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The Location and Source Parameters of the Lompoc, California, Earthquake of 4 November 1927

Helmberger, D. V. and Somerville, P. G. and Garnero, E. J. (1992) The Location and Source Parameters of the Lompoc, California, Earthquake of 4 November 1927. Bulletin of the Seismological Society of America, 82 (4). pp. 1678-1709. ISSN 0037-1106.

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In this paper, we address the relocation, magnitudes, and the style of faulting of the Lompoc earthquake from a sparse assortment of teleseismic and regional seismograms. The highest quality teleseismic waveform data come from a station at De Bilt (Netherlands) that remains in operation. Thus, recordings of numerous modern events in central coastal California (i.e., the 1969 Santa Lucia Banks, 1983 Coalinga, 1978 Santa Barbara, and 1989 Loma Prieta earthquakes) have been used for comparison with the 1927 records. Location constraints for the Lompoc event were established from the De Bilt recording by comparing S-P and SSS-S waveform matches against the above master events to avoid the effect of unknown clock errors on locations that use absolute times. These same seismograms were modeled to estimate the depth, faulting parameters, and source strength. A similar approach using observational comparisons and numerical modeling was applied to the regional waveform data obtained from the stations at Berkeley, Tucson, and Pasadena. Our results indicate a north-northwesterly striking reverse event located about 40 km west of Point Conception, which is in excellent agreement with the recent tsunami modeling results by Satake and Somerville (1992). This location is 25 km south of that proposed by Hanks (1979) and well within his error bars. We obtain a body-wave moment of 1 × 10^(26) dyne-cm, a trapezoidal time history of (2, 2, 2) sec. and a source depth of 10 km. The weak beginning of the Pnl wavetrain at Berkeley indicates some source complexity, which is characteristic of many large events. The fault parameters are strike = N20°W, dip = 66°NE, and rake = 95°. Most seismicity catalogs report a M_s = 7.3 for this event, after Gutenberg and Richter (1956), but this was a long-period body-wave magnitude and not a surface-wave result. Their original worksheets indicate a smaller Ms = 7.0. The body waves of the Loma Prieta event (M_s = 7.1) appear distinctly larger than those of the Lompoc event at De Bilt, in agreement with our lower estimate of source strength.

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Additional Information:© 1992 The Seismological Society of America. Manuscript received 12 July 1991. We would like to thank Allison Bent, Larry Burdick, Gladys Engen, Jim McLaren, and Lianshe Zhao for assistance and for reviewing this manuscript. Support for this study was provided in part by Pacific Gas and Electric Company and by USGS contract number 14-08-0001-G1872 at the California Institute of Technology, Division of Geological and Planetary Science contribution no. 4992.
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Caltech Division of Geological and Planetary Sciences4992
Issue or Number:4
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Official Citation:Helmberger, D. V., Somerville, P. G., & Garnero, E. (1992). The location and source parameters of the Lompoc, California, earthquake of 4 November 1927. Bulletin of the Seismological Society of America, 82(4), 1678-1709.
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:48055
Deposited On:06 Aug 2014 17:58
Last Modified:03 Oct 2019 06:59

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