SITE VELOCITIES BEFORE AND AFTER THE LOMA-PRIETA AND GULF OF ALASKA EARTHQUAKES DETERMINED FROM VLBI

We use geodetic data from VLBI to determine the pre- and postseismic v elocities of two sites. We then place limits on variations in intersei smic strain buildup. The 1987 and 1988 Gulf of Alaska earthquakes (eac h Ms=7.6) broke the Pacific plate interior. During the earthquakes the Cape Yakataga site moved 78 mm toward southwest. During the 1989 Loma Prieta earthquake (Ms=7.1) the Fort Ord site moved 48 mm toward north . Baselines (a) from Fairbanks to Cape Yakataga and (b) from Mojave to Fort Ord change at nearly the same rate before and after the earthqua kes. Postseismic transients, which we determine from differences betwe en post- and preseismic rates, are minor: at Cape Yakataga the transie nt is 3 +/- 4 mm in a postseismic interval of 23 months, and at Fort O rd the transient is 6 +/- 5 mm in 21 months. The slip beneath the Loma Prieta rupture needed to generate the Fort Ord transient is 0.22 +/- 0.19 m, one-tenth the coseismic slip (2 m). We analyze elastic lithosp here-viscous asthenosphere models to determine that the characteristic time describing exponential decay in deep fault slip is longer than 6 years. The VLBI measurements are consistent with uniform interseismic strain buildup. They disagree with fast postseismic rates caused by a n asthenosphere with very low viscosity.