Bulletin of the Seismological Society of America

Recordings from tire strong-motion accelerograph stations have been used to derive a three-dimensional dislocation model for the Parklield Earthquake. The model consists of a buried fault which extends from a depth of 3 km to a depth of 9 km below the ground surface. It appears from the analysis, which considers various fault lengths, that the zone of significant faulting was the 20-kin-long northwestern section of the fault. The rupture velocity has been found to be between 2.4 and 2.5 km/sec and the dislocation amplitudes have been found to be about 120 era. There have been comparisons made of the model results with geodetic data on static deformations and creep measurements following the event. In contrast with several other source mechanism studies of the Parklield event, this model yields a picture which appears to be very consistent with both the dynamic strong-motion measurements as well as the available geodetic and creep data. INTRODUCTION This paper represents the continuation of our efforts at gaining a deeper understanding of the details of energy release during earthquakes. By using the elementary form of the dynamic dislocation theory (Haskell, 1969), a suitable three-dimensional dislocation model has been worked out for the Parkfield event. As demonstrated earlier (Trifunac, 1974), such a model can explain the gross long-period features of the measured near-field strong ground motions. Although the search for a suitable model involves a nonunique inverse approach, the choice of the "best" dislocation model can be substantially guided by independent inferences based on static and dynamic computations. The Parkfield Earthquake is a well-documented event. The main shock and several aftershocks were recorded at five strong-motion stations of the Cholame-Shandon array (Cloud and Perez, 1967) in addition to other teleseismiz data. Even though the strongmotion stations were located only on one side of the fault, the excellent quality accelerograms obtained can be used for source mechanism studies (Trifunac et al. 1973). The purpose of this paper is two-fold. First, through a study of the dynamic faulting process, we will analyze the mechanism of energy release, thereby clarifying and resolving some of the contradictory interpretations of the event (e.g., Scholz et al., 1969 and Aki, 1968). Second, the inversion technique, so far tested only on the San Fernando, California, earthquake, which was a thrust fault, will be applied to a strike-slip fault along the San Andreas. The applicability of such a simple inversion scheme will be tested through comparisons with available information from geodetic and creep data. SUMMARY OF FIELD OBSERVATIONS, AFTERSHOCK STUDIES AND PREVIOUS SOURCE MECHANISM WORK The earthquake occurred on June 27, 1966 at 9:26 p.m. Pacific daylight time (04:26: 13.4 GMT, June 28, 1966) beginning with two light foreshocks (magnitudes 2-3) at 511 512 M. D. TRIFUNAC AND F. E. UDWADIA 6:00 p.m. and 6:15 p.m. on June 27 (McEvilly, 1966), being followed by a stronger foreshock of magnitude 5.1 at 9:09 p.m. Magnitude estimates of the main shock range from 5.3 to 6.5 [rob = 5.3 (USGS), M L = 5.5 (Berkeley), ML = 5.8 (Pasadena), M s = 6.5 (Palisades)], and numerous aftershocks continued for many weeks (Eaton et al., 1970). Since the earthquake occurred at night, nothing is known about the surface faulting which immediately followed the main shock. The first observations that were made about 10 hr after the main shock (Allen and Smith, 1966) indicated that the white line on Highway 46 (Figure 1) had been offset by 4.5 cm in a right-lateral sense. The extrapolation of the displacement curve back to the origin time indicates that there was essentially zero offset at the time of the main shock (Wallace and Roth, 1967 ; Figure 25). 35' 30' 25 ' 20 ' 15' I0 ' 120 ° 5' 36 ° 00' 36 o 00'