by J. Vidale and 0. Bonamassa
Vidale, J., and 0. Bonamassa (1991). Strong Shaking Directions From the 18 October 1989 Loma Prieta Earthquake and Aftershocks in San Francisco and Oakland. SMIP91
Seminar on Seismological and Engineering Implications of Recent Strong-Motion Data, p. 3-1 - 3-10.
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The direction of strong shaking observed at 13 California Division of Mining and Geology sites across San Francisco and
Oakland at frequencies less than one Hz roughly agrees with a prediction calculated from the well-determined long-period
focal mechanism. The directions of shaking at frequencies higher than one Hz, however, show little resemblance to the
simple prediction, suggesting that the near-surface geology interacts with the higher frequency seismic waves in a
complicated way. This interpretation is reinforced by the observation that aftershocks recorded at the CDMG sites show
similar directional effects from earthquakes with a variety of mechanisms. This propagational complication suggests
that the focal mechanism does not determine the direction of strongest shaking in an earthquake at this range of about
100 km at frequencies above about one Hz.