Influence of focal mechanism on peak accelerations of strong motions of the Whittier Narrows, California, earthquake and an aftershock

Focal mechanisms affect the pattern of the peak accelerations of the October 1, 1987, Whittier Narrows earthquake and its October 4 aftershock. The peak accelerations observed on 21 Strong Motion Instrumentation Program and 22 U.S. Geological Survey accelerograms correlate well with the ratio of shear wave amplitude computed from the thrust mechanism of the main shock and the strike‐slip mechanism of the aftershock. This correlation means that seismic energy is radiated from the fault with close to the standard double‐couple radiation pattern at the frequencies 3–6 Hz corresponding to the peak accelerations.