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Directional site resonances and the influence of near‐surface geology on ground motion
Author(s) -
Bonamassa Ornella,
Vidale John E.,
Houston Heidi,
Schwartz Susan Y.
Publication year - 1991
Publication title -
geophysical research letters
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.007
H-Index - 273
eISSN - 1944-8007
pISSN - 0094-8276
DOI - 10.1029/91gl01063
Subject(s) - geology , seismology , aftershock , azimuth , shear (geology) , ground motion , focal mechanism , surface wave , magnitude (astronomy) , geodesy , structural geology , geophysics , induced seismicity , physics , paleontology , optics , astronomy
We examine the horizontal motions at close stations from earthquakes in the Loma Prieta and Whittier Narrows sequences to study the shear wave polarizations. We use a dense, six station array recording 10 aftershocks for the former, and use two events and 11 stations across the Los Angeles area for the latter. We compute the average azimuth of strongest shaking in the shear wave as a function of frequency from 1 to 18 Hz for each record of each earthquake. The direction of shaking at a given frequency often correlates much better with an empirical site resonance direction than with the direction of shaking expected from the focal mechanism of the earthquake. The effect tends to be greatest at the frequencies that are the most amplified. This phenomenon can complicate determination of the earthquake source at frequencies higher than 1 Hz. Further, since sites only 25 meters apart show different preferred directions, very near‐surface geology is probably responsible. Estimation of directional site resonances may prove useful for seismic design.