LARSE II: TOWARDS AN UNDERSTANDING OF THE SUBSURFACE STRUCTURE IN THE SANTA MONICA AREA

The city of Santa Monica sustained concentrated damage from the anomalous amplification of seismic energy during the 1994 Northridge earthquake. Several hypotheses have been developed to explain the high amplitudes of ground motion. These include 1) focusing by a deep geological structure which acted like an acoustic lens, 2) a combination of focusing and shallow basin effects, 3) shallow (less than 1 km) basin edge effects involving constructive interference of surface and body waves. As part of LARSE II we conducted a high resolution seismic survey of Santa Monica to test the various hypotheses. The experiment took place in two parts: 1) a refraction survey in October 1999 which involved recording arrivals from shots and earthquakes (including Hector Mine) on ~200 sensor array, and 2) a 10 km Vibroseis reflection survey in June 2000 through Santa Monica and Pacific Palisades with vibes every 60 m and geophones every 30m. In addition to local shots to study the structure, two distant (4000lb) shots were detonated designed to reproduce the focusing of seismic energy that occurred during the Northridge Earthquake. The wave amplitudes of these shots along with Northridge aftershocks were examined to confirm the existence of focusing. Travel times of first p arrivals for shots and vibes have been used to obtain a 2D velocity structure. The velocity structure confirms existence of sub-basin in Santa Monica beneath the damage zone bounded to the north by the Santa Monica fault. The vibroseis experiment shows coherent reflections south of the Santa Monica fault related to sedimentary layers in the basin. We locate the Santa Monica fault at depth about 1km north of the location of a surface trace that was previously mapped. We also locate the unmapped Potrero Canyon fault using shots detonated in the Santa Monica Mountains.