The Santa Barbara, California, earthquake of13

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 followed between 13 August and 30 September by 373 after-shocks 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 stres