Seismic Response of Two Specific Sites in the New Madrid Seismic Zone

Scientists estimate that there is a 9-in-10 chance of a magnitude 6 to 7 temblor occurring in the New Madrid Seismic Zone (NMSZ) within the next 50 years. Due to the presence of the deep soil and the closeness of the faults to the studied sites, the site response is different and worse than those provided in National Earthquake Hazards Reduction Program (NEHRP) (1997). A major task is to characterize the site and evaluate the dynamic soil properties using field tests. Three different field tests have been used to study the maximum shear modulus: seismic cone penetration, cross-hole velocity, and spectral analysis of surface wave (SASW). Meanwhile, an empirical equation, using results from laboratory tests, was used to compare and calibrate the field test results. Since deep soil is the typical feature of the NMSZ, attention is concentrated particularly on the effect of confining pressure on the variation of shear modulus and damping ratio with wide range shear strain (10^–6–10^–1). The results are compared by different methods. The typical curves of strain dependent modulus degradation and damping ratio at different confining pressures are presented. On the aforementioned results, the influence of confining pressure on the seismic site response in deep soil is analyzed accounting for the near-fault effect.