SOUTHERN CALIFORNIA PERMANENT GPS GEODETIC ARRAY - SPATIAL-FILTERING OF DAILY POSITIONS FOR ESTIMATING COSEISMIC AND POSTSEISMIC DISPLACEMENTS INDUCED BY THE 1992-LANDERS-EARTHQUAKE

The June 28, 1992 (M-W = 7.3) Landers, California, earthquake was the first earthquake to be surveyed by a continuously operating Global Pos itioning System (GPS) array. The coordinate time series of seven sites are evaluated for station displacements during an interval of 100 day s centered on the day of the earthquake. We employ a new spatial filte ring technique that removes common-mode errors from the coordinate tim e series. This approach provides precise estimates of site-specific di splacements compared to the cumbersome method of analyzing baselines b etween pairs of stations. All sites indicate significant coseismic hor izontal displacements of 5-65 mm with uncertainties of 1-2 mm. Horizon tal displacements are in general agreement with elastic dislocation mo dels, in particular for sites closer to the epicenter. Vertical displa cements range from -13 to +7 mm with uncertainties of 2-4 mm. The obse rved vertical displacements in all cases show 5-10 mm more subsidence than expected from geodetic and seismic/geologic models. Significant p ostseismic horizontal displacements totaling 6 +/- 2 mm (10-20% of the coseismic displacement) are detected at the three sites closest to th e epicenter. These displacements are modeled as a short-term exponenti al relaxation with a decay time of 22 +/- 10 days superimposed on a lo nger-term linear interseismic trend. Scaling the observed coseismic an d postseismic displacements at one of the sites with the distance to t he epicenter provides a measure of site strain, which agrees well with the direction and magnitude determined from more precise laser strain meter data. The time series do not show any detectable preseismic dis placements.