Towards Construction of the Central United States Seismic Observatory and Calibration Site: Defining the Geologic Site Model

The University of Kentucky has drilled, geophysically logged, and cased a borehole that penetrated the thick Mississippi embayment soil/sediment deposits and terminated 9.5 meters into Paleozoic bedrock near the most active segment of the New Madrid seismic zone. The borehole will be instrumented with a vertical accelerometer array by Fall 2007. As part of the ongoing construction of the new central United States seismic observatory and calibration site, this study proposes defining the subsurface geologic configuration in the immediate vicinity (e.g., 1.5 km radius) of the deep borehole in order for future research to effectively evaluate the 3-D soil/sediment transfer function. Specifically, 3-D ground-motion modeling and observational comparison cannot be accurately performed without knowledge of the largescale subsurface geometry. In addition, smaller scale geologic anomalies can affect the spatial coherence of the observed ground motions between instruments, again degrading the accuracy of the modeled and observational comparison. Subsequent to understanding the transfer function at the new observatory, data collected at regional free-field surface instrumentation can be calibrated and extrapolated to understand the spatial variance of site effects in the central U.S. Consequently, the purpose of this proposal is to perform a series of north- and east-oriented commonmidpoint (CMP) surveys to image and define the subsurface configuration in a 1.5 km radius of the observatory site. The objective of these profiles is to define the larger scale geometry in the vicinity of the site for future 3-D ground motion modeling, and to also define smaller scale near-surface anomalies that might affect spatial coherence. In addition, a series of long-offset S-wave refraction surveys will be acquired to compare and constrain velocity data collected from the downhole PIS velocity measurements that has been acquired. The refraction lines will be collected in a 120-degree fan array in order to evaluate the near-surface velocity variation. Specific acquisition parameters for both the reflection and refraction surveys will be determined in the field from initial matrix testing. The completed observatory and calibration site will provide a unique field laboratory for seismic hazard assessment in the central United States, and the nation as a whole. An accurate site characterization is essential for maximum effectiveness, however.