HILL, David P., Volcano Hazards Team, U.S. Geol Survey, MS 910, 345 Middlefield Rc, Menlo Park, CA 94025, hill@usgs.gov.

Long Valley caldera in east-central California has shown twenty years of volcanic unrest beginning with an intense earthquake swarm in May 1980 that included four M6 earthquakes accompanied by a 25-cm upwarping of the resurgent dome. Since 1980, notable unrest episodes have included 1) a seismic swarm in January 1983 with two M=5.3 earthquakes; 2) a swarm of 8-months duration beneath Mammoth Mountain on the southwest rim of the caldera in 1989, followed by diffuse carbon dioxide degassing around the flanks of the mountain that continues through the present, and 3) the strong swarm of July 1997 -- January 1998 that included nine M>4 earthquakes accompanied by an additional 10-cm uplift of the resurgent dome. The resurgent dome currently stands over 70 cm higher than in l978. Each of these unrest episodes appears to have been associated with a magmatic intrusion to depths as shallow as 3 to 4 km beneath the surface. Initial notification of the potential volcanic hazards associated with this unrest in May 1982, which used the lowest of a 3-level USGS warning system then in effect, was met with disbelief and anger by local residents. This 3-level warning system was officially replaced by a “WARNING / no-WARNING” scheme in 1984. In response to the request from a frustrated local official for more systematic emergency-response guidance (short of a full WARNING) during the 1989 unrest episode, we developed a five-level alphabetic code (E…A) for local volcanic hazards. This code worked well between scientists and the civil authorities but was widely misunderstood by both the media and the public. In an attempt to make the notification system more understandable to all concerned, we converted this alphabetic code to a four-level color code (green, yellow, orange, red) in June 1997. This new system worked well during the strong unrest episode of July 1997 – January 1998. While relative quiescence in caldera unrest since mid-1999 has not further tested the color-code scheme, it has emphasized the challenge in maintaining awareness of local volcanic hazards and the hazards communication system by emergency response agencies and the public over the long term.

Cordilleran Section - 98th Annual Meeting (May 13–15, 2002)
Session No. 5
Natural Hazard Monitoring and Warning Systems I
CH2M Hill Alumni Center: Ballroom 110B
8:00 AM-12:00 PM, Monday, May 13, 2002

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