Central United States Seismic Observatory: Phase I, Borehole Installation

Short-term Objective (this Proposal) The thick Mississippi embayment soil/sediment deposits are expected to produce significant ground-motion site effects. Consequently, the primary objective of this proposal is to acquire assistance in the drilling, casing, and geophysical logging of a 640- m borehole through the sediment overburden and into bedrock for the future establishment of a vertical seismic observatory. The observatory will ultimately evaluate the soil transfer function of the unlithified post-Paleozoic sediments at a site near the center of the New Madrid seismic zone (NMSZ). In addition to constraining existing and future site response models in the region (Fig. A), the final array will act as a calibration site for regional strong-motion stations. The site of the proposed seismic observatory is near the most active part of the NMSZ; therefore, it should provide the maximum amount of data in the shortest period of time. The existing free-field strong-motion station at VSAB is, on the average, triggered 1 to 2 times a month by an earthquake. In addition, the proposed site is at the site of a shallow (250 m) vertical array, and midway between free- field strong-motion stations VSAB and RIGD (Street et aI., 1995). Long-term Objectives The subsequent research for the borehole is to rigorously evaluate the effect of soil conditions on earthquake ground motions near the center of the New Madrid Seismic Zone in order to evaluate existing and constrain future site response models in the Upper Mississippi Embayment. These objectives include: 1. Acquire, install and operate a 3-D array of borehole and surface accelerometers. 2. Measure the dynamic soil properties at the array site. 3. Perform downhole and surface shear-wave velocity measurements for backcalculation of in situ soil properties at the site. 4. Utilize 3-D vertical array as calibration site for existing free-field accelerometers in the region. 5. Predict expected site response, based on dynamic soil properties measured at the site. 6. Perform refraction and reflection seismic imaging to obtain a comprehensive understanding of subsurface structural and stratigraphic geologic features in three dimensions in order to provide information regarding vertical and lateral variations in Gmax at the site. 7. Measure deep borehole ground motion while shaking at the ground surface to obtain synthetic site response data. 8. Investigate the seismic response of deep soil sites in order to validate current analytical modeling techniques used to assess deep (i.e., ~100 m) soil sites.