Geodetic measurement of deformation in the Loma Prieta, California Earthquake with very long baseline interferometry

Following the Loma Prieta earthquake, two mobile Very Long Baseline Interferometry (VLBI) systems operated by the NASA Crustal Dynamics Project and the NOAA National Geodetic Survey were deployed at three previously established VLBI sites in the earthquake area: Fort Ord (near Monterey), the Presidio (in San Francisco) and Point Reyes. From repeated VLBI occupations of these sites since 1983, the pre-earthquake rates of deformation have been determined with respect to a North American reference frame with 1σ formal standard errors of ∼1 mm/yr. The VLBI measurements immediately following the earthquake showed that the Fort Ord site was displaced 49 ± 4 mm at an azimuth of 11 ± 4° and that the Presidio site was displaced 12 ± 5 mm at an azimuth of 148 ± 13°. No anomalous change was detected at Point Reyes with 1σ uncertainty of 4 mm. The estimated displacements at Ford Ord and the Presidio are consistent with the static displacements predicted on the basis of a coseismic slip model in which slip on the southern segment is shallower than slip on the more northern segment of the fault rupture. We also give the Cartesian positions at epoch 1990.0 of a set of VLBI fiducial stations and the three mobile sites in the vicinity of the earthquake.