Kentucky Geological Survey Report of Investigations

Abstract

The Central United States Seismic Observatory is a vertical seismic array in southwestern Kentucky within the New Madrid Seismic Zone. It is intended to record the effects of local geology, including thick sediment overburden, on seismic-wave propagation, particularly strong ground 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 provides a unique opportunity to describe the geology and geophysically measure the complete Late Cretaceous through Quaternary stratigraphy in the northern Mississippi Embayment.

Based on the surface and borehole geophysical measurements, the thick sediment overburden and its complex heterogeneous stratigraphy have been partitioned into a seven-layer sediment velocity model overlying a bedrock half-space. The S- and P-wave sediment velocities range between 160 and 875 m/s, and 1,000 and 2,300 m/s, respectively, and bedrock velocities between 1,452 and 3,775 m/s, respectively. In addition, high-resolution seismic-reflection profiles acquired within a 1-km radius of the array have imaged a complex geologic model, including steeply dipping N30°E-striking faults that have uplifted and arched post-Paleozoic sediments in a manner consistent with a dextral transpression component of displacement. The subparallel fault strands have been traced 1.4 km between reflection profiles and are adjacent to the array. The fault deformation extends above Paleozoic bedrock, affecting the Late Cretaceous and Eocene Mississippi Embayment sediments, as well as the base of the Quaternary. The Paleozoic and Cretaceous horizons show as much as 75 and 50 m of relief, respectively, with the middle Eocene and basal Quaternary disrupted 25 and 15 m, respectively. The differential fault offsets suggest episodic activity during the post-Paleozoic, and represent the first indications of Quaternary neotectonics in this part of Kentucky. More important, these faults may be the first evidence for a hypothesized northeast extension of the strike-slip Axial Fault Zone from a through-going intersection with the left-stepover Reelfoot Fault (i.e., thrust).

Seismometers and accelerometers were both installed at the surface, 30 m, 259 m, and 526 m depths, and at 2 m into bedrock in three separate boreholes. The instrumentation elevation in the boreholes was determined by the major impedance boundaries within the stratigraphic section. Although the array operation has been frequently interrupted by the large hydrostatic pressures on the deeper instrumentation, the full array has recorded weak motions from 95 earthquakes at local, regional, and teleseismic distances. Initial observations reveal a complex spectral mix of amplification and deamplification 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 (HV) experiments show the bedrock vertical and horizontal amplitudes are not equal, violating a required assumption for site characterization. Furthermore, there are marked differences between spectral ratios from the directly measure transfer function (HH) and HV for particular earthquakes. On average, however, the HH and HV methods yield similar results within a narrow band of frequencies ranging between 0.35 and 1.1 Hz.

Publication Date

2016

Series

Series XII

Report Number

Report of Investigations 37

Digital Object Identifier (DOI)

https://doi.org/10.13023/kgs.ri37.12

Funding Information

This work was financially supported by U.S. Geological Survey–NEHRP (awards 08HQGR0094 and G11AP20156), U.S. Department of Energy/ Kentucky Research Consortium for Energy and Environment (award DE-FG05-03OR23032), and the Kentucky Geological Survey.

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