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

Dreger, Douglas S. and Helmberger, Donald V. (1991) Complex faulting deduced from broadband modeling of the 28 February 1990 Upland earthquake (M_L = 5.2). Bulletin of the Seismological Society of America, 81 (4). pp. 1129-1144. ISSN 0037-1106.

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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 θ = 216°, δ = 77°, λ = 5.0°, M_0 = 2.5 × 10^(24) 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 km^2) 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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Alternate Title:Complex faulting deduced from broadband modeling of the 28 February 1990 Upland earthquake (ML = 5.2)
Additional Information:© 1991, by the Seismological Society of America. Manuscript received 20 August 1990. The authors acknowledge Chandan Saikia for his efforts in developing the F-K code used in this study. We thank David Boore, Robert Graves, Egill Hauksson, and Bradley Woods for their meaningful reviews. We benefited from discussions with Hiroo Kanamori regarding the Pasadena broadband, high-dynamic range instrument and data processing codes he made available. This research was supported by the National Science Foundation grant EAR9014787. Contribution No. 4910, Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California.
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Caltech Division of Geological and Planetary Sciences4910
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Official Citation:Douglas S. Dreger and Donald V. Helmberger Complex faulting deduced from broadband modeling of the 28 February 1990 Upland earthquake (ML = 5.2) Bulletin of the Seismological Society of America, August 1991, v. 81, p. 1129-1144
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:48103
Deposited By: Ruth Sustaita
Deposited On:06 Aug 2014 20:43
Last Modified:03 Oct 2019 07:00

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