Seismic wave observations with the Global Positioning System

We describe the direct measurement of ground displacement caused by the Hec tor Mine earthquake in southern California (M-w 7.1, October 16, 1999). We use a new method of instantaneous positioning, which estimates site coordin ates from only a single epoch of Global Positioning System (GPS) data, to m easure dynamic as well as static displacements at 24 stations of the Southe rn California Integrated GPS Network (SCIGN), with epicentral distances fro m 50 to 200 km. For sites outside the Los Angeles basin the observed displa cements are well predicted by an elastic half-space model with a point shea r dislocation; within the sedimentary basin we observe large displacements with amplitudes up to several centimeters that last as long as 3-4 min. Sin ce we resolve the GPS phase ambiguities and determine site coordinates inde pendently at each epoch, the GPS solution rate is the same as the receiver sampling rate. For the SCIGN data this is 0.033 Hz (once per 30 s), though sample rates up to 2 Hz are possible with the SCIGN receivers. Since the GP S phase data are largely uncorrelated at I s, a higher sampling rate would offer improved temporal resolution of ground displacement, so that in combi nation with inertial seismic data, instantaneous GPS positioning would in m any cases significantly increase the observable frequency band for strong g round motions.