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

Abstract

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 km2, 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 × 1024 dyn cm and the potency is 0.01 km3. Using the area of significant slip estimated from the finite fault inversion, the resulting stress drop is on the order of 150–200 bars.

Slip History of the 1995 Kobe, Japan, Earthquake Determined from Strong Motion, Teleseismic, and Geodetic Data.

“…Examination of the simulated ground motion reveals that the source contribution to the most damaging ground motion in Kobe is from the relatively low slip beneath Kobe and not from regions of greater slip on Awaji Island. This can be attributed to the additional distance to the large slip Wald and Heaton (1994); Kobe , this study; Northridge, Wald et al (1996); Sierra Madre, Wald (1992); and Loma Prieta , Wald et al (1991).…”

Microscopic and macroscopic physics of earthquakes

“…However, higher stress drops have been reported for some earthquakes for which the source dimension was determined well [e.g. Kanamori et al, 1990;Wald, 1992]. Also, there is evidence that the stress drop can be locally very high (up to 25 kbar) around small asperities [Nadeau and Johnson, 1998].…”

Prospects for Larger or More Frequent Earthquakes in the Los Angeles Metropolitan Region

“…(A) Rupture area (square kilometers) plotted as a function of moment magnitude (M,) for southern California earthquakes(Table 1). Data sources as in(35)(36)(37)(38)(39)(40). (B) Moment magnitude versus average slip over the rupture plane for southern California events was determined by solving the equation M, = pAD.which we then used to calculate the average coseismic slip for each earthquake (Table 2).The two remaining large fault systems (Santa Monica Mountains and Los Angeles basin) involve multiple faults with varying slip rates and slip directions (49).…”