Analysis of regional broadband recordings of the 1987 Whittier Narrows, California, earthquake

Abstract

Recordings of the Whittier Narrows earthquake of October 1, 1987 (BRK ML = 6.0), and the large aftershock of October 4 (BRK ML = 5.5) from broadband seismographs (10 mHz to 10 Hz) at six stations in the Berkeley Seismograph Network in northern California at 430 < Δ < 825 km demonstrate the range of information provided on source mechanisms in such circumstances. Analysis of the waveforms shows that estimation of source mechanisms in this case requires incorporation of the significantly different velocity structures along the Coast Ranges and the Sierra Nevada indicated by the broadband signals. Inversion of the transverse component waveforms, after allowance for regional wave dispersion, provides the following source estimations for the main shock: seismic moment M0 of (1.0±0.06) × 1025; preferred fault plane (strike, rake, and dip) of (259°±33°, 128°±16°, 11°±15°). The main shock mechanism is interpreted as a shallow north dipping reverse fault with the hanging wall moving southeasterly relative to the footwall. Sources of error and of differences with the more robust solution from locally recorded first motions include wave mixing and depth bias at the source. The broadband recordings indicate clearly that the mechanism of the aftershock (M0 of (1.5±0.11) × 1024 dyn cm), although not well resolved by these regional measurements, is quite different from the main shock mechanism.