Rupture History of the 1984 Morgan Hill, California, Earthquake from the Inversion of Strong Motion Records By

Near-source strong motion velocity records and teleseismic short-period P waveforms are modeled to obtain the spatial and temporal distribution of slip for the 1984 Morgan Hill earthquake. Both forward modeling and constrained, leastsquares inversion techniques are used to interpret the strong motion velocity waveforms in the frequency range of approximately 0.2 to 2.0 Hz. These data support a heady unilateral rupture to the southeast with a rupture propagation velocity of nine-tenths of the local S-wave velocity. The majority of the slip occurs over a fault length of 25 km and to a first approximation can be interpreted as two main source regions, each with an extent of about 5 km with their centers separated by about 12 kin. However, each of the sources has detailed structure of its own, and a simple two-point-source model is not an accurate representation of the Morgan Hill earthquake. The second source occurs about 4.5 sec after the first and is approximately 3 times larger. The maximum dislocation on the fault plane is about 1 m. The total moment of the earthquake is estimated to be 2.1 x 10 ~ dyne-cm. The Morgan Hill earthquake offers convincing evidence for very inhomogeneous slip and stress distributions on shallow strike-slip faults. INTRODUCTION The Morgan Hill earthquake of 24 April 1984 (21:15:18.8 UTC) is a moderatesized event (ML 6.2 USGS) which occurred on the Calaveras fault approximately 17 km southeast of San Jose, California. The epicenter is located at 37°18.56'N, 121°40.68'W with a hypocentral depth of 8.5 km (Cockerham and Eaton, 1984). This earthquake produced a significant set of near-source strong motion records; 12 ground-level, mostly free-field stations with accelerations greater than 0.1 g were recorded within 40 km of the epicenter. One of these stations (Coyote Lake Dam) recorded a peak acceleration of 1.3 g and a peak velocity of 79 cm/sec on a horizontal component. Other records of the strong motion data set suggest that this earthquake consisted of two main sources, or alternatively, a single source which produced a prominent "starting phase" and "stopping phase." In this paper, the strong motion velocity records and teleseismic short-period P waveforms are modeled to determine the distribution of coseismic slip. This slip distribution is then used to determine which of the above source models is correct, as well as, increase our general understanding of the build-up and release of stress on shallow strike-slip faults. Recent seismicity along this section of the Calaveras fault is discussed by Bakun (1980). Larger recent events are the 6 August 1979 Coyote Lake earthquake (ML 5.9), located just south of the Morgan Hill rupture, the 29 August 1978 Halls Valley earthquakes (ML 4.2 and ML 3.9), and the 8 May 1979 Halls Valley earthquake (ML 4.5). DATA The near-source strong motion velocity records are shown in Figure 1, a to c (Shakal et al., 1984; Brady et al., 1984). Unfortunately, most of the stations are not naturally rotated with respect to the fault orientation, with the majority of stations 649 650 S T E P H E N H. H A R T Z E L L A N D T H O M A S H. H E A T O N