Abstract

Approximately 3.6 km of P-wave seismic-reflection data were acquired along two orthogonal profiles in Sherman Oaks, California to determine whether shallow (less than 1-km depth) geologic structures contributed to the dramatic localized damage resulting from the 1994 Northridge earthquake. Both lines, one along Matilija Avenue and one along Milbank Street, crossed areas of both high and low damage. We believe these data reveal a geologic structure in the upper 600 m that contributed to the increased earthquake ground shaking in the high-damage areas south of and along the Los Angeles River. Of interest in these data is a reflection interpreted to be from bedrock that can be traced to the north along the Matilija Avenue profile. This reflecting interface, dipping northward at 15°–22°, may be an important impedance boundary because it is the lower boundary of a wedge of overlying low-velocity sediments. The wedge thins and terminates in the area where we interpret down-warped reflections as evidence of a shallow subbasin. The low-velocity subbasin sediments (Vs of 200 m/sec Vp of 500 m/sec) may be up to 150 m thick beneath the channelized Los Angeles River. The area across the subbasin experienced greater earthquake damage from possible geometric focusing effects. Three-dimensional basin effects may be responsible for the variable damage pattern, but from these seismic profiles it is not possible to determine the regional structural trends. Two-dimensional elastic and SH-mode finite-difference modeling of the imaged structural geometry along Matilija Avenue suggests that a peak horizontal-velocity amplification factor of two-and-over can be explained in the high-damage area above the shallow subbasin and sediment wedge. Amplification factors up to 5 were previously observed in aftershock data, at frequencies of 2 to 6 Hz. Amplification in the elastic simulation at the Santa Monica Mountains range-front on the southern end of the Matilija profile, with the geologic layering and geometry interpreted from the seismic data, is also consistent with aftershock observations.

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