Field Measurements of Shear Wave Velocity Profiles in the New Madrid Seismic Zone Using the NEES Field Shaker

The low‐frequency field shaker developed as part of the Network for Earthquake Engineering Simulation (NEES) program was used in field measurements of actively‐generated surface waves in the deep soils overlying the New Madrid Seismic Zone. The objectives of this work were to: (1) study active and passive surface wave methods for deep shear wave velocity (VS) profiling applications, and (2) characterize the stiffness properties of the deep soils in this region. The focus of this presentation will be on the second objective. Bedrock depths in this region of the central United States reach nearly 1000 m in some areas. The softer overlying sediments are generally poorly characterized at depths below about 70 m. Using actively‐generated surface wave energy from the NEES shaker at frequencies as low as about 1 Hz, VS profiles were developed to depths of over 200 m at eleven sites. Lithological information from nearby wells was used to relate the measured VS values to soil formations in the region. Using this information, average VS and coefficient of variation (COV) values were determined for near‐surface alluvial deposits, the Upper‐Claiborne formations, and the Memphis Sand formation. In addition, single‐station spectral ratio measurements (H/V) were performed at each of the eleven sites. The site frequency and bedrock depth were used to interpret the average full‐depth shear wave velocity at each site. The VS profiles developed from the surface wave measurements were useful in identifying limitations of the H/V approach for determining average shear wave velocity values.