Site Effects on Regional Seismograms Recorded in the Vicinity of Weston Observatory
WESTON OBSERVATORY MA
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This report describes a study in which we investigated the variation of amplitudes of seismic waves recorded in the vicinity of Weston Observatory. The data used for this study consisted of seismograms recorded from events primarily quarry blasts located at regional distances from the Observatory. The data were recorded on a 0.25 km aperture array that surrounds the Observatory. Seismometers were installed at four concrete piers anchored into bedrock and four sites where the sensors are in soil overlying bedrock. We calculated spectral amplitude ratios between channels for cases in which the type of siting varied. We found that spectral amplitudes varied by nearly a factor of two at bedrock sites and by as much as a factor of five at soil- covered sites where the soil layer thickness was about 5 to 6 m. Based on the lack of an obvious trend toward greater variation in amplitudes for sites separated by greater distances, our preliminary results suggest that on the scale of a few tenths of a kilometer, site effects play a larger role than propagation effects.
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