Relations between seismicity and deformation during unrest in Long Valley Caldera, California, from 1995 through 1999

Unrest in Long Valley Caldera and the adjacent Sierra Nevada from 1995 through 2000 was dominated by three major episodes: (1) the March-April 1996 earthquake swarm in the east lobe of the south moat; (2) the July 1997-January 1998 caldera-wide unrest; and (3) a sequence of three M > 5 earthquakes (9 June 1998, 13 July 1998, and 15 May 1999 UT) located in the Sierra Nevada block immediately south of the caldera. These three unrest episodes each had distinct characteristics with distinct implications for associated hazards. Seismicity developed as earthquake swarms for the 1996 and 1997-98 episodes, both of which were within the caldera. In contrast, the series of three M > 5 earthquakes south of the caldera, in 1998-99 each developed as a mainshock-aftershock sequence. Marginal deformation within the caldera associated with the 1996 swarm and the 1998-99 M > 5 earthquakes is consistent with the cumulative seismic moments for the respective sequences. Deformation associated with the 1997-98 episode, however, was roughly five times larger than can be accounted for by the cumulative seismic moment of the associated earthquake swarm. We conclude that the 1997-98 episode was associated with mass transport (local intrusion of magma or magmatic brine) and that the associated earthquake swarm activity, which had a relatively high b-value of 1.2, was largely driven by the intrusive process. In contrast, the 1996 earthquake swarm and the 1998-99 M > 5 mainshock-aftershock sequences, both with 'normal' b-values of similar to0.9, represent brittle relaxation to previously accumulated stresses associated with little or no mass transport. These relations emphasize the importance of simultaneous, real-time monitoring of both seismicity and deformation as a basis for judging whether an evolving unrest episode has the potential for culminating in a volcanic eruption. (C) 2003 Elsevier B.V. All rights reserved.