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

by J. Vidale, 0. Bonamassa and H. Houston

Vidale, J., O. Bonamassa and H. Houston (1990). Directional Site Resonances Observed From the 1 October 1987 Whittier Narrows, California Earthquake and the 4 October Aftershock. SMIP 1990 Seminar on Seismological and Engineering Implications of Recent Strong-Motion Data, p. 2-1 - 2-18.

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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 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 focal mechanisms. The coincidence of the polarizations from the two events was greatest for the frequencies at peaks in the 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.