CONTINUOUS BOREHOLE STRAIN IN THE SAN-ANDREAS FAULT ZONE BEFORE, DURING, AND AFTER THE 28 JUNE 1992, M(W)7.3 LANDERS, CALIFORNIA, EARTHQUAKE

High-precision strain was observed with a borehole dilational strainme ter in the Devil's Punchbowl during the 11:58 UT 28 June 1992 M(w) 7.3 Landers earthquake and the large Big Bear aftershock (M(w) 6.3). The strainmeter is installed at a depth of 176 m in the fault zone approxi mately midway between the surface traces of the San Andreas and Punchb owl faults and is about 100 km from the 85-km-long Landers rupture. We have questioned whether unusual amplified strains indicating precursi ve slip or high fault compliance occurred on the faults ruptured by th e Landers earthquake, or in the San Andreas fault zone before and duri ng the earthquake, whether static offsets for both the Landers and Big Bear earthquakes agree with expectations from geodetic and seismologi c models of the ruptures and with observations from a nearby two-color geodimeter network, and whether postseismic behavior indicated contin ued slip on the Landers rupture or local triggered slip on the San And reas. We show that the strain observed during the earthquake at this i nstrument shows no apparent amplification effects. There are no indica tions of precursive strain in these strain data due to either local sl ip on the San Andreas or precursive slip on the eventual Landers ruptu re. The observations are generally consistent with models of the earth quake in which fault geometry and slip have the same form as that dete rmined by either inversion of the seismic data or inversion of geodeti cally determined ground displacements produced by the earthquake. Fina lly, there are some indications of minor postseismic behavior, particu larly during the month following the earthquake.