Strong motion and broadband teleseismic analysis of the 1991 Sierra Madre, California, earthquake

Short period and broadband teleseismic waveform data and three‐component strong motion records were analyzed to obtain the source parameters of the 1991 Sierra Madre earthquake. Close‐in strong motion velocity records (analyzed from 5 s to 5 Hz) show two distinct pulses about 0.35 s apart, requiring some rupture complexity. The near‐field shear wave displacement pulse from this event has a relatively short duration (about 1 s) for the magnitude of the event, requiring a particularly high average stress drop. To further constrain the rupture process, the data were used in a finite fault source inversion to determine the temporal and spatial distribution of slip. We chose a fault plane orientation striking S 62°W and dipping 50° toward the northwest as required by the distribution of aftershocks, the first motion mechanism and the teleseismic bodywave point source inversion. In addition to the aftershock locations, depth constraints are provided by teleseismic short period and broadband recordings which require a centroid depth of 10–11 km. Our inverse modeling results indicate that both the teleseismic and strong motion data sets can be fit with a compact rupture area, about 12 km 2 , southwest and up‐dip from the hypocenter. The average slip is approximately 50 cm, and the maximum slip is 120 cm. The seismic moment obtained from either of the separate data sets or both sets combined is about 2.8 ± 0.3 × 10 24 dyn cm and the potency is 0.01 km 3 . Using the area of significant slip estimated from the finite fault inversion, the resulting stress drop is on the order of 150–200 bars.