USING MICROTREMORS TO ANALYZE SITE CHARACTERISTICS IN THE NEW MADRID SEISMIC ZONE

We use Nakamura's method to assess site characteristics in the New Madrid seismic zone (NMSZ). Nakamura's method uses microtremors, or seismic noise, to extract fundamental resonant periods for basin sediments. Broadband seismic data were collected at 19 sites in the central part of the zone, in an area known to have experienced intense liquefaction and ground failure during the 1811 to 1812 earthquake sequences. Sites for the study were chosen to represent four major lithologic groups identified by Saucier (1984): (1) meandering stream and floodplain deposits, (2) braided stream deposits, (3) transitional areas, and (4) Western Lowland deposits. Each province contains varying fractions of sediment types (e.g., clay, silt, sand, etc.) that combine to influence the site's response to strong ground motion. Using the broadband data, we calculate HVPSRs (spectral ratios) to extract fundamental periods across the embayment. Peaks in the spectra are correlated with subsurface units. Lowest periods typically represent depth to basement, while higher frequencies likely reflect shallower units. Results from the study will be used to examine possible wave amplification from large earthquake sequences. Results will be compared to previous studies conducted in other parts of the Mississippi embayment to provide a better understanding of how basin sediments might respond to strong ground motion from large earthquakes.