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Volcanic Hazard Downgraded at Mammoth--at Least on Paper

January 07, 1985|GEORGE ALEXANDER | Times Science Writer

MAMMOTH LAKES — Of all the changes that have taken place recently at this popular ski resort--and there have been many--none has been more warmly received by the local community than the U.S. Geological Survey's downgrading of the area's volcanic hazard.

The revision was made by the highest officials of the agency with little participation by working-level scientists who have been studying the area. The status of the hazard has changed more on paper--because of the agency's new criteria for defining geological situations--than in reality, according to many of those scientists.

Nevertheless, merchants, politicians and townspeople alike approve of the change. They have tended to blame their economic woes of the last few years on what they considered to be a greatly overstated threat of lava movement below the surface, just a few miles southeast of the center of this newly incorporated town.

But now that the sales of condominiums and other properties have perked up, and the deep snows on the slopes of nearby Mammoth Mountain hold the promise of another profitable skiing season, the talk of the town has turned to the future. There are ambitious plans afoot for a new ski bowl near Mammoth's often-crowded slopes, an 18-hole golf course for summer and autumn use and a high-rise hotel with a convention center.

Amid all this optimism, the last thing local people want to talk about is the volcanic situation--even the geological survey's de-emphasis of its likelihood.

"Is volcano a dirty word?" one man said. He pondered the question and then answered: "Very much so." Said another: "Boy, you just never hear the word spoken around here anymore. It's almost as if they passed a law against saying it aloud."

Displeasure, however, cannot brush aside the reality that violent upheavals have played a major role in shaping the Eastern High Sierra--a reality that was forcefully underscored on Nov. 23, 1984, when the region was shaken by a magnitude 5.7 earthquake and an unusually vigorous string of smaller aftershocks. Indeed, it was a temblor in 1978 of almost the same size and almost in the same place that set off the train of events leading to the issuance of the volcanic notice.

This oval-shaped valley, a corner of which is occupied by the town and its popular ski mountain, is really a caldera--a volcanic crater that was created about 700,000 years ago by a mighty eruption. Since then, other eruptions have reshaped the local terrain. The Inyo craters and domes, for example, were formed by smaller blasts a few hundred years ago.

During the 1980 Memorial Day weekend, the region was jack-hammered by four earthquakes of magnitude 6 to 6.2 on the Richter scale. It was unusual to have that many strong temblors within such a short span, and when those shocks were accompanied by new hot springs and hotter-than-usual water in existing hot pools, scientists took notice.

Measurements showed that the valley floor was both rising rapidly and spreading outward. When several swarms of small quakes hit the area in late 1981 and early 1982, the most logical interpretation of all these phenomena seemed to be that magma--molten rock--was once again forcing its way toward the surface. Concerned, the Geological Survey issued a volcano notice, the lowest of three levels of hazard warning.

Concern turned to alarm in January, 1983, when a large and prolonged swarm of temblors hit the valley. Scientists thought it looked a lot like the swarms that had preceded the 1980 explosion of Mt. St. Helens volcano in Washington, and they were on the verge of upping the hazard stakes to a watch, the second level of warning.

But the Geological Survey scrapped the three-level warning system--notice, watch and warning--last spring and replaced it with a two-tier system--informal and formal alert.

With the criteria changed, with seismic activity greatly reduced from the previous 18 months and with the valley floor not deforming as much as it had earlier, the Geological Survey declared last summer that "a volcanic eruption does not now pose an immediate threat to public safety in the (Mammoth Lakes) region."

Since that statement, "morale has gone up and people feel things are moving ahead, " said Gary Flynn, the mayor pro tem of Mammoth Lakes.

Alan O'Connor, an architect, agreed: "The atmosphere here is changing very quickly, and it's because the USGS has taken the volcano notice off the area."

Indeed, in making the revisions, the Geological Survey has given many here the impression that it has totally reversed itself about the possibility of volcanism in Long Valley.

Not so. "The geology hasn't really changed," said Roy Bailey, a geophysicist with the Geological Survey who has been studying Long Valley for several years. "We're still dealing with the same problem, a progressing situation."

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