Abstract Measurements of ellipticity of Rayleigh waves recorded in the U.S. Midwest have been examined for azimuth dependence, effects of interference, and repeatability, as well as the hypothesis that a single station may be used to determine local structure. Time- and frequency-domain analyses were performed for each event, with more consistent results from the time-domain method. Results indicate that for the period range of 10 to 50 sec, ellipticity depends primarily upon local structure and does not exhibit significant azimuthal dependence. Most ellipticity values for a given period are repeatable within 5 per cent of other measured values from all source regions, with the greatest deviation being about 10 per cent. The cause of the deviations is attributed to interfering waves and/or poor signal-to-noise ratios. Interference effects result in scatter in ellipticity values. An ellipticity peak in the period range of 18 to 22 sec has variable magnitude for different events, depending upon the amount of interference present and the signal-to-noise ratio. Interference effects also manifest themselves as sharp decreases in group-velocity observations even after filtering. Model studies show that ellipticity peaks can exist, which are due to the layered structure and not necessarily to interference effects. Ellipticity measurements (10- to 50-sec-period range) from a single station are useful for determination of a crustal model for the vicinity of the recording station, but should be used in conjunction with other available geophysical and geological data. Ellipticity measurements are shown to be of special value for model determination in areas with sedimentary layering, a result in agreement with the Boore-Toksöz 1969) study.
Tension-free hernia repair plus recovery expectancy statements return personnel to work more quickly. On the day of primary inguinal hernia repair, patients were given statements about their likelihood of returning from convalescent leave after 7 days and performing nonstrenuous work. Similar statements were given to them by telephone at 72 hours postoperatively and at a 1-week follow-up appointments. Seventy-four percent of the 73 patients returned to nonstrenuous work within 7 days, and 90% returned to strenuous work within 30 days. In this small sample, 385 work days were saved from the Navy's recommended 14 days of convalescent leave. By combining recovery expectancy statements with an effective surgical procedure, it is possible to avoid prolonged convalescence, thereby enhancing military readiness.
The interpretation of 1047 km of seismic reflection data collected in western Lake Superior is presented along with reflection traveltime contour maps and gravity models to understand the overall geometry of the Midcontinent Rift System beneath the lake. The Douglas, Isle Royale, and Keweenaw fault zones, clearly imaged on the seismic profiles, are interpreted