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The Mammoth Lakes activity is especially worrisome to the scientists, to say nothing of local residents, who saw a severe drop in tourism immediately after the USGS put the area on its volcano-hazard list last year. The study notes that during the past 1,500 years, the Mammoth Lakes area has been second only to Mount St. Helens in volcanic activity, on the average erupting every two or three centuries along a 15-mile chain of lava domes and old vents. The most recent major eruption took place about 250 years ago, when the area was showered with flaming ash that was, in one scientist's words, "hot enough to incinerate entire forests." Moreover, the resort has been repeatedly shaken in the past few years by minor earthquakes, ineluding one in July that registered 5.2 on the Richter scale, a moderately powerful jolt capable of cracking walls, knocking dishes off shelves and causing other minor damage. Scientists have measured at least a 13-in. swelling of the ground near Mammoth Lakes in the past two years, and found that one new hot spring has formed and three other long-dormant hot springs have become resurgent. All this is interpreted as an indication that magma under the region is gradually working its way to the surface.
Some volcanologists caution that far too little is known about volcano behavior to make any firm forecasts about imminence of eruptions. In their view, what seems like multiplying danger signs may, in fact, be a byproduct of the recently intensified monitoring of the earth. Other scientists, however, are sure that the signals are both new and significant. Moreover, because of their recent experience with Mount St. Helens, these volcano watchers have become more optimistic about their ability to make reasonably accurate short-term predictions about the likelihood of an outburst.
Since that May morning three years ago when the Washington peak blew its top, experts have been probing the mountain with every sort of instrument, including seismometers planted within the crater that was formed by the big blast. Every time the volcano twitches, the rumblings are recorded and sent by radio to the University of Washington at Seattle. Mount St. Helens has cooperated by continuing to shake, vent gases and debris and emit lava flows. So thoroughly have the scientists analyzed the interplay of these events that they can now see recognizable patterns in the volcano's behavior. Some disturbances have served as early warning signs, and over the past three years the scientists have been able to predict 13 minor Mount St. Helens eruptions. The last seven, starting in April 1982, were announced up to three weeks before the actual occurrence. Equally important, none of the predictions were false alarms. With understandable pride, the researchers announced last month in Science: 'Such repeated accuracy is uncommon if not unparalleled in volcanology."