Deformation and seismicity triggered beneath Mammoth Mountain, California, by surface waves from the M7.9 Denali fault, Alaska, earthquake of 3 November 2002

The November 3, 2002, MW 7.9 Denali Fault earthquake triggered deformational offsets and microseismicity in Long Valley Caldera, California some 3200 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 strainmeter and tiltmeters at three sites around the western part of the caldera that includes Mammoth Mt. (MM) - a young volcano on the south-western rim of the caldera. The largest recorded strain offsets were 0.1 microstrain at POPA on the west side of MM, -0.05 microstrain at MX to the south-east of MM, and -0.025 microstrain at BS to the north-east of MM with negative strain contractional. High sample-rate strain data show initial triggering of the offsets began at 22:30 UT and within the S-wave coda and the early arriving surface waves from the Alaskan earthquake with peak dynamic strain amplitudes of ~2 microstrain. The strain offsets grew to their final values in the next 5-10 seconds. The associated triggered seismicity was centered at about 3 km beneath the south flank of MM and also began at 22:30 UT and died away over the next 15 to 30 minutes. This relatively weak seismicity burst included a dozen small events all with magnitude less than M=1. While poorly constrained, triggered slip and/or intrusive opening on a north-striking normal fault centered at a depth of 6 km with a moment of 3x1015 Nm, or the equivalent of a M~4.3 earthquake is consistent with these data. The cumulative seismic moment for the associated seismicity burst was more than three orders of magnitude smaller. This model is similar to that for the triggered deformation and slip that occurred following the October 16, 1999, M7.1 Hector Mine, California, earthquake except that we have yet to see a decay of the strain offset following the Alaska earthquake. It is distinctly different from the more wide-spread and energetic seismicity and deformation triggered by the 1992 M7.3 Landers earthquake in the Long Valley Caldera. Each of the three instances of remotely triggered unrest in Long Valley caldera recorded to date differ in detail, although in each case, the deformation moment inferred from the strain meter data is an order of magnitude or more larger than the cumulative moment for the associated triggered seismicity.