The Central United States seismic observatory: Site characterization, instrumentation, and recordings

The Central United States SeismicObservatory is a vertical seismic array located in southwestern Kentucky within the New Madrid seismic zone. It is intended to describe the effects of local geology, including thick sediment overburden, on seismic-wave propagation, particularly strong motion. The three-borehole array is composed of seismic sensors placed on the surface, in the bedrock, and at various depths within the 585-m-Thick sediment overburden. The array's deep borehole also provided a unique opportunity in the northern Mississippi embayment for the direct geologic description and geophysical measurement of the complete Late Cretaceous-Quaternary sediment column. A seven-layer intrasediment velocity model is interpreted from the complex, inhomogeneous stratigraphy. The S-and P-wave sediment velocities range between 160 and 875 m=s and between 1000 and 2300 m=s, respectively, and their bedrock velocities range between 1452 and 3775 m=s, respectively. Seismometers and accelerometers operate both at the surface and 2 m into bedrock, with strongmotion accelerometers at depths of 30, 259, and 526 m. The array operation has been frequently interrupted by the large hydrostatic pressures on the deeper instrumentation; however, the full array has recorded weak-motion response from 95 earthquakes at local, regional, and teleseismic distances. Initial observations reveal a complex spectral mix of amplification and de-Amplification across the array, indicating the site effect in this deep-sediment setting is not simply generated by the shallowest layers. Preliminary horizontal-to-vertical spectral ratio (H/V) experiments show the bedrock vertical and horizontal amplitudes are not equal, a required assumption for site characterization. Further, there are marked differences between spectral ratios from the directly measured transfer function (H/H) and H/V for particular earthquakes. On average, however, the H/H and H/Vmethods are coincident within a narrow band of frequencies ranging between 0.35 and 1.1 Hz.