7. Directional Site Resonances Observed from the 1 October 1987 Whittier Narrows, California, Earthquake and the 4 October Aftershock

Abstract

We present evidence that sites often resonate preferentially in a particular compass direction. The 1 October 1987 mainshock and 4 October 1987 aftershock in the Whittier Narrows, California, sequence had very different focal mechanisms. Nevertheless, at 8 of 11 strong‐motion stations for which digitized records of both events are available, the direction of strongest shaking in the two events was much more similar than would be expected from their different mechanisms. The coincidence of the polarizations from the two events was greatest at frequencies with peaks in the amplitude spectra, suggesting that site amplification and directional resonances are linked. Knowledge of directional site resonances may aid in predicting the directions of damaging earthquake motions.