THE SLIP HISTORY OF THE 1994 NORTHRIDGE, CALIFORNIA, EARTHQUAKE DETERMINED FROM STRONG-MOTION, TELESEISMIC, GPS, AND LEVELING DATA

We present a rupture model of the Northridge earthquake, determined fr om the joint inversion of near-source strong ground motion recordings, P and SH teleseismic body waves, Global Positioning System (GPS) disp lacement vectors, and permanent uplift measured along leveling lines. The fault is defined to strike 122 degrees and dip 40 degrees to the s outh-southwest. The average rake vector is determined to be 101 degree s, and average slip is 1.3 m; the peak slip reaches about 3 m. Our est imate of the seismic moment is 1.3 +/- 0.2 x 10(26) dyne-cm (potency o f 0.4 km(3)). The rupture area is small relative to the overall afters hock dimensions and is approximately 15 km along strike, nearly 20 km in the dip direction, and there is no indication of slip shallower tha n about 5 to 6 km. The up-dip, strong-motion velocity waveforms are do minated by large S-wave pulses attributed to source directivity and ar e comprised of at least 2 to 3 distinct arrivals (a few seconds apart) . Stations at southern azimuths indicate two main S-wave arrivals sepa rated longer in time (about 4 to 5 sec). These observations are best m odeled with a complex distribution of subevents: The initial S-wave ar rival comes from an asperity that begins at the hypocenter and extends up-dip and to the north where a second, larger subevent is centered ( about 12 km away). The secondary S arrivals at southern azimuths are b est fit with additional energy radiation from another high slip region at a depth of 19 km, 8 km west of the hypocenter. The resolving power of the individual data sets is examined by predicting the geodetic (G PS and leveling) displacements with the dislocation model determined f rom the waveform data, and vice versa, and also by analyzing how well the teleseismic solution predicts the recorded strong motions. The gen eral features of the geodetic displacements are not well predicted fro m the model determined independently from the strong-motion data; like wise, the slip model determined from geodetic data does not adequately reproduce the strong-motion characteristics. Whereas a particularly s mooth slip pattern is sufficient to satisfy the geodetic data, the str ong-motion and teleseismic data require a more heterogeneous slip dist ribution in order to reproduce the velocity amplitudes and frequency c ontent. Although the teleseismic model can adequately reproduce the ov erall amplitude and frequency content of the strong-motion velocity re cordings, it does a poor job of predicting the geodetic data. Conseque ntly, a robust representation of the slip history and heterogeneity re quires a combined analysis of these data sets.