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Complex faulting deduced from broadband modeling of the 28 February 1990 Upland earthquake (M_L = 5.2)

SEISMOLOGY LABORATORY CALIFORNIA INSTITUTE OF TECHNOLOGY 252-21, PASADENA, CALIFORNIA 91125

Abstract

The 1990 Upland earthquake was one of the first sizable local events to be recorded broadband at Pasadena, where the Green's functions appropriate for the path are known from a previous study. The synthetics developed in modeling the 1988 Upland sequence were available for use in rapid assessment of the activity. First-motion studies from the Caltech-USGS array data gave two solutions for the 1990 main shock based on the choice of regional velocity models. Although these focal mechanisms differ by less than 5° in strike and 20° rake, it proved possible to further constrain the solution using these derived Green's functions and a three-component waveform inversion scheme. We obtain from long-period waves a fault-plane solution of = 216°, = 77°, = 5.0°, M₀ = 2.5 x 10²⁴ dyne-cm, depth = 6 km, and a source duration of 1.2 sec, for which the orientation and source depth are in good agreement with the first-motion results of Hauksson and Jones (1991). Comparisons of the broadband displacement records with the high-pass Wood-Anderson simulations suggests the 1990 earthquake was a complicated event with a strong asperity at depth. Double point-source models indicate that about 30 per cent of the moment was released from a 9-km deep asperity following the initial source by 0.0 to 0.5 sec. Our best-fitting distributed fault model indicates that the timing of our point-source results is feasible assuming a reasonable rupture velocity. The rupture initiated at a depth of about 6 km and propagated downward on a 3.5 by 3.5 km (length by width) fault. Both the inversion of long-period waves and the distributed fault modeling indicate that the main shock did not rupture the entire depth extent of the fault defined by the aftershock zone. A relatively small asperity (about 1.0 km²) with a greater than 1 kbar stress drop controls the short-period Wood-Anderson waveforms. This asperity appears to be located in a region where seismicity shows a bend in the fault plane.

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