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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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.