Vs30 and spectral response from collocated shallow, active- and passive-source Vs data at 27 sites in Puerto Rico

Abstract

Shear‐wave velocity (V S ) and time‐averaged shear‐wave velocity to 30 m depth (V S30 ) are the key parameters used in seismic site response modeling and earthquake engineering design. Where V S data are limited, available data are often used to develop and refine map‐based proxy models of V S30 for predicting ground‐motion intensities. In this paper, we present shallow V S data from 27 sites in Puerto Rico. These data were acquired using a multimethod acquisition approach consisting of noninvasive, collocated, active‐source body‐wave (refraction/reflection), active‐source surface wave at nine sites, and passive‐source surface‐wave refraction microtremor (ReMi) techniques. V S ‐versus‐depth models are constructed and used to calculate spectral response plots for each site. Factors affecting method reliability are analyzed with respect to site‐specific differences in bedrock V S and spectral response. At many but not all sites, body‐ and surface‐wave methods generally determine similar depths to bedrock, and it is the difference in bedrock V S that influences site amplification. The predicted resonant frequencies for the majority of the sites are observed to be within a relatively narrow bandwidth of 1–3.5 Hz. For a first‐order comparison of peak frequency position, predictive spectral response plots from eight sites are plotted along with seismograph instrument spectra derived from the time series of the 16 May 2010 Puerto Rico earthquake. We show how a multimethod acquisition approach using collocated arrays compliments and corroborates V S results, thus adding confidence that reliable site characterization information has been obtained.