The Loma Prieta, California, Earthquake of October 17,1989: Earthquake Occurrence Main-shock Characteristics Strong-motion and Broadband Teleseismic Analysis of the Earthquake for Rupture Process and Hazards Assessment*

We have used broadband records from 18 teleseismic stations and three-component records from 16 local strongmotion stations in a formal inversion to determine the temporal and spatial distribution of slip during the earthquake. Separate inversions of the teleseismic (periods, 3Contribution No. 4935, Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91 125. 30 s) and strong-motion (periods, 1-5 s) data sets result in similar source models. The data require bilateral rupture, with relatively little slip in the region directly updip from the hypocenter. Slip is concentrated in two patches: one centered 6 km northwest of the hypocenter at 12-km depth with an average slip amplitude of 250 cm, and the other centered about 5 km southeast of the hypocenter at 16-km depth with an average slip amplitude of 180 cm. This bilateral rupture results in large-amplitude ground motions at sites both to the northwest and southeast along the fault strike. The northwestern patch, however, has a larger seismic moment and overall stress drop and thus is the source of the highest ground-motion velocities, a result consistent with observations. The bilateral rupture also results in relatively moderate ground motion directly updip from the hypocenter, in agreement with the ground motions observed at Corralitos, Calif. Furthermore, there is clear evidence of a foreshock (M-4.5-5.0) or slow rupture nucleation about 2 s before the main rupture; the origin time implied by strong-motion trigger times is systematically nearly 2 s later than that predicted from the high-gain regional-network data. The seismic moment obtained from either or both data sets is about 3 . 0 ~ 1 0 ~ ~ dyne-cm, and the seismic potency is 0.95 km3. Our analysis indicates that the rupture model determined from the teleseismic data set alone, independent of the strong-motion data set, is adequate to predict many characteristics of the local-strong-motion recordings.