation of that dome has increased by 10 inches - a rate of growth meaningful to few nongeologists. A series of earthquakes occurred in the area in the fall of 1978, and in May 1980 there were four quakes registering 6 or more on the Richter scale. (The scale goes to 10, and each number signifies about a 60-fold increase in magnitude. Quakes of magnitude 6 can cause severe damage.)

In May 1982 the Geological Survey, acting on the basis of a report by a team of its scientists, issued a notice of ''potential volcanic hazard'' in the Long Valley-Mono Lake area. That is the mildest of three kinds of alerts issued by the USGS, the second being a ''watch'' and the third a ''warning.''

So far the geologists have found no reason to escalate the hazard notice. Later this week they hope to have sufficient data to determine whether or not the dome has grown recently and whether the hazard has increased.

The scientists know there is a pool or ''tongue'' of magma below the surface of the caldera. Most interpret the seismic activity in recent years as evidence that this magma is getting closer to the surface and might erupt at some point.

There are other possibilities. One is that the current seismic activity is the result of subsidence, rather than upward thrust, of the magma. Another is that the quakes are tectonic in origin - that is, caused by stresses along fault lines. Besides being close to the frontal fault of the Sierras, the Long Valley-Mono Lake area is laced with minor fault lines.

Geologist Guffanti says, ''We're not necessarily expecting that the deformation (bulging) of the caldera dome will continue. We can't predict earthquakes or volcanic eruptions. Although in the past people have considered that maybe the seismic swarms meant a rise of magma to the surface, that is still a hypothesis.''

Actually, the Geological Survey recently reported that ''deformation measurements show no significant changes between last August and December 1982 .''

The scientists are not advising either local officials or potential visitors to the area about what to do. Rather, says Miss Gaffanti, they are ''just trying to keep them informed as to what is going on, so far as we can determine.''

She points out that US Forest Service personnel, who are close to the scene, are best able to advise skiers, hikers, and others about possible hazards such as avalanches and landslides resulting from the quakes.

Whether a new volcano will some day rise near Mammoth Lakes is not likely to be known soon. But if and when it does, geologists will be waiting.

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