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

Abstract

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.