Two weeks after the 18 October 1989 Loma Prieta, California, earthquake, 18 three-component digital seismometers were deployed in the epicentral area to form three six-station subarrays. The subarray configuration allowed us to investigate the presence of direction- and frequency-dependent site resonances. We measured the shear-wave polarization from the recordings of 10 aftershocks from the Loma Prieta earthquake.
Our observations show that the site response has a strong azimuthal dependence and that both the shear-wave polarization and the spectral amplitude of the ground motions are affected by site characteristics. In the frequency range from 1 to 18 Hz, the majority of stations examined showed preferred azimuths of ground motion for the scattered waves that did not depend either on the earthquake location or on the polarization of the shear waves expected from the known focal mechanism. The measurements were made from 5-sec windows that included direct and scattered shear waves, which contain the largest amplitude motions in the near-source region and are therefore of most interest to earthquake engineers. However, in the 0- to 2-Hz frequency range, the first pulse of shear waves shows a polarization that is well predicted by the mechanism and location of the earthquake.
The rapid spatial variation of the preferred directions and their corresponding frequencies indicate that geologic structures within a distance of the order of 50 m probably control these site effects. We suggest that the site amplified the motion of scattered waves in one preferred direction, altering the resulting polarization and modulating the spectral amplitude.