Comparison of Geophysically-derived and Surficial Sediment-based Estimates of Seismic Risk in Hartford County, Connecticut

Seismic hazard classifications developed for Hartford County, Connecticut are based primarily on surficial materials and depositional environment to estimate classifications specified by the National Earthquake Hazard Reduction Program (NEHRP). A study using near-surface seismic techniques to measure sediment shear-wave velocities (Vs) in Connecticut was conducted in support of broader seismic hazard mapping efforts undertaken by New England State Geologists. Thirty field sites in Hartford County representative of the range of mapped seismic hazard classes were chosen based on the availability of boring logs and adequate open space for geophysical surveys. Because it can be difficult to acquire multi-channel seismic data in urban areas due to unwanted noise and open space restrictions, the use of passive single-station seismometer measurements was also investigated as a compact supplement and potential alternative to long-offset multi-channel measurements. Here the results of active-source multi-channel analysis of surface waves (MASW) and passive horizontal-to-vertical spectral ratio (HVSR) seismic methods are compared to determine shear-wave velocity profiles and seismic hazard classification based on Vs30 and the shear-wave velocity of glacial sediments throughout Hartford County, Connecticut where Vs30 refers to an averaged shear-wave velocity of the earth’s uppermost thirty meters. For additional verification, active-source vertical seismic profiling (borehole-VSP) was used at sites in two of the five hazard classifications. HVSR-derived seismic resonances were used as a constraint during inversion of the MASW dispersion curve to reduce model misfit and improve model comparison to site lithology. From the calculated Vs30 profiles, we observed that most sites fall under a C or D seismic hazard level where bedrock velocities are included in the Vs30 estimate. For the HVSR method, reliable results were obtained from surveys conducted at sites where the sediment profile met or exceeded the 2:1 velocity contrast between sediment and underlying layers (e.g. bedrock). We also observed that MASW surveys generated suitable velocity profiles when the active seismic source induced sufficient energy into the subsurface. When HVSR and MASW results were combined, a relationship between the observed resonance frequencies at each site corresponded to the dispersion curve fundamental mode. In favorable conditions, the VSP results determine average velocities that compare favorably with the HVSR and MASW results. The active and passive seismic methods used for this study provided the first field-derived shear-wave velocity values for sediments underlying Harford County, Connecticut, necessary for quantitative assessment of seismic hazard class