2202 Edward

The 5.1 ML Santa Barbara earthquake of 13 August 1978 occurred at 22h54 rn 52.8 s UTC. The epicenter was located 3 km southeast of Santa Barbara at 34 ° 23.9 'N latitude and 119°40.9'W longitude with a focal depth of 12.7 km. The main shock was fol lowed between 13 August and 30 September by 373 aftershocks that were located with the Caltech-USGS array. The aftershock zone extended 12 km WNW from the epicenter and was 6 km wide in the N-S direction, and it had a very clear temporal development. During the first 20 min of activity, all the aftershocks were located in a cluster 7 km WNW of the main shock epicenter. During the next 24 hr, the aftershock zone grew to 11 km in the WNW direction and 4 km in the N-S direction. During succeeding weeks, the zone extended to 12 by 6 km. This temporal-spatial development relative to the main shock epicenter may indicate that the initial rupture propagated 7 km unilaterally to the WNW, and the initial rupture plane may have been considerably smaller than the eventual aftershock zone. This smaller area suggests that the stress drop may have been significantly greater than that derived from the final aftershock zone. In cross section, the aftershock hypocenters outline a nearly horizontal plane (dipping 15 ° or less) at 13 km depth. The main shock focal mechanism indicates NNE-SSW compression and vertical extension. The preferred fault plane strikes N80°W and dips 26°NNE, indicating north-over-south thrusting with a component of left-lateral movement. Focal mechanisms for 40 aftershocks also indicate compression in the general N-S direction. For most of these events, the northdipping nodal plane dips between 7 ° and 45 ° , with most dipping 25 ° or more, which is significantly steeper than the plane delineated by the hypocenters themselves. These observations are consistent with a tectonic model in which much of the slip during the Santa Barbara earthquake occurred on a nearly horizontal plane. The after shocks then might represent movement on a complex series of imbricate thrust faults that flatten into the plane of primary slip. Hence, the Santa Barbara earthquake may be taken as evidence for mid-crustal horizontal shearing in the western Transverse Ranges. INTRODUCTION On the afternoon of 13 August 1978, the Santa Barbara area was shaken by a magnitude 5.1 ML earthquake which caused moderate damage. The epicenter was located just offshore, 3 km southeast of Santa Barbara, with a hypocentral depth of nearly 13 kin. The event was preceded by two foreshocks and was followed by over 300 aftershocks that were locatable using the Caltech-USGS seismographic network. This earthquake has been previously studied by Lee et al. (1978), but their locations for the main shock and aftershocks are systematically 4 km SW of our locations. This discrepancy is probably due to a strong contrast between the velocity structure in the Santa Barbara Channel and that on the mainland. We fee] we have successfully corrected for that factor in this study. Two fortunate circumstances allowed us to obtain high-quality locations for the main shock and many of its aftershocks. First, University of Southern California (USC) seismologists began deploying ocean bottom seismographs (OBS) in the 2201 2202 EDWARD J. CORBETT AND CARL E. JOHNSON Santa Barbara Channel on the day before the earthquake, and four of them were in operation when the earthquake occurred (Henyey et al., 1978). These stations were located almost directly above the activity and provided excellent data for controlling focal depths: Secondly, 1 month before the earthquake, a group from the University of California at Santa Barbara (UCSB) shot a seismic refraction line in the Santa Barbara Channel (Crandall et al., 1979), and the arrival times at the onshore stations allowed us to calibrate the network for the velocity gradient in the Santa Barbara region. These two data sets greatly enhanced the location accuracy of the main shock and aftershocks, which enabled us to delineate the source mechanism involved in the Santa Barbara earthquake. Geologic setting. The Santa Barbara earthquake occurred in the western end of