Fault History and Architecture of the Southernmost San Andreas Fault and Brawley Seismic Zone: New Constraints from CHIRP Data Acquired in the Salton Sea (original) (raw)

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Abstract

The Salton Trough is the boundary between spreading-center dominated extension in the Gulf of California and dextral strike-slip deformation along the San Andreas Fault (SAF) system. The Salton Trough provides an ideal opportunity to image this transition in modes of deformation. The critical portion of this system, namely the intersection of the SAF and the Brawley Seismic Zone (BSZ) in the southern Salton Sea has not been imaged by geophysical methods. To address this problem, we conducted a pilot, high-resolution seismic CHIRP survey in the Salton Sea offshore Bombay Beach. CHIRP imaging, together with onshore field mapping and paleoseismic investigations, has the potential to define the interaction between the SAF and the BSZ, as well as delineate fault architecture and strain partitioning in the central Salton Trough. Preliminary onshore examination of Lake Cahuilla sediments reveal lake-level changes and earthquake event chronology for the last ~1,000 years, and suggest a relatively long period of seismic quiescence for the southern SAF preceded by several events with shorter recurrence intervals. Fault excavations have revealed several lake episodes separated by terrestrial horizons that include distinct features such as mud-cracks. New CHIRP data show potential correlation of faulted offshore stratigraphy with paleoseismic deformation documented at an excavation site 15 km to the north adjacent to Salt Creek. Profiles image a well-defined fault trending obliquely to the strike of the onshore SAF, and the observed trend is sub-parallel to the BSZ. Offset stratigraphy across the fault imaged in CHIRP profiles increases with depth, with a maximum vertical offset of ~6-8 m. Relief of ~.5 m exists across the post-1905 surface and most likely represents deposition mantling an older scarp. Assuming high amplitude reflectors observed in CHIRP data correlate with lowered lake levels associated with weathering and/or desiccation horizons, then we can correlate the offshore CHIRP with the five most recent lake episodes observed at the excavation site. Predicted sedimentation rates from the CHIRP data, based on this assumption, are consistent with rates determined from nearby URS borings in the Salton Sea. The incremental, down-section increase in offset shown in CHIRP profiles is consistent with three to four events over the last four lake cycles, which is nearly equivalent to the timing and number of events observed at the Salt Creek paleoseismic site. If our interpretations are correct, the onshore excavation and offshore CHIRP data suggest that faults in the BSZ rupture in concert with the southern SAF. Furthermore, based on the limited imaging offshore, we suggest that the BSZ consists of a series of relatively short en-echelon normal- dominated faults. Such onshore-offshore studies can potentially answer important questions regarding linkage and slip partitioning between the southernmost San Andreas Fault, Brawley Seismic Zone, Superstition Hills Fault and cross faults such as the Elmore Ranch Fault, and will strengthen interpretations of the individual data sets.

Publication:

AGU Fall Meeting Abstracts

Pub Date:

December 2006

Bibcode:

2006AGUFM.T41D1599B

Keywords: