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

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

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

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

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

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

Linear site-response characteristics at central and eastern U.S. seismic stations

Earthquake S waves can become trapped, or resonate, between the free surface and high-impedance basal layers, strongly contributing to site response at specific frequencies. Strong S-wave resonances have been observed in the central and eastern U.S., where many sites sit on unlithified sediments underlain by stiff bedrock. To evaluate S-wave resonances in this region, we calculated 1D linear site-responses at 89 seismic stations with developed S-wave velocity profiles into bedrock. We found that S-wave resonances at the fundamental and strongest (peak) modes occur across large ranges of frequencies, each spanning more than two orders of magnitude — 0.21–54.0 Hz and 0.29–71.5 Hz, respectively. Amplifications of ∼5 and ∼6 are common at the fundamental frequency and peak modes, respectively; the largest amplification calculated was 13.2. Using simple regression analyses, we evaluated the skills of six proxies derived from the S-wave velocity profiles to predict the frequencies and corresponding amplifications of the fundamental and peak modes. We found that the depths to the 1.0 km/s and 2.5 km/s horizons, consistent with other studies, and to the maximum impedance contrasts strongly correlate with the resonance frequencies and that the fundamental-mode and peak amplifications correlate with the maximum impedance ratios. Correlations improved for data subsets based on the number and magnitude of impedance ratios underlying the sites and are the strongest at sites underlain by a single impedance ratio of 3.0 or greater. Finally, we calculated the S-wave horizontal-to-vertical spectral ratios (HVSR) at each possible seismic station and found, consistent with other studies, that the first peak can be used to estimate fundamental-mode frequencies and the corresponding amplifications. Thus, S-wave HVSR, can provide useful estimates of the fundamental-mode linear site response parameters at sites lacking S-wave velocity profiles. Furthermore, S-wave HVSR curves appear to be useful to broadly categorize impedance-ratio profiles

Seismic Monitoring and Baseline Microseismicity in the Rome Trough, Eastern Kentucky

In the central and eastern United States, felt earthquakes likely triggered by fluid injection from oil and gas production or wastewater disposal have dramatically increased in frequency since the onset of the unconventional shale gas and oil boom. In the Rome Trough of eastern Kentucky, fracture stimulations and wastewater injection are ongoing and occur near areas of historical seismic activity. Unlike in surrounding and nearby states (Ohio, West Virginia, and Arkansas), in Kentucky, no seismic events related to subsurface fluid injections have been reported as felt or detected by regional seismic networks, including the Kentucky Seismic and Strong-Motion Network. Oil and gas development of the deep Cambrian Rogersville Shale in the Rome Trough is in a very early stage, and will require horizontal drilling and high-volume hydraulic fracturing. To characterize natural seismicity rates and the conditions that might lead to induced or triggered events, the Kentucky Geological Survey is conducting a collaborative study, the Eastern Kentucky Microseismic Monitoring Project, prior to large-scale oil and gas production and wastewater injection. A temporary network of broadband seismographs was deployed near dense clusters of Class II wastewater-injection wells and near the locations of new, deep oil and gas test wells in eastern Kentucky. Network installation began in mid-2015 and by November 2015, 12 stations were operating, with data acquired in real time and jointly with regional network data. Additional stations were installed between June 2016 and October 2017 in targeted locations. The network improved the monitoring sensitivity near wastewater-injection wells and deep oil and gas test wells by approximately an entire unit of magnitude: With the temporary network, the detectable magnitudes range from 0.7 to 1.0, and without it, the detectable magnitudes range from 1.5 to 1.9. Using the real-time recordings of this network in tandem with the recordings of other temporary and permanent regional seismic stations, we generated a catalog of local seismicity and developed a calibrated magnitude scale. At the time this report was prepared, 151 earthquakes had been detected and located, 38 of which were in the project area, defined as the region bounded by 37.1°N to 38.7°N latitude and 84.5°W to 82.0°W longitude. Only six earthquakes occurred in the Rome Trough of eastern Kentucky, none of which were reported in regional monitoring agency catalogs, and none of which appear to be associated with the deep Rogersville Shale test wells that were completed during the time the network was in operation or with wastewater-injection wells