USING PASSIVE SINGLE-STATION SEISMIC DATA FOR SHEAR-WAVE VELOCITY PROFILING AND SEISMIC HAZARD ASSESSMENT IN HARTFORD COUNTY, CONNECTICUT

The goal of this research is to measure shear-wave velocity in glacial materials in Hartford County, Connecticut, in order to improve seismic hazard assessments. In standard geophysical practice, collection of data for determination of a subsurface shear-wave velocity profile requires two or three field personnel and a vehicle full of heavy equipment including induced ground-shaking equipment. In this study, single-station seismic sensors provided a compact, low maintenance, inexpensive alternative for collecting shear-wave velocity data. In addition, no supplementary ground-shaking equipment was needed. With this passive seismic technique, three-component velocity data were collected and processed using either the Horizontal-to-Vertical Spectral Ratio (HVSR) or Frequency-Wavenumber (FK) algorithms. The HVSR method calculates the Fourier amplitude spectra of the horizontal and vertical components where the peak frequency (f₀) can be used to estimate depth to bedrock. The FK method uses a FK-domain approach based on Capon’s method to also estimate depth to bedrock. Both methods produce dispersion curves of surface-wave energy in a velocity-frequency plot; the dispersion curve is the input data for inversion of shear-wave velocity depth dependence.

These methods were applied to data collected at Connecticut sites in Hartford County. Field sites near selected wells that had well logs available that indicated depth to bedrock. The results of these passive seismic surveys were grouped according to geologic setting, degree of saturation, and depositional environment using site characterizations specified by the National Earthquake Hazard Reduction Program (NEHRP). This work supports previous efforts of New England State Geologists by providing field measurements used to calculate shear-wave velocities of surficial geologic materials for improved seismic hazard assessment.