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

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 Pn l 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 M s = 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. INTRODUCTION The 1927 Lompoc ear thquake (4 November 0551 Pacific Standard Time, M 8 = 7.0) occurred before the deployment of regional seismic arrays in California, with the result tha t the location and mechanism of the ear thquake have been subject to considerable uncertainty. Byerly (1930) used regional travel-time data to locate the event offshore Point Arguello at 34.5°N, 121.4°W (Fig. 1). Gawthrop (1978, 1981) located it at 34.9°N 120.7°W, much closer to the coast near Point Sal, using teleseismic travel-time data, and suggested that this ear thquake occurred on the Hosgri fault. Hanks et al. (1975) and Hanks (1979, 1981) located it at an intermediate position of 34.6°N, 120.9°W using regional seismic data from the mainshock and aftershocks. This uncertainty in location has resulted in uncertainty in the tectonic interpretat ion of the event and its association with active offshore faults. However, the development of synthetic seismogram techniques in recent years in conjunction with a set of recent ear thquakes has provided an opportunity to obtain more accurate est imates of 1678 THE 1927 LOMPOC, CALIFORNIA, E A R T H Q U A K E 1679 35 o EXPLANATION 1927 Lompoc Earthquake Epicenters (~) Byerly 119301 ( ~ Hanks (19791 ~ ) Gawthrop 119781 (~) This study ----~--'------'_-------'~ Arcs drawn from De Bilt and Santa Barbara seismograph stations I 34 o 122