SEDIMENT-INDUCED AMPLIFICATION IN THE NORTHEASTERN UNITED-STATES - A CASE-STUDY IN PROVIDENCE, RHODE-ISLAND

We employed ambient-noise measurements to assess the potential for sei smic site response in sediment-filled valleys that intersect beneath d owntown Providence, Rhode Island, At eight valley stations and at two sites on an adjacent bedrock highland, we recorded ground motion from two types of sources: pile drivers at a local construction site and am bient microtremors. At all valley sites where sediment thicknesses exc eed 10 m, spectral ratios contain amplitude peaks at frequencies of 1. 5 to 3.0 Hz. In contrast, spectral ratios from the two sites on the be drock highland where sediment cover is less than 4-m thick are relativ ely flat within this frequency range. A variety of borehole logs ident ified two fundamental sediment types (soft sediment and a consolidated glacial till) and were used to map layer thicknesses over the entire study region. Refraction data constrained P-wave velocity in the bedro ck to be 3680 +/- 160 m/sec and indicated two soft-sediment layers wit h P-wave velocities of 300 +/- 50 and 1580 +/- 120 m/sec. Using a one- dimensional reflection matrix technique, we matched the spectral-ratio peak observed at each valley site with the frequency of fundamental r esonance predicted for local layer thicknesses and velocities. A posit ive correlation between the best-fitting soft-sediment velocities and bedrock depth may reflect greater compaction in the deepest sediments or a locally two-dimensional sediment resonance at the deepest sedimen t sites. We conclude that unconsolidated sediment layers under downtow n Providence have the potential to amplify earthquake ground motion at frequencies damaging to engineered structures.