Source complexity of the October 1, 1987, Whittier Narrows earthquake

Abstract

Long‐ and short‐period body waveform data recorded at regional and teleseismic distances are modeled to obtain the source parameters of the Whittier Narrows earthquake of October 1, 1987. Although the long‐period data can be satisfied by a simple thrust earthquake on a plane striking 280° at a depth of 14 km, we require a double source solution to satisfy both the long‐ and short‐period teleseismic data. The first subevent has a focal mechanism of strike (θ) = 250°, dip (δ) = 30°N, and a slip vector (λ) = 90°; the second is θ = 280°, δ = 40°N, and λ = 98°. The second event occurred 1.0 s after the first, was 5 times larger (1.2 × 1025 dyn cm for the second, 0.23 × 1025 for the first) in terms of seismic moment and was also shallower (12 km, whereas the first subevent was at 15 km). The two subevents probably occurred on a north dipping listric fault but may have occurred on two subparallel faults. These features were determined by studying a variety of phases including teleseismic P and SH and regional Pnl. The Whittier Narrows earthquake was a high stress drop event, but a single value for stress drop cannot be obtained because of the complex nature of the source. If we define the fault area as the aftershock area, we obtain a stress drop of about 750 bars.