Rupture model of the 1989 Loma Prieta earthquake from the inversion of strong-motion and broadband teleseismic data

Abstract We have used 24 broadband teleseismic and 48 components of local strong-motion velocity records of the 1989 Loma Prieta earthquake in a formal inversion to determine the temporal and spatial distribution of slip. Separate inversions of the teleseismic data (periods of 3 to 30 sec) or strong-motion data (periods of 1 to 5 sec) result in similar 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 a depth of 12 km and with a maximum slip of 350 cm, and the other centered about 5 km southeast of the hypocenter at a depth of 16 km and with a maximum slip of 460 cm. The bilateral nature of the rupture results in large amplitude ground motions at sites located along the fault strike, both to the northwest and the southeast. However, the northwestern patch has a larger moment and overall stress drop and is, consequently, the source of the largest ground motion velocities, consistent with the observed recordings. This bilateral rupture also produces relatively modest ground motion amplitudes directly updip from the hypocenter, which is in agreement with the velocity ground motions observed at Corralitos. There is clear evidence of a foreshock (magnitude between 3.5 and 5.0) or a slow rupture nucleation about 2 sec before the main part of the rupture; the origin time implied by strong-motion trigger times is systematically 2 sec later than the time predicted from the high-gain regional network data. The seismic moment obtained from either of the separate data sets or both sets combined is about 3.0 × 1026 dyne-cm and the potency is 0.95 km3.