VOLCANIC WARNINGS AT LONG VALLEY

David P. Hill

View of the resurgent dome, Long Valley Caldera, 1998.

Left photo: View of the resurgent dome (dark, tree-covered hills in center of photo). Town of Mammoth Lakes in the middle foreground.

View of the resurgent dome looking toward the east-southeast from along the west caldera rim.

Right photo: View of the resurgent dome looking towards the east-southeast from along the west caldera rim.

On May 25, 1980, a series of three strong earthquakes (magnitude 6) struck the Mammoth Lakes area of eastern California, a beautiful and popular region of ski resorts, forest-covered mountains, and hot springs. There was minor damage in the town of Mammoth Lakes. On May 27, the USGS warned of possible additional damaging earthquakes in the region--and later that same day another magnitude 6 shock occurred.

Geologists already knew that Mammoth Lakes sat on the edge of a large, ancient volcanic caldera, the Long Valley Caldera, but no large volcanic eruptions had occurred for a long time. In light of the series of earthquakes, the USGS undertook further studies to understand what was happening. In the summer of 1980 it found that the center of the caldera was rising, perhaps being pushed up by new molten rock at depth.

By 1982, as this uplift continued, and swarms of small earthquakes occurred, USGS scientists began to worry that this activity represented volcanic unrest. A proposed "Hazard Watch" statement was suggested to the USGS Director. News of this somehow leaked to the media, and a May 1982 article in the Los Angeles Times about an impending warning of volcanic hazards in the Mammoth Lakes region caught local authorities and residents by surprise. When the official USGS notice was released, residents were angry, resentful, and in denial--there are no volcanoes here! Memories of the violent eruptions of Mount St. Helens in 1980 helped stir fears and concerns. Worries about tourists staying away and real estate values declining ran so strong that some USGS geologists were refused service in restaurants and motels!

As caldera uplift and occasional strong earthquakes continued through the 1980’s and into the 1990’s, USGS geologists worked hard to explain the geology and volcanic hazards of Long Valley Caldera to local officials and residents of Mammoth Lakes. Seminars, discussions, and field trips around the caldera gradually convinced the local people that the USGS scientists knew what they were talking about. At the request of the Town Manager, they devised a five-level alphabetic warning scheme to allow authorities to judge the appropriate level of civil response to varying conditions. Because of alarmist misinterpretations of this alphabetic scheme by both the media and the public, it was replaced in 1997 by a more intuitive, four-level color code.

The color-code warning scheme has worked well, though it has not been tested by signs of an impending eruption so far. Meanwhile, the economy of the Mammoth Lakes area is again growing strongly, with much new investment in the tourist facilities of the region. USGS monitoring of Long Valley Caldera has been formalized as the Long Valley Volcano Observatory, one of five such observatories in the USGS. One indication of local acceptance of the volcanic nature of the area is shown by current plans for an interpretive center on the summit of Mammoth Mountain that will include a major component on the geology and volcanology of the region.