SITE RESPONSE IN THE UPPER MISSISSIPPI EMBAYMENT

摘要

The New Madrid Seismic Zone (NMSZ) is a seismically active region in the Central United States. The low-velocity deposits in the Upper Mississippi Embayment and the large impedance contrast at the base of the embayment contribute to ground motion amplification, particularly at long periods. To quantify the effect of the deep soil column and age of near-surface geologic deposits on ground motion amplification, site response analyses were performed for a scenario earthquake of moment magnitude 6.5 located at an epicentral distance of 50 km. Two soil profiles representative of the Holocene and Pleistocene-age deposits in the Upper Mississippi Embayment were used in the site response analyses. Due to the lack of strong motion records for the Central U.S., rock motions at the base of the soil column were modeled using band-limited white noise and random vibration theory (BLWN-RVT). The response spectra of the soil sites were compared with the Paleozoic rock site to obtain amplification spectra. The results indicate that nonlinear soil behavior controls site response at short periods (< 0.2 second) whereas amplification due to low-velocity deposits and resonances within the soil column control site response at long periods (> 0.2 second). Peak ground accelerations were similar for the rock and soil sites, but the soil sites amplified spectral accelerations at periods longer than 0.2 second by factors of 2 to 3. Resonances due to the large impedance contrast at the base of the soil column produced significant amplifications at periods near 5 to 6 seconds. Soil sites in Holocene-age deposits produced more amplification than sites in Pleistocene-age deposits due to amplification from the lower-velocity deposits.