Basin Effects in the Upper Mississippi Embayment

Site effects play a very important role in characterizing earthquake ground surface motions. At the edges of sedimentary basins, the large velocity contrast between soil and rock generates basin-induced surface waves that propagate in the horizontal direction inside the basin resulting in ground motions with long duration and high low-frequency energy. The 2-D non-linear seismic response of the Upper Mississippi Embayment has been computed to evaluate these basin effects in the region. The analyses have been performed using an indirect boundary integral — discrete wave number method. The non-linear soil behavior has been incorporated by using an equivalent linear approach. For the range of periods implemented in the analyses, 2 to 10 seconds, no significant difference was observed between the soil amplification of 1-D and 2-D models of the Upper Mississippi Embayment, except for sites located in a narrow region along the basin edge and periods longer than 4 seconds, where higher ground amplification was observed due to basin-edge effects. Possible differences in ground motion duration due to basin effects are not included in these analyses and must be accounted for separately if necessary.