ABSTRACT

The Central United States Seismic Observatory 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 strong‐motion 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/V methods are coincident within a narrow band of frequencies ranging between 0.35 and 1.1 Hz.

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