SH-Mode Seismic-Reflection Imaging of Earthfill Dams

Assessing subsurface characteristics and imaging geologic features (e.g., faults, cavities, low-velocity layers, etc.) are typical problems in near-surface geophysics. These questions often have adverse geotechnical engineering implications, and can be especially acute when associated with high-hazard structures such as large earthen flood-control dams. Dam-related issues are becoming more frequent in the United States, because a large part of this major infrastructure was designed and constructed in the early- to mid-twentieth century; these dams are thus passing into the latter stages of their design life, where minute flaws that were overlooked or thought to be insignificant in design/construction are now proving problematic. The high-hydraulic heads associated with these structures can quicken degradation of weak areas and compromise long-term integrity. Addressing dam-related problems solely with traditional invasive drilling techniques is often inadequate (i.e., lack of lateral resolution) and/or economically exorbitant at this scale. However, strategic geotechnical drilling integrated with the broad utility of near-surface geophysics, particularly the horizontally polarized shear-wave (SH-mode) seismic-reflection technique for imaging the internal structural detail and geological foundation conditions of earthfill embankment dams can cost-effectively improve the overall subsurface definition needed for remedial engineering. Demonstrative evidence for this supposition is provided in the form of SH-wave seismic-reflection imaging of in situ and engineered as-built components of flood-control embankment dams at two example sites in the central United States

Northeast‐Oriented Transpression Structure in the Northern New Madrid Seismic Zone: Extension of a Shear Zone across the Reelfoot Fault Stepover Arm

High‐resolution seismic‐reflection profiles recently acquired 12 km northeast of the New Madrid seismic zone’s Reelfoot thrust and along the central axis of the Reelfoot rift, imaged steeply dipping N30°E striking faults that have uplifted and arched post‐Paleozoic sediments in a manner consistent with a dextral strike‐slip component of displacement. The subparallel fault strands have been traced 1.4 km between reflection profiles. In order to evaluate the structure’s potential regional scale, the strike was projected northeast 22 km to its intersection with a nearby industry profile. At the intersection, this lower‐resolution profile exhibits a discrete 0.75 km wide structure with style and offset similar to the high‐resolution lines. The high‐resolution images indicate the 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. Geologic and geophysical logs from a borehole adjacent to the seismic lines constrain the depth, velocity, and stratigraphic interpretations. We interpret the faults as a minimum 34 km northeast extension of the Axial fault zone from a throughgoing intersection with the left‐stepover Reelfoot thrust

Source Scaling, Subevent Distributions, and Ground-Motion Simulation in the Composite Source Model

Predicting strong ground motion from a large earthquake depends to a large extent on the development of a realistic source model. Strong ground motion was simulated using the composite source model. F0or comparison purposes, two different approaches were implemented in the source procedure simulation. For the first approach, the source was taken as a superposition of circular subevents with a constant stress drop. The number of subevents and their radii followed fractal law distribution, specified as a spatial random field, and subevents were allowed to overlap. This resulted in the total area of the subevents being much greater than the area of the main event, in order to catch the total seismic moment conservation. For the second approach, the number of subevents and their characteristic dimensions still obeyed fractal law, but subevents were distributed randomly over the main fault and did not overlap. The total area of subevents equaled the area of the main fault. In the second approach, the subevent stress drop was left as a free parameter to be adjusted, so that the sum of the subevents’ seismic moment equalled the seismic moment of the main event. Using these two approaches, broadband ground motion was predicted from scenario earthquakes. The numerical simulations from these two approaches gave us similar results in waveform, peak ground motions, and frequency contents. The major purpose of these simulations was to address some recent criticism of the overlapping procedure (e.g., numerical implementation) used in the previous composite source model. The generally good agreement between simulated and observed ground motions from the Mw4.6 June 18, 2002, Darmstadt, Ind., earthquake and the Mw4.0 June 6, 2003, Bardwell, Ky., earthquake shown in this study indicates that the numerical techniques of the composite source model are capable of reproducing the main characteristics of ground motion, both in the near field and the far field, in the central United States

An Update of Seismic Monitoring and Research in the Vicinity of the Paducah Gaseous Diffusion Plant: January 2018–December 2019

From January 2018 to December 2019, the Kentucky Geological Survey monitored earthquakes and conducted research on seismic hazards in the vicinity of the Paducah Gaseous Diffusion Plant, a former uranium enrichment facility, in McCracken County, western Kentucky. Six hundred forty-four earthquakes with magnitude between 0.5 and 3.7 were recorded in the area during this period. Research focused on the influence of the thick sediments on earthquake ground motion, the so-called site response, through theoretical and data analysis of borehole seismic records. Our research has shown that the National Earthquake Hazards Reduction Program site classification, which is based on Vs30, and correction factors currently being used in earthquake engineering design and other safety evaluations are not appropriate to account for site response in the area

