MULTISCALE SEISMIC IMAGING OF ACTIVE FAULT ZONES FOR HAZARD ASSESSMENT - A CASE-STUDY OF THE SANTA-MONICA FAULT ZONE, LOS-ANGELES, CALIFORNIA

High-resolution seismic reflection profiles at two different scales we re acquired across the transpressional Santa Monica Fault of north Los Angeles as part of an integrated hazard assessment of the fault. The seismic data confirm the location of the fault and related shallow fau lting seen in a trench to deeper structures known from regional studie s. The trench shows a series of near-vertical strike-slip faults benea th a topographic scarp inferred to be caused by thrusting on the Santa Monica fault. Analysis of the disruption of soil horizons in the tren ch indicates multiple earthquakes have occurred on these strike-slip f aults within the past 50000 years, with the latest being 1000 to 3000 years ago. A 3.8-km-long, high-resolution seismic reflection profile s hows reflector truncations that constrain the shallow portion of the S anta Monica Fault (upper 300 m) to dip northward between 30 degrees an d 55 degrees, most likely 30 degrees to 35 degrees, in contrast to the 60 degrees to 70 degrees dip interpreted for the deeper portion of th e fault. Prominent, nearly continuous reflectors on the profile are in terpreted to be the erosional unconformity between the 1.2 Ma and olde r Pico Formation and the base of alluvial fan deposits. The unconformi ty lies at depths of 30-60 m north of the fault and 110-130 m south of the fault, with about 100 m of vertical displacement (180 m of dip-sl ip motion on a 30 degrees-35 degrees dipping fault) across the fault s ince deposition of the upper Pico Formation. The continuity of the unc onformity on the seismic profile constrains the fault to lie in a rela tively narrow (50 m) zone: and to project to the surface beneath Ohio Avenue immediately south of the trench. A very high-resolution seismic profile adjacent to the trench images reflectors in the 15 to 60 m de pth range that are arched slightly by folding just north of the fault. A disrupted zone on the profile beneath the south end of the trench i s interpreted as being caused by the deeper portions of the trenched s trike-slip faults where they merge with the thrust fault.