Installation of a Vertical Strong-motion Array: PGDP, McCracken County, Kentucky

Local soil conditions have a profound influence on the characteristics of ground shaking experienced during an earthquake. Damage patterns in the 1989 Loma Prieta (Mw=6.9) and the 1994 Northridge (Mw=6.7) earthquakes generally correspond with areas underlain by deep and/or soft soil deposits, indicating that these soil deposits intensified ground motions and enhanced damage (Holzer T., 1994; Chang et aI., 1996). These earthquakes highlight the importance of understanding how soil conditions affect ground motions. The thick soil/sediment deposits of the Upper Mississippi Embayment can also be expected to produce significant site effects. Bedrock is concealed throughout the NMSZ by post-Paleozoic sediments that range from 100 to 1,000 m in depth. These sediments consist of deltaic, shallow marine, and fluvial gravel, silt, and clay. The geometric configuration of these units is enigmatic due to the complicated structural and depositional geology. Consequently, surface recordings in the region consist of a . complex mixture of source, path, and site effects (including anisotropy). The only reliable means of separating source and path effects from site effects is to simultaneously record the earthquake at the base rock and ground surface. As noted by Field et al. (1998), the best source of information for characterizing the input motion at a site comes from a downhole array of accelerometers. Downhole array recordings are essential to accurately resolve the source, path, and site effects (including nonlinearity). Although the earthquake-induced ground motions are expected to be predominantly a function of the shear-wave velocity, unexpected ground motions that are not accounted for in traditional engineering analysis (one-dimensional) can include: . P/SV wave coupling of non-vertically incident waves . trapped body waves . locally generated surface waves arising from the conversion of body S-waves to surface waves at the boundary of a sediment-filled basin (Le., Mississippi Embayment) . spatial variations caused by irregular geometry at the sediment-bedrock interface, horizontal velocity/density variations, or focusing. Possible effects may include broadband amplification, resonance at frequencies other than those predicted by one-dimensional modeling, and substantially longer duration of ground motions (particularly at the longer periods). Multi-dimensional effects of this kind were observed by Carver and Hartzell (1996) in the Santa Cruz Basin during aftershocks associated with the 1989 Loma Prieta earthquake. Consequently, the primary objective of this proposal is to rigorously ~valuate the total soil transfer function of the post-Paleozoic sediments at the PGDP site for the purpose of constraining existing and future site response models.