Triggered Deformation and Seismic Activity under Mammoth Mountain in Long Valley Caldera by the 3 November 2002 Mw 7.9 Denali Fault Earthquake

Abstract

The 3 November 2002 Mw 7.9 Denali fault earthquake triggered defor- mational offsets and microseismicity under Mammoth Mountain (MM) on the rim of Long Valley caldera, California, some 3460 km from the earthquake. Such strain offsets and microseismicity were not recorded at other borehole strain sites along the San Andreas fault system in California. The Long Valley offsets were recorded on borehole strainmeters at three sites around the western part of the caldera that includes Mammoth Mountain—a young volcano on the southwestern rim of the caldera. The largest recorded strain offsets were 0.1 microstrain at PO on the west side of MM, 0.05 microstrain at MX to the southeast of MM, and 0.025 microstrain at BS to the northeast of MM with negative strain extensional. High sample rate strain data show initial triggering of the offsets began at 22:30 UTC during the arrival of the first Rayleigh waves from the Alaskan earthquake with peak-to-peak dynamic strain amplitudes of about 2 microstrain corresponding to a stress amplitude of about 0.06 MPa. The strain offsets grew to their final values in the next 10 min. The associated triggered seismicity occurred beneath the south flank of MM and also began at 22:30 UTC and died away over the next 15 min. This relatively weak seismicity burst included some 60 small events with magnitude all less than M 1. While poorly constrained, these strain observations are consistent with triggered slip and intrusive opening on a north-striking normal fault centered at a depth of 8 km with a moment of 10 16 N m, or the equivalent of a M 4.3 earthquake. The cumulative seismic moment for the associated seismicity burst was more than three orders of magnitude smaller. These observations and this model resemble those for the triggered deformation and slip that occurred beneath the north side of MM following the 16 October 1999 M 7.1 Hector Mine, California, earthquake. However, in this case, we see little post-event slip decay reflected in the strain data after the Rayleigh-wave arrivals from the Denali fault earthquake and onset of triggered seismicity did not lag the triggered defor- mation by 20 min. These observations are also distinctly different from the more widespread and energetic seismicity and deformation triggered by the 1992 M 7.3 Landers earthquake in the Long Valley caldera. Thus, each of the three instances of remotely triggered unrest in Long Valley caldera recorded to date differ. In each case, however, the deformation moment inferred from the strain meter data was more than an order of magnitude larger than the cumulative moment for the associated triggered seismicity.