RESPONSE OF LONG-VALLEY CALDERA TO THE M(W)=7.3 LANDERS, CALIFORNIA, EARTHQUAKE

Of the many sites in the western United States responding to the June 28, 1992, Landers earthquake (M(w) = 7.3) with remotely triggered seis micity, only Long Valley caldera is monitored by both seismic and cont inuous deformation networks. A transient strain pulse and surge in sei smicity recorded by these networks began within tens of seconds follow ing arrival:of the shear pulse from Landers. The cumulative strain and number of triggered earthquakes followed the same exponentially decay ing growth rate (time constant 1.8 days) during the first 6 days follo wing Landers. The strain transient, which was recorded on a borehole d ilatometer at the west margin of the caldera and a long-base tiltmeter 20 km to the east, peaked on the sixth day at ;approximate to 0.25 pp m and gradually decayed over the next 15-20 days. The absence of a cle ar strain signal exceeding 0.4 ppm in data from the two-color geodimet er deformation lines, which span the central section of the caldera, i ndicates that the strain transient cannot be due solely to pressure ch anges in the concentrated pressure source 7 km beneath the central par t of the caldera that accounts for most of the uplift of the resurgent dome since 1980. The triggered seismicity occupied the entire seismog enic volume beneath the caldera. The focal mechanisms, the frequency-m agnitude distribution, and the spatial distribution of the triggered e arthquakes are typical of other swarms in Long Valley caldera. The cum ulative seismic moment of the triggered earthquakes through the first 2 weeks after the Landers earthquake corresponds to a single M = 3.8 e arthquake, which is too small by nearly 2 orders of magnitude to accou nt for the 0.25-ppm peak amplitude of the observed strain transients. Evidently, the strain transient represents the dominant response mode, which precludes direct triggering of local earthquakes by the large d ynamic stresses from Landers as the dominant process. Conditionally vi able models for the triggering process beneath the caldera include (1) the transient pressurization of magma bodies beneath the resurgent do me and Mammoth Mountain by the advective overpressure of rising bubble s, (2) a surge in fluid pressure within the seismogenic zone due to up ward cascading failure of isolated compartments containing superhydros tatic pore fluids, (3) relaxation (fluidization) of a partially crysta llized magma body or dike intrusion in the deep crustal roots of Long Valley magmatic system, or (4) aseismic slip on midcrustal faults. Eit her the deep, relaxing-magma body or lower crustal dike intrusion sati sfy all the strain observations with a single deformation source. The latter model admits the possibility that large, regional earthquakes c an trigger the episodic recharge of the deep roots of crustal magmatic systems.