Using Microtremors to Assess Site Characteristics in the New Madrid Seismic Zone

Abstract

Results are presented from a study in the New Madrid seismic zone (NMSZ) utilizing the microtremor method. The study analyzes and interprets horizontal to vertical (H/V) spectral ratios to determine resonant periods, wave amplification factors, and liquefaction vulnerability at sites across the Mississippi embayment. Data were collected from areas that experienced earthquake-induced soil liquefaction due to strong ground motion during historic and prehistoric earthquake sequences. Results from 15 sites show resonant fundamental periods of 0.5 s to 4.5 s for embayment thicknesses of ~ 100 m to 900 m and average shear-wave velocity of 800 m/s. These fundamental periods are associated with a strong impedance contrast between embayment sediments and underlying basement rock. The basin configuration and a strong impedance contrast between sediments and basement rocks could contribute significantly to wave amplification at these resonant periods. Other spectral peaks are correlated with major stratigraphic boundaries within basin sediments. Results also indicate higher amplification factors and liquefaction vulnerability at sites located in meandering stream deposits near the basin axis, where embayment thickness is greatest.