HIGH-RESOLUTION SEISMIC REFRACTION AND REFLECTION INVESTIGATION OF THE SANTA MONICA FAULT, LOS ANGELES, CALIFORNIA

The Santa Monica fault (SMF), part of a greater system of east-west trending faults along the southern edge of the Transverse Ranges in southern California, is seismically active and has generated at least six surface-rupturing earthquakes in the past 50,000 years (Dolan, 1992). Its activity and location in the densely populated Los Angeles area make it a potentially high seismic hazard. In April 1997, we acquired a north-south trending high-resolution seismic reflection/refraction profile across the fault to determine the nature (depth, dip, etc.) and the velocity structure of the shallow fault zone. The 158-m-long seismic profile consisted of 83 shot points and 60 geophones, each spaced at 2-m increments. Seismic sources were generated by a Betsy SeisgunTM. The data were recorded for 2 s at a sampling rate of 0.5 ms. Inversion of first-arrivals indicate that the shallow subsurface velocities range from about 500 m/s at the surface to about 2500 m/s at 40 m depth with a higher-velocity gradient on the southern end of the profile. We interpret a north-dipping (20o) structure that disrupts sub-horizontal reflectors to be the SMF. Southward projection of the inferred SMF suggests that it intersects the ground surface south of Ohio Ave. In addition to the SMF, the seismic reflection image shows subhorizontal reflectors that are apparently offset in the near surface by steeply dipping faults in the upper 50 m. The steeply dipping faults also appear to offset the SMF, suggesting a complex faulting relationship. High-resolution seismic imaging studies such as this study, combined with trenching studies, can help to better determine the seismic hazard associated with the SMF in the Los Angeles area.