Wabash Valley Fault Research Articles

Comparison of Seismicity Rates in the New Madrid and Wabash Valley Seismic Zones

The Wabash Valley seismic zone in southern Illinois and Indiana is a northeastern extension of the New Madrid seismic zone (Figure 1). Like New Madrid, the Wabash zone is underlain by a failed Precambrian rift, which plays a role in controlling the recent faulting (Braile et al. 1986; Sexton et al. 1986; Bear et al. 1997). Paleoliquefaction deposits indicate the past occurrence of large earthquakes in the Wabash zone (Obermeier 1998) that may have been comparable to those that occurred in the New Madrid zone in 1811–1812 (Hough et al. 2000). The two areas seem likely to be mechanically coupled in that stress transfer following large earthquakes in one could affect earthquake occurrence in the other (Mueller et al. 2004). Numerical modeling indicates that stress transfer following the 1811–1812 New Madrid earthquakes may be loading faults in the Wabash zone (Li et al. 2005; 2007). Despite their similarities, the two zones have an intriguing difference in seismicity rates (Stein and Newman 2004). This difference is shown in Figure 2A by comparison of frequency-magnitude plots. The plots combine the Center for Earthquake Research and Information (CERI) catalog of seismologically recorded small earthquakes spanning January 1975–June 2010 (http://www.ceri.memphis.edu/seismic/catalogs/cat_nm.html) and the Nuttli catalog of historic earthquakes for 1804–1974 with magnitudes out to magnitude 6.2 (Nuttli 1974). Both areas show a Gutenberg-Richter distribution of seismicity, log …

Geodetic Observations of Active Intraplate Crustal Deformation in the Wabash Valley Seismic Zone and the Southern Illinois Basin

Research Article| September 01, 2010 Geodetic Observations of Active Intraplate Crustal Deformation in the Wabash Valley Seismic Zone and the Southern Illinois Basin Gerald A. Galgana; Gerald A. Galgana 1Department of Geological Sciences, Indiana University, Bloomington, Indiana 2Now at Lunar and Planetary Institute, Universities Space Research Association, Houston, Texas Search for other works by this author on: GSW Google Scholar Michael W. Hamburger Michael W. Hamburger 1Department of Geological Sciences, Indiana University, Bloomington, Indiana Search for other works by this author on: GSW Google Scholar Seismological Research Letters (2010) 81 (5): 699–714. https://doi.org/10.1785/gssrl.81.5.699 Article history first online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Gerald A. Galgana, Michael W. Hamburger; Geodetic Observations of Active Intraplate Crustal Deformation in the Wabash Valley Seismic Zone and the Southern Illinois Basin. Seismological Research Letters 2010;; 81 (5): 699–714. doi: https://doi.org/10.1785/gssrl.81.5.699 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietySeismological Research Letters Search Advanced Search Seismically active intraplate continental settings are characterized by relatively low strain rates, but they nonetheless display evidence of active faulting and seismicity. Examples of these tectonic environments include seismically active areas of northern Eurasia, central India, Australia, and the U.S. mid-continent (e.g., Sykes 1978; Johnston 1996). They are characterized by low heat flow (e.g., Kusznir and Park 1984), relatively thick lithosphere, and presumably low strain rates (∼10–10–10–12 yr–1) (Gordon 1998). Yet in only a handful of these environments have geodetic measurements been used to provide constraints... You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

GIS-Based Virtual Geotechnical Database for the St. Louis Metro Area

Research Article| May 01, 2010 GIS-Based Virtual Geotechnical Database for the St. Louis Metro Area JAE-WON CHUNG; JAE-WON CHUNG 1Department of Geological Sciences and Engineering, 1400 North Bishop Avenue, Missouri University of Science and TechnologyRolla, MO 65409 Search for other works by this author on: GSW Google Scholar J DAVID ROGERS J DAVID ROGERS 1Department of Geological Sciences and Engineering, 1400 North Bishop Avenue, Missouri University of Science and TechnologyRolla, MO 65409 Search for other works by this author on: GSW Google Scholar Author and Article Information JAE-WON CHUNG 1Department of Geological Sciences and Engineering, 1400 North Bishop Avenue, Missouri University of Science and TechnologyRolla, MO 65409 J DAVID ROGERS 1Department of Geological Sciences and Engineering, 1400 North Bishop Avenue, Missouri University of Science and TechnologyRolla, MO 65409 Publisher: Association of Environmental & Engineering Geologists First Online: 02 Mar 2017 Online ISSN: 1558-9161 Print ISSN: 1078-7275 Copyright © 2010 EEGS Environmental & Engineering Geoscience (2010) 16 (2): 143–162. https://doi.org/10.2113/gseegeosci.16.2.143 Article history First Online: 02 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation JAE-WON CHUNG, J DAVID ROGERS; GIS-Based Virtual Geotechnical Database for the St. Louis Metro Area. Environmental & Engineering Geoscience 2010;; 16 (2): 143–162. doi: https://doi.org/10.2113/gseegeosci.16.2.143 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyEnvironmental & Engineering Geoscience Search Advanced Search Abstract The St. Louis metropolitan area is the focus of the U.S. Geological Survey's Earthquake Hazard Program's plan for assessing the likely risks of an earthquake emanating from the New Madrid Seismic Zone or the Wabash Valley Seismic Zone, which are the most active seismic zones recognized in the Midwestern United States. The St. Louis metro area includes portions of eight counties located both in Missouri and Illinois that abut the state boundary formed by the Mississippi River. The Illinois and Missouri geological surveys have prepared geologic maps and data sets that employ dissimilar geodata information, differing map units, map scales, and storage formats, with geodata stored in analog or digital formats. This research sought to combine the dissimilar geodata sets and to integrate them into a single Virtual Geotechnical Database within an accepted geographic information system (ArcGIS), which could be manipulated to retrieve subsurface data and to perform an array of spatial analyses. The geodatabase is intended to promote standardization of geologic interpretations between Missouri and Illinois and will be available to other researchers. The existing body of data was manipulated to extract useful information on the surficial geology, loess thickness, bedrock geology, well locations, and measured values of shear wave velocity, which could be conjoined to create stand-alone GIS information layers. “Depth-to-bedrock” refers to the lithologic contact with early Paleozoic-age strata recognized by formational assignments across the two states. Depth-to-bedrock and groundwater table elevations underling the study area were interpolated using geostatistical methods of ordinary kriging and cokriging, respectively. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Three‐dimensional crustal structure of eastern North America extracted from ambient noise

Group velocity dispersion curves of surface waves extracted from ambient seismic noise are inverted to find 3‐D shear wave structure of the crust beneath eastern North America. The 3‐D model consists of one sediment layer and another six layers with fixed depths at 5, 7.5, 10, 15, 25, and 43 km. Velocities of the seven layers together with the thickness of the sediment layer are determined by the simulated annealing method. We found that almost all failed ancient rifting events (e.g., the Reelfoot rift, Ouachita triple junction, and the Midcontinent rift) and rifting related events (e.g., the Ozark uplift and the Nashville dome) are associated with high‐velocity bodies in the middle and lower crust. Our results also suggest the existence of a triple junction‐like high‐velocity body centered around the New Madrid and the Wabash Valley seismic zones with the Reelfoot rift, the Ozark uplift, and the Nashville dome being on its southwestern, northwestern, and southeastern arms, respectively. We also found that the western limb of the Midcontinent rift (MCR) extends southwestward to western Oklahoma and Texas, and the eastern limb of the MCR extends southeastward into western Ohio. The Appalachian Mountains are characterized by high‐velocity upper crust underlain with relatively low velocity middle and lower crust. All major seismic zones are associated with either divergent or convergent events. The New Madrid seismic zone and Wabash Valley seismic zone are clearly associated with the failed Reelfoot rift. Both the eastern Tennessee seismic zone and Ouachita Orogen seismic zone are located along convergent boundaries.

Small temblor rattles Missouri

A magnitude 2.7 earthquake shook part of central Missouri at about 6:25 A.M. local time on 5 May. The event, which caused no reported damage, follows an 18 April magnitude 5.2 earthquake whose epicenter was in Illinois near the border of Indiana. That earlier quake, classified as moderate by the U.S. Geological Survey (USGS), occurred in the Wabash Valley seismic zone and was the strongest earthquake in southern Illinois since 1968.

The April 18, 2008 Illinois Earthquake: An ANSS Monitoring Success

The largest-magnitude earthquake in the past 20 years struck near Mt. Carmel in southeastern Illinois on Friday morning, 18 April 2008 at 09:36:59 UTC (04:37 CDT). The Mw 5.2 earthquake was felt over an area that spanned Chicago and Atlanta, with about 40,000 reports submitted to the U.S. Geological Survey (USGS) “Did You Feel It?” system. There were at least six felt aftershocks greater than magnitude 3 and 20 aftershocks with magnitudes greater than 2 located by regional and national seismic networks. Portable instrumentation was deployed by researchers of the University of Memphis and Indiana University (the first portable station was installed at about 23:00 UTC on 18 April). The portable seismographs were deployed both to capture near-source, high-frequency ground motions for significant aftershocks and to better understand structure along the active fault. The previous similar-size earthquake within the Wabash Valley seismic zone (WVSZ) of southeastern Illinois and southwestern Indiana was a magnitude 5.0 in June 1987. The seismicity associated with the WVSZ is thought to occur in a complex horst and graben system of Precambrian igneous and metamorphic units at depths between 12 and 20 km. Paleoliquefaction evidence suggests several major shaking events have occurred within the past 12,000 years (Munson et al. 1997). Table 1 lists the significant earthquakes in this part of southeastern Illinois and southwestern Indiana. The locations with intensities are from a catalog of Nuttli and Brill (1981). The location of most earthquakes in the 1900s benefited from the increasing number of seismograph stations in the area. The recurrence rate of the significant events is roughly one every 10 years. The distinctive difference between the earlier earthquakes and the 18 April mainshock is the presence of modern digital instruments near the earthquake source, which permits improved location and, for the first time, rapid determination …

Deep faulting and structural reactivation beneath the southern Illinois basin

The investigation of deep fault structure and seismogenesis within “stable” continental interiors has been hindered by the paucity of detailed subsurface information and by low levels of seismicity. Outstanding seismotectonic questions for these areas include whether pre-existing structures govern the release of seismic energy as earthquakes, can reactivation of such structures be recognized, and to what extent have Precambrian basement structures exerted long-lived controls on the development of overlying Phanerozoic features. The southern portion of the Illinois basin provides a premier area in which to study the relation between contemporary seismicity and pre-existing structures due to the frequency of seismic events, the concentration of available geophysical data, and the wealth of borehole information. We have integrated the study of this information in order to create a 2.5-dimensional picture of the earth for local seismogenic depths (0–15 km) for a study area of moderate 20th century earthquake activity. The area is located along the western flanks of two of the major structures within the Illinois basin, the Wabash Valley fault system (WVFS) and the La Salle anticlinal belt (LSA). The results of reprocessing seismic reflection profiles, combined with earthquake hypocenter parameters, suggest three distinct seismotectonic environments in the upper crust. First, we have delineated a fault pattern that appears to correspond to the steep nodal plane of a strike-slip mechanism event (1974.04.03; m b = 4.7). The fault pattern is interpreted to be a deeply buried rift zone or zone of intense normal faulting underpinning a major Paleozoic depocenter of the Illinois basin (Fairfield basin). Second, a similar event (1987.06.10; m b = 5.2) and its well-located aftershocks define a narrow zone of deformation that occurs along and parallel to the frontal thrust of the LSA. Third, the hypocenter of the largest event in the study area (1968.11.09; m b = 5.5) may be spatially associated with a prominent zone of dipping middle crustal reflections, just west of the WVFS, which have been interpreted as a deeply buried blind thrust. The proposed correlation of pre-existing structures with earthquakes having consistently oriented structural parameters supports the reactivation of old deformation zones by contemporary stresses as previously proposed by earlier workers. However, the degree to which deformation has propagated upward from Precambrian basement into the Paleozoic rocks varied significantly even over a small study area. The societal value of associating an earthquake with a specific pre-existing deformation zone in the seismogenic crust is to improve the assessment of seismic hazard or to assess the integrity of a stratigraphic formation, being considered as a target for natural gas storage or carbon sequestration.

Evidence of Possible Induced Seismicity in the Wabash Valley Seismic Zone from Improved Microearthquake Locations

To evaluate the spatial structure of seismicity in the Wabash Valley of southern Indiana and Illinois, we analyzed data from a temporary seismic network that included 10 three-component, short-period seismometers and a 10-component dense phased array. We produced the first comprehensive catalog of local earth- quakes recorded during the 211-day deployment, from November 1995 through June 1996. The results are dominated by a cluster of 534 microearthquakes, with magni- tudes ranging from 0.6 to 1.8, near the town of New Harmony, Indiana, that were detected and recorded only by the phased array. The remarkable similarity of the cluster events in terms of waveform, magnitude, and temporal distribution suggests a tightly spaced grouping from a single seismic source. We relocated the earthquakes using improved relative arrival times of P and S waves computed by time-domain cross-correlation of the vertical beam traces and by complex cross-correlation of the horizontal beam traces, respectively. Additional constraints on absolute locations were applied using a graphical method of arrival-time difference analysis for six earthquakes recorded by two network stations outside the phased array. The resulting locations define a tight spatial grouping about a zone of post-Paleozoic faulting just west of the Indiana-Illinois border, in White County, Illinois. Average source depths from phased-array location methods and waveform modeling with synthetic seis- mograms indicate, in contrast to larger events in the region, that the earthquakes occurred at depths less than 4 km, within the sedimentary section of the Illinois Basin. We propose that these earthquakes are artificially induced events, likely related to water injection for the purpose of secondary recovery of petroleum in the Illinois Basin. The primary evidence for this includes: (1) tight spatial clustering of earth- quakes; (2) unusually shallow earthquake depths; (3) good spatial correlation of the relocated hypocenters to existing wells and oil fields; and (4) highly repetitive events with only a small range in magnitude.

Geophysical and Geological Evidence of Neotectonic Deformation along the Hovey Lake Fault, Lower Wabash Valley Fault System, Central United States

High-resolution seismic (shear-wave) reflection profiles were collected over a segment of the Hovey Lake fault, a known Paleozoic fault within a system of faults in the southernmost Wabash River valley of the central United States. Although the fault zone, called the Wabash Valley fault system, lies in an area of recognized prehistoric and contemporary seismicity, the seismogenic potential of this system remains poorly defined. Consequently, the objectives of this study are to assess the Hovey Lake fault, one of the more prominent fault strands in the system, for neotectonic reactivation and if present, collect sediment samples for dateable material from the disrupted horizons to provide an age constraint for the movement. The resultant stacked profiles show high-angle deformation extending above the Paleozoic bedrock and into Upper Quaternary sediment. Time-displacement calculations from the data show approximately 10.5 m of offset on the top-of-bedrock horizon and 2 m of inverted displacement along the earliest-arriving Quaternary soil reflector. Preliminary correlative coring and carbon-14 age dating of a disrupted soil horizon, located 7.7 m below ground surface, suggest fault movement at this site as late as approximately 37,000 years before present.

Revised Magnitude-bound Relation for the Wabash Valley Seismic Zone of the Central United States

Seismic hazard assessment in the central United States, and in particular the Wabash Valley seismic zone of Indiana-Illinois, frequently relies on empirical estimates of paleoearthquake magnitudes ( M ). In large part these estimates have been made using the magnitude-bound method. Existing region-specific magnitude-bound relations rely heavily on only a few historical earthquakes in the central United States and eastern Canada that induced reported liquefaction features. Recent seismological studies have suggested smaller magnitudes than previously presumed for some of these earthquakes, however, and other studies have reinterpreted site-to-source distances to liquefaction features associated with some of these earthquakes. In this paper, we re-examine historical earthquakes ( M > ∼5) that occurred in the central and eastern United States and eastern Canada; some of these earthquakes triggered liquefaction and others did not. Based on our findings, we reinterpret the region-specific magnitude-bound relation for the Wabash Valley. Using this revised magnitude-bound relation, we present magnitude estimates for four prehistoric earthquakes that occurred in the Wabash Valley seismic zone during Holocene time.

Short Note: Magnitude Scaling of Prehistorical Earthquakes in the Wabash Valley Seismic Zone of the Central United States

The moment magnitude determination for prehistorical earthquakes in the Wabash Valley seismic zone traditionally has relied on modified magnitude-bound liquefaction curves. The calibration of the moment magnitude and maximum distance to surface evidence of liquefaction curve for the central United States depended, in part, on the scaling of the 31 October 1895 earthquake as an M 6.8 event. Returning the M of the 1895 earthquake to a previously accepted value range of M 5.9 to M 6.2, and permitting the distance between the epicenter and the resulting liquefaction sites to vary, lowers the M of the prehistorical earthquakes by at least 0.6 units.

The 18 June 2002 Caborn, Indiana, Earthquake: Reactivation of Ancient Rift in the Wabash Valley Seismic Zone?

The 18 June 2002 Caborn, Indiana, earthquake ( M W 4.6) occurred on a steeply dipping fault at a depth of about 18 km. Regional and teleseismic waveform data were used to obtain an accurate hypocenter with uncertainties better than about ±2 km in both vertical and horizontal directions. The source mechanism determined from regional waveform analysis is predominantly strike slip along near-vertical nodal planes (dips 82° and 84°) striking 28° and 297°. The nearly horizontal P axis trends 252° and plunges 10°. The close proximity of the epicenter to the trace of the compound Caborn fault, and good agreement between the strike and dip of that fault and the source mechanism of the 2002 event, suggests that the earthquake may have occurred on that fault. Most of the Wabash Valley faults as mapped from the shallow subsurface data extend into the Precambrian basement, to at least 7 km depth, without significantly changing their orientation. The June 2002 event at 18 km depth and the south-central Illinois earthquake on 9 November 1968 ( m w 5.3), which occurred at 25 km depth, suggest that the seismogenic depth in the Wabash Valley seismic zone extends down to at least 18 km depth. This earthquake is unique because it is the largest instrumented event to date that is clearly located within the area spanned by the Wabash Valley fault system. Its focal mechanism—vertical strike-slip faulting at depth—may suggest that buried faults associated with a possible Precambrian rift system are being reactivated by the contemporary east–east-northeast–trending regional horizontal compressive stress.

Interpreting the Earthquake Source of the Wabash Valley Seismic Zone (Illinois, Indiana, and Kentucky) from Seismic-reflection, Gravity, and Magnetic-intensity Data

Reprocessing of seismic-reflection data reveals new images of upper- to middle-crustal structures beneath the Wabash Valley seismic zone, located north of the New Madrid seismic zone within the seismically active southern Illinois basin. Four intersecting deep seismic profiles (243 km total) indicate an anomalous, 5–10-km-wide zone of dipping reflections and diffractions below the western flank of the Wabash Valley fault system (WVFS). The zone corresponds in places to gently arched regions of Paleozoic strata. The reflector zone can be interpreted as a result of either (or a combination of) magmatic intrusion or structural deformation. The area encompassing the reflection profiles has experienced several moderate magnitude (3.0 ≤ mbLg ≤ 5.5) earthquakes during the past 50 years, defining the central part of the Wabash Valley seismic zone. The hypocenter of the largest 20th-century earthquake in the central USA midcontinent (9 November 1968, mbLg 5.5) corresponds to the most prominent zone of dipping middle-crustal reflections, just west of the WVFS. Both the focal mechanism (moderately dipping reverse fault) and the expected rupture zone size (∼2.9 km fault length) of this earthquake are consistent with the orientation and size of observed reflectors. Dipping reflector patterns in the Precambrian crust are not collinear with fault surfaces updip in the Paleozoic sedimentary section. This indicates that shallow Paleozoic structures are effectively “decoupled” from deeper, possibly seismogenic structure, which suggests that understanding Paleozoic structure is not the key to understanding the earthquake source. The complex dipping crustal reflectivity beneath the WVFS is typical of Paleozoic continental convergent zones observed elsewhere ( e.g. , Appalachian orogen) and thus may suggest a preserved Proterozoic suture, possibly associated with the distal Grenville orogeny or an older event. Although magnetic intensity, Bouguer gravity, and seismic-reflection data present different means of understanding the deep geology of the area, their integration aids in limiting the number of admissible interpretations. The reflection profiles indicate a variable zone of anomalous crustal structure, including the dipping reflector zone, along a trend of northeast-trending gravity and magnetic highs locally defining the Commerce geophysical lineament (CGL), which is a suspected source of seismic hazard in the central USA midcontinent. Three-dimensional inverse modeling of the residual isostatic gravity anomaly values indicates that the upper part of the dipping reflector zone beneath the CGL lies near an important density boundary in the upper Precambrian crust. The results of our study suggest that the seismogenic source just north of the New Madrid seismic zone consists, in part, of a pre-existing fabric of blind thrusts localized along pre-existing igneous intrusions, locally coincident with the CGL. This suggests that the CGL may be seismogenic in places and thus a potential seismic hazard. The variation in the expression of the CGL using reflection and potential-field data sets is probably partly related to the differing geologic features by which it is expressed, but would be also consistent with its reactivation numerous times under varying stress regimes.

Preliminary Results from a GPS Geodetic Network in the Southern Illinois Basin

Observations from a new high-precision GPS geodetic network in the southern Illinois basin provide evidence for present-day tectonic strain in the Wabash Valley seismic zone, an area associated with a concentration of historical and instrumentally recorded earthquakes, paleoseismic evidence of repeated, large-magnitude earthquakes, and possible Quaternary faulting. The GPS network consists of 56 sites distributed over a 100,000 km 2 area of Illinois, Indiana, and Kentucky. The preliminary results reported here are based on a one-year measurement interval, from 1997–1998, and suggest statistically significant horizontal motions at 28 of the sites surrounding the Wabash Valley seismic zone. The inferred velocities are highly variable, presumably influenced by systematic and random geodetic errors, as well as significant nontectonic deformation sources, such as mine- and solution-related subsidence. Nonetheless, the individual site velocities, as well as a formal inversion for tectonic strain, suggest a systematic pattern of shear strain that may be interpreted either as sinistral shear along the north-northeast-trending Wabash Valley fault system or as dextral shear along the northeast-trending Commerce geophysical lineament. While most of our strain estimates remain statistically indistinguishable from zero, the averaged shear strain for the entire network is estimated at 12.7 ± 6.0 × 10 −9 yr −1 . The shear strain rate estimated for the area surrounding the Wabash Valley fault system is estimated at 3.6 ± 4.8 × 10 −9 yr −1 , in a similar direction but at a significantly smaller magnitude than previously measured rates in the New Madrid seismic zone. This observed present-day tectonic strain signal, while close to the level of noise, suggests a systematic pattern that may be tested by future GPS measurements in the region.

Seismicity of the Wabash Valley Seismic Zone Based on a Temporary Seismic Array Experiment

We operated an eleven-station network of digital instruments in the Wabash Valley region from November 1995 through June 1996 in order to investigate seismic activity in the Wabash Valley seismic zone. One station of the network was a ten-element, three-component, high-frequency, phased array The array was primarily responsible for lowering the detection threshold by approximately 1.5 magnitude units below that achieved previously, to magnitudes of 1.2 to 1.5. We observe a significant excess of events in the region from that expected by extrapolation of the historical and earlier instrumental catalogs. We show that the excess is related to a cluster of earthquakes near New Harmony, Indiana. We argue that their shallow depth, similarity of waveform characteristics, and proximity to producing oil wells and underground coal mines suggest that these small-magnitude events may be artificially induced. We find that discarding the events from this cluster leads to seismicity rates more consistent with previous data.

Reduction of earthquake risk in the United States: bridging the gap between research and practice

Continuing efforts under the auspices of the National Earthquake Hazards Reduction Program are under way to improve earthquake risk assessment and risk management in earthquake-prone regions of Alaska, California, Nevada, Washington, Oregon, Arizona, Utah, Wyoming, and Idaho, the New Madrid and Wabash Valley seismic zones in the central United States, the southeastern and northeastern United States, Puerto Rico, Virgin Islands, Guam, and Hawaii. Geologists, geophysicists, seismologists, architects, engineers, urban planners, emergency managers, health care specialists, and policymakers are having to work at the margins of their disciplines to bridge the gap between research and practice and to provide a social, technical, administrative, political, legal, and economic basis for changing public policies and professional practices in communities where the earthquake risk is unacceptable.

Free and Ambient Vibration of Brent-Spence Bridge

The main objective of this study is to assess the structural integrity of the double-deck through-truss Brent-Spence bridge when subjected to a seismic event from the New Madrid, Wabash Valley, or Anna seismic zones. To achieve this, the scope of work was divided into several tasks as follows: field testing of the main bridge, analytical modeling, determination of the site-specific ground motion, and seismic response analysis. This paper deals with the first two tasks. The ambient vibration properties of the main bridge were determined through field testing under traffic induced excitation. The purpose of measuring the ambient vibration properties was to determine the mode shapes and the associated natural frequencies. These vibration properties were subsequently used as the basis for calibrating the finite-element model for seismic response analysis. A three-dimensional finite-element model of the main bridge was developed for the free vibration analysis. The seismic response analysis using this calibrat...

Liquefaction Evidence for Holocene and Latest Pleistocene Seismicity in the Southern Halves of Indiana and Illinois: A Preliminary Overview

Clastic dikes filled with sand and gravel, interpreted to be of seismic liquefaction origin, occur throughout much of the southern halves of Indiana and Illinois. Nearly all of the liquefaction features originated from earthquakes centered in the southern halves of Indiana and Illinois, rather than further south in the nearby source region for the great 1811–12 New Madrid earthquakes. At least eight earthquakes strong enough to induce liquefaction have occurred during the past 20,000 years. At present, the best dated paleoliquefaction evidence lies in Indiana. More paleoliquefaction features have been found in Illinois, but studies there have not yet narrowly bracketed the age of the liquefaction evidence at most sites. Estimated magnitudes of at least two prehistoric events are higher than M 7, which greatly exceeds the largest historic earthquake of M 5.5 that has originated in Indiana and Illinois. The strongest prehistoric earthquakes have been centered in the general vicinity of strongest historic seismicity, in and near the Wabash Valley seismic zone in the lower Wabash Valley of Indiana and Illinois. However, in several cases prehistoric events greater than M 6 have struck far outside the Wabash Valley seismic zone, in areas where there has been little or no historic seismicity. Numerous other strong paleoearthquakes could have struck elsewhere in the southern halves of Indiana and Illinois but not be recognized because of the lack of liquefiable deposits in many areas.

Geophysical Setting of the Wabash Valley Fault System

Interpretation of existing regional magnetic and gravity data and new local high-resolution aeromagnetic data provides new insights on the tectonic history and structural development of the Wabash Valley Fault System in Illinois and Indiana. Enhancement of short-wavelength magnetic anomalies reveal numerous NW- to NNE-trending ultramafic dikes and six intrusive complexes (including those at Hicks Dome and Omaha Dome). Inversion models indicate that the interpreted dikes are narrow (≤3 m), lie at shallow depths ( Based on the interpretation of both the regional magnetic and gravity data and the high-resolution magnetic data, we propose that the shallow faults and deep-seated rift structures in the Wabash Valley terminate at or near the Rough Creek-Shawneetown Fault System. The Grayville Graben (∼20 km wide, ∼700 m maximum basement relief, and et al. , this volume]) underlying the Wabash Valley developed during rifting, perhaps in response to stress concentrations generated by a bend in the Reelfoot-Rough Creek-Rome rift system. We therefore hypothesize that although the Reelfoot Rift and Rough Creek Graben represent tectonic intraplate structures of large areal extent (>500 km long and generally >50 km wide) and with deep basins (locally >3 km thick), the ancestral Wabash Valley faults express, in comparison, minor tectonic structures and probably do not represent a failed rift arm. There is a lack of any obvious relation between the Wabash Valley Fault System and the epicenters of historic and prehistoric earthquakes. Five prehistoric earthquakes lie conspicuously near structures associated with the Commerce geophysical lineament, a NE-trending magnetic and gravity lineament lying oblique to the Wabash Valley Fault System and possibly extending over 600 km from NE Arkansas to central Indiana.

Seismic Interpretation of the Deep Structure of the Wabash Valley Fault System

Interpretations of newly available seismic reflection profiles near the center of the Illinois Basin indicate that the Wabash Valley Fault System is rooted in a series of basement-penetrating faults. The fault system is composed predominantly of north-northeast-trending high-angle normal faults. The largest faults in the system bound the 22-km wide 40-km long Grayville Graben. Structure contour maps drawn on the base of the Mount Simon Sandstone (Cambrian System) and a deeper pre-Mount Simon horizon show dip-slip displacements totaling at least 600 meters across the New Harmony fault. In contrast to previous interpretations, the N-S extent of significant fault offsets is restricted to a region north of 38° latitude and south of 38.35° latitude. This suggests that the graben is not a NE extension of the structural complex composed of the Rough Creek Fault System and the Reelfoot Rift as previously interpreted. Structural complexity on the graben floor also decreases to the south. Structural trends north of 38° latitude are offset laterally across several large faults, indicating strike-slip motions of 2 to 4 km. Some of the major faults are interpreted to penetrate to depths of 7 km or more. Correlation of these faults with steep potential field gradients suggests that the fault positions are controlled by major lithologic contacts within the basement and that the faults may extend into the depth range where earthquakes are generated, revealing a potential link between specific faults and recently observed low-level seismicity in the area.