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 Positioning System (GPS) array. The coordinate time series of seven sites are evaluated for station displacements during an interval of 100 days centered on the day of the earthquake. We employ a new spatial filtering technique that removes common-mode errors from the coordinate time series. This approach provides precise estimates of site-specific displacements compared to the cumbersome method of analyzing baselines between pairs of stations. All sites indicate significant coseismic horizontal displacements of 5-65 mm with uncertainties of 1-2 mm. Horizontal displacements are in general agreement with elastic dislocation models, in particular for sites closer to the epicenter. Vertical displacements range from -13 to +7 mm with uncertainties of 2-4 mm. The observed vertical displacements in all cases show 5-10 mm more subsidence than expected from geodetic and seismic/geologic models. Significant postseismic horizontal displacements totaling 6 +/- 2 mm (10-20% of the coseismic displacement) are detected at the three sites closest to the epicenter. These displacements are modeled as a short-term exponential relaxation with a decay time of 22 +/- 10 days superimposed on a longer-term linear interseismic trend. Scaling the observed coseismic and postseismic displacements at one of the sites with the distance to the 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 displacements.