THREE DIMENSIONAL PALEOSEISMIC INVESTIGATION ON THE SOUTH BREAK OF THE COYOTE CREEK FAULT, SOUTHERN CALIFORNIA

High-resolution paleoseismic studies have been utilized to better understand segment controls on the rupture history of the southern San Jacinto fault (Gurrola, 1996; Rockwell et al., 2000). Trenches have been excavated at seven sites along the southern San Jacinto fault zone. Our study, located on the south break of the Coyote Creek fault, has the unusual status that it is possibly the transitional element between the two above noted faults to the northwest and southeast, respectively. In order to get accurate recognition and the slip-per-event measurements for each individual earthquake, three-dimensional trenching was utilized to expose paleo-surfaces, linear sedimentological features, and to resolve slip. In the main two-dimensional trench excavated across the 1968 rupture trace, we exposed 10 discrete lacustrine deposits representing highstand phases of ancient Lake Cahuilla. Unfortunately, between the third lake and the previous one believed to be the fourth lake, we found a depositional hiatus of about 2500-3000 years. Nevertheless, five earthquake events can be demonstrated above that hiatus. Evidence for those events was established and quantified by excavating trenches both parallel and across the 1968 fault trace. Three-dimensional trenching analysis confirmed that the 1968 slip is the only event recognized as post-dating the 300 year-old lake stand. Previous events are represented by liquefaction, increased displacement, and faults truncated at the same stratigraphic level with fault-rooted fissure fillings. Most events are recognizable at the Middle Break site to the north as well as at Carrizo Wash and Northern Shoreline sites to the south. From these observations it appears that only some of the frequent, moderate1968-type earthquakes rupture the southern segment of the Coyote Creek fault, and almost all of the less frequent larger Superstition Mountain events rupture through this segment. With the application of concepts of the relationship between seismicity at the surface and at depth, the above noted assumption is important in the understanding of fault segmentation and the forecasting of future earthquakes.