SITE EFFECT OBSERVATIONS FROM BROAD-BAND ARRAYS

We present evidence of significant variations in time-domain amplitude and spectral estimates of local earthquake recorded by small-aperture arrays. We examine data from three arrays: two arrays of different sc ales deployed at Pinon mat, California, in 1990 and 1991 and an array deployed in the summer of 1993 in the former Soviet republic of Turkme nistan. We find consistent evidence in all cases for significant varia tions in measured amplitudes over scale lengths comparable to feasible wavelengths of incident wave fields. This phenomenon, however, is str ongly frequency dependent. At the Pinon Flat site, variations in power spectral estimates exceed a factor of 100 at frequencies over 4 Hz. A nalysis of teleseismic signals, however, demonstrates that these varia tions diminish rapidly at lower frequencies and falls to negligible le vels below 0.6 Hz. The Turkmenistan site shows similar overall charact eristics; high-frequency variations are less dramatic. Variations comp arable to the Pinon mat site do not occur below 20 Hz. Analysis of tel eseismic signals yield results similar to Pinon Flat, although the tra nsition to negligible variability seems to occur at a slightly lower f requency of 0.3 Hz. The 1990 Pinon Flat experiment utilized simultaneo us recording in two boreholes directly beneath the array. Comparison o f spectral estimates from these boreholes to the surface sensors stron gly suggests that the deviations in high-frequency spectral estimates we observe across the array are due to interaction of the wave field w ith the near-surface, weathered layer. We suggest the differences in h igh-frequency variations of signals recorded at Pinon Flat compared wi th that of the Turkmenistan site can be explained by differences in ne ar-surface conditions at the two sites. The low-frequency transition t o negligible signal variation seen at both sites occurs when the array aperture becomes small compared with the wavelength of surface waves, suggesting that signal variations in intermediate frequencies may be influenced by body-wave to surface-wave conversions caused by crustal velocity variations and topography.