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Description
Paleoseismic studies along the southern San Jacinto fault zone (SJFZ) were initiated to better understand the interactions between individual fault segments, particularly in terms of earthquake recurrence, and the potential for an individual fault segment to control rupture initiation, propagation, and termination of other fault segments across their mutual boundaries. This study better constrains the slip distribution of past earthquakes along the Superstition Mountain fault (SMF), and relates the earthquake history to that of the Coyote Creek fault (CCF). This was accomplished through detailed mapping of offset geomorphic features along the SMF, and a paleoseismic study at the intersection of the southeastern SMF and the shoreline berm for ancient Lake Cahuilla, where offsets suggest Late Holocene activity. The location of the SMF was mapped utilizing aerial photography and 4m-resolution multispectral Ikonos satellite imagery. Offset geomorphic features were analyzed to determine the probable amount and frequency of fault slip that occurred during past earthquakes. These observations suggest multiple events. The most recent earthquake produced slightly greater than 2 m of right lateral slip, which is consistent with a paleoseismic study north of Superstition Mountain. The penultimate event produced about the same amount of slip, suggesting that the SMF probably ruptures characteristically with about 2 m of slip. The paleoseismic study at the intersection of the SMF and the shoreline berm for ancient Lake Cahuilla, informally called the Southern Shoreline site, suggests that three earthquake events can be correlated with previous paleoseismic studies on the northern SMF and the south break of the CCF. The two oldest events, referred to as events A and M, may correlate to the poorly constrained South Break site events Zz and H, respectively. Radiocarbon data suggests that Event A has a minimum age of B.C. 2300-2010, and Event M is poorly constrained to B.C. 1130 -A.D. 1290. The most recent earthquake noted at the Southern Shoreline site, referred to as Event Y, occurred after A.D. 1430 and before A.D. 1680. This event correlates to the most recent event (Event 1) at the Northern Shoreline site, Event T at the Carrizo Wash site, and Event M at the South Break site. A recent, previously undocumented slip event was noted at the Southern Shoreline site. This slip, referred to as Event Z, probably resulted as triggered slip from the 1987 Superstition Hills event (M6.6). Reconstruction of a fractured surface suggests that there was approximately 2.3 cm of right lateral displacement. Previous paleoseismic studies suggest that rupture on the CCF will only propagate as afterslip to the northern SMF. Event Y at the Southern Shoreline site confirms that rupture on the SMF will likely involve both the SMF and the south break of the CCF. This along with other findings from previous studies suggests that the south break of the CCF is probably the northern continuation of the SMF. An earthquake on the SMF resulting in slip of slightly greater than 2 m, and propagating to the northern extent of the south break of the CCF, would probably be on the order of M7.0 or greater.
