Abstract

We conducted an ambient noise survey to assess the potential for seismic site response in Providence, Rhode Island. Providence is built on bedrock valleys that are partially filled with unconsolidated Holocene sediments. Because similar valley structures exist in other heavily populated coastal and river cities of the northeastern United States, the results of this study are relevant to seismic hazards throughout the region. At sites located in the river valleys beneath downtown Providence, we found peaks in ambient noise frequency spectra that suggest amplification of energy in the range of 2–3 Hz. This spectral peak occurs in the spectra from sediment sites and in spectral ratios that reference sediment to hardrock stations. We used geotechnical borehole records to constrain valley structure and seismic refraction data to determine bedrock velocity, and we calculated the theoretical response to ground motion for one-dimensional models that approximate basin structure. The modeling shows that the 2–3 Hz ambient noise peak can be interpreted as the fundamental resonance frequency of low-impedance surface sediment layers.

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