Salton Seismic Imaging Project Line 7: Data and Analysis to Date

The Salton Seismic Imaging Project (SSIP) is a large-scale, active- and passive-source seismic-imaging project designed to image the San Andreas Fault (SAF) and adjacent basins (Imperial and Coachella Valleys) in southernmost California. Data and preliminary results from many of the seismic profiles are reported elsewhere (Fuis et al., Catchings et al., Rymer et al., this meeting). Here, we focus on SSIP Line 7, one of four 2-D NE-SW-oriented seismic profiles that were acquired across the SAF, parts of the Coachella Valley, and/or the Salton Sea. Seismic sources for Line 7 include both land-based downhole explosive sources and airgun sources within the Salton Sea. Data were recorded by 189 Texan seismographs on land (50 m spacing), 102 channels of a multi-channel cabled recording system near the San Andreas fault on land (10 m spacing), and nine ocean bottom seismographs (OBS) within the Salton Sea (1.3 km spacing). The Texans and OBS's recorded both airgun and explosive sources, and the cable array recorded explosions only. Data from the Texan and the multi-channel seismographs were organized as shotgathers, and the OBS data were arranged as receiver gathers. All data were merged into a single profile for analysis. The seismic profile is approximately 23 km long and crosses approximately normal to the SAF, but an approximately 2-km-long segment of the profile at the northeastern edge of the Salton Sea, does not have either seismograph or seismic source coverage due to limited OBS data. Because the gap in the seismic profile was within about 500 m of the surface trace of the SAF, imaging of the shallow part of the SAF was limited. First arrivals from all data sets were combined to develop a refraction tomography velocity image of the upper crust. From the surface to about 6 km depth, P-wave velocities range from about 2 km/s to about 6 km/s, with basement (~6 km/s) shallower northeast of the SAF. The SAF also marks the southwestern boundary of a relatively high-velocity body in the near surface. Southwestward of the SAF, low-velocity (< 4 km/s) sediments thicken to ~ 4 km beneath the Salton Sea. Northeast of the SAF, near-surface (upper 2 km) velocity variations infer complex structures that likely arise from multiple faults. Data were also stacked to develop low-fold reflection images, and we used pre-stack depth migration of automated line drawings to develop migrated images of the upper crust. Stacked and migrated line drawings suggest a northeast dip of sediments on the southwest side of the SAF and sub-horizontal reflectors on the northeast side of the SAF. Velocity contours in the near surface (< 2 km) suggest a low-angle (23o to 33o), northeast-dipping low-velocity zone that extends to the surface near the SAF, and potential fields models support a high-angle (~75o) northeast dip of the SAF.