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Description
The southern San Jacinto fault zone consists of three main fault strands or segments: the Coyote Creek (CCF), Superstition Mountain (SMF) and Superstition Hills (SHF) faults. The CCF is divided into northern, central and southern segments, defined after its rupture in the 1968 Borrego Mountain earthquake. The boundaries of these segments are delineated by step-overs and/or fault bends. In contrast, the segment boundary between the southern segment of the CCF and the northern end of the SMF is defined only by a l0° bend or change in strike. However, the main reason that these two faults are considered separate is that the 1968 rupture terminated along the southern segment of the CCF. The only way to demonstrate how individual segments have behaved in the past and how segment boundaries work is to resolve their past rupture histories through high-resolution paleoseismic studies. We studied the earthquake history of the northern SMF to obtain a complete record of how and which boundaries have controlled past ruptures. We exposed faulted sediments of the regionally extensive Lake Cahuilla at Carrizo Wash along the northernmost SMF, and correlated the stratigraphy and earthquake history to sites along the CCF using radiocarbon dates and sequence stratigraphy. We exposed a 5.5 m-thick section of very well stratified fluvial, deltaic and lacustrine sediments, part of which have been ruptured by the fault. Six and probably seven surface rupturing events have been identified in this section but none of them affected the last two lake Cahuilla high-stand deposits, indicating that the northern Superstition Mountain fault has not ruptured for at least 330 years and possibly as long as 500 years. Using high resolution 3D trenching techniques we obtained information of fault geometry and slip for the last three ground rupturing events. The last earthquake rupture (event V) consisted of en echelon faults with a minimum horizontal displacement of 6 to 9 cm of slip in each segment, which sums to a minimum total slip across the fault zone off 15cm ±4cm. Events 2 and 3 (events T and R) are concentrated in a narrow fault zone that displace a channel margin about 6±1 m. The slip measured for events T and R at Carrizo Wash may represent three separated events, but fluvial erosion related to units 180-195 have apparently removed evidence for the additional event. Based on radiocarbon results alone, this larger earthquakes dates between A.D. 1085 and 1680. Correlation to Gurrola and Rockwell's (1996) site 2 km south along the SMF suggests that the penultimate event at Carrizo wash correlates with the two last SMF events between A.D. 1280 and 1680 (Gurrola and Rockwell, 1996). This is essentially the same result as for the southern CCF, suggesting that in most large earthquakes, the SMF and southern CCF rupture together. In contrast, the central CCF has ruptured repeatedly during the past 300 years whereas the southern CCF ruptured only in 1968. It thus appears that the southern CCF is actually the northern end of the SMF and the 3 km step-over separating the central and southern strands of the CCF is the primary structural and seismic boundary.