An Update of Seismic Monitoring and Research in the Vicinity of the Paducah Gaseous Diffusion Plant: January 2013–December 2017

From January 2013 to December 2017, the Kentucky Geological Survey monitored earthquakes and conducted research on seismic hazards in the vicinity of the Paducah Gaseous Diffusion Plant, a former uranium enrichment facility, in western Kentucky. Fifteen earthquakes with magnitude greater than 3.0 occurred in the area during this period, and data were collected from the Central U.S. Seismic Observatory and the vertical seismic array at the gaseous diffusion plant. This monitoring improved our understanding of seismic-wave propagation through thick sediments and ground-motion site effects, as well as fault locations in the New Madrid Seismic Zone, ground-motion attenuation, and seismic-hazard assessment. Results have been communicated through publications and presentations at workshops and conferences. The data will contribute to the development of design ground motions for western Kentucky, and specifically for buildings and facilities at the Paducah Gaseous Diffusion Plant

The \u3cem\u3eM\u3c/em\u3e\u3csub\u3ew\u3c/sub\u3e 4.2 Perry County, Kentucky, Earthquake of 10 November 2012: Evidence of the Eastern Tennessee Seismic Zone in Southeastern Kentucky

The 10 November 2012 Mw 4.2 Perry County earthquake may represent a continuation of the seismically active Eastern Tennessee seismic zone (ETSZ) farther north than previously recognized into southeastern Kentucky. The mainshock and aftershock data from regional seismic networks and EarthScope’s Transportable Array stations allowed high‐quality determinations of the source parameters. The focal mechanism, depth, and proximity of the mainshock to the New York–Alabama magnetic lineament, a subsurface, crustal‐scale structure that spatially correlates with central ETSZ seismicity, suggest that this earthquake may share the same type of causal geologic structures as the more‐active ETSZ region to the south

Site Characteristics, Instrumentation, and Recordings of the Central United States Seismic Observatory

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\u27s 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

Comparison of the Ground-Motion Attenuation Relationship Between the Wenchuan, China, Area and the Central and Eastern United States

An Mw-7.9 earthquake occurred in Wenchuan, China, in 2008, along the Longmenshan Fault, which is located on the western border of the South China stable continental region. A detailed comparison of the Wenchuan ground-motion attenuation relationships with the relationships for the central and eastern United States (also a stable continental region) showed that the ground-motion prediction equation for the Wenchuan area is similar to those for the central and eastern United States. Thus, the strong-motion records from the Wenchuan earthquake can be used for constraining the ground-motion prediction equation and engineering analysis for the central and eastern United States

Earthquake Hazard Mitigation in the New Madrid Seismic Zone: Science and Public Policy

In the central United States, earthquake sources that are not well defined, long earthquake recurrence intervals, and uncertain ground-motion attenuation models have contributed to an overstatement of seismic hazard for the New Madrid Seismic Zone on the national seismic hazard maps published by the U.S. Geological Survey. A series of informal interviews in western Kentucky with local businesspersons, public officials, and other professionals in occupations associated with seismic-hazard mitigation discussed seismic-mitigation policies in relation to depressed local economy. Scientific and relative economic analysis was then performed using scenario earthquake models developed with the Federal Emergency management Agency\u27s Hazus-MH software. The ground-motion hazard generated by the 2008 Wenchuan, China, earthquake and seismic mitigation policies in that area were compared with those of the New Madrid Seismic Zone. Continued scientific research, additional educational opportunities for laymen and engineering professionals, and changes in the application of current earthquake science to public policy in the central United States should help improve public safety and economic development

Geologic Characterization, Hydrologic Monitoring, and Soil-Water Relationships for Landslides in Kentucky

Complex spatial and temporal variables control the movement of water through colluvial soils in hillslopes. Some of the factors that influence soil-moisture fluctuation are soil type, thickness, porosity and permeability, and slope morphology. Landslide-characterization and field-monitoring techniques were part of a method to connect hydrologic and geotechnical data in order to monitor long-term hydrologic conditions in three active landslides in Kentucky, establish hydrologic relationships across the slope, and analyze specific soil-water relationships that can predict shear strength. Volumetric water content, water potential, and electrical conductivity were measured between October 2015 and February 2019. The duration and magnitude of drying and wetting within the soil varied for each slope location and soil depth, suggesting that differences in slope morphology, soil texture, and porosity influence the water-infiltration process, as well as shear strength and general landslide dynamics. The parameters measured and soil-water relationships were also compared to rainfall and slope movement at one of the landslides. The method used to acquire hydrologic data was cost-effective, and the field techniques may be useful for subsequent projects, such as slope-stability assessments and landslide-susceptibility modeling. Hydrologic parameters, volumetric water content, and water potential are pertinent to investigating the stability of landslides, which are often triggered or reactivated by rainfall. These methods can be used to support landslide-hazard assessment and improve our understanding of the long-term influence of moisture conditions in hillslope soils