High Resolution Seismic Imaging of the Brawley Seismic Fault Zone

In March 2010, we acquired a series of high-resolution P-wave seismic reflection and refraction data sets across faults in the Brawley seismic zone (BSZ) within the Salton Sea Geothermal Field (SSGF). Our objectives were to determine the dip, possible structural complexities, and seismic velocities within the BSZ. One dataset was 3.4 km long trending east-west, and consisted of 334 shots recorded by a 2.4 km spread of 40 hz geophones placed every 10 meters. The spread was initially laid out from the first station at the eastern end of the profile to roughly 2/3 into the profile. After about half the shots, the spread was shifted from roughly 1/3 into the profile to the last station at the western end of the profile. P-waves were generated by Betsy-Seisgun 'shots' spaced every 10 meters. Initial analysis of first breaks indicate near-surface velocities of ~500-600 meters/sec, and deeper velocities of around 2000 meters/sec. Preliminary investigation of shot gathers indicate a prominent fault that extends to the ground surface. This fault is on a projection of the Kalin fault from about 40 m to the south, and broke the surface down to the west with an approximately north-south strike during a local swarm of earthquakes in 2005 and also slipped at the surface in association with the 2010 El Mayor-Cucapah earthquake in Baja California. The dataset is part of the combined Obsidian Creep data set, and provides the most detailed, publicly available subsurface images of fault structures in the BSZ and SSGF.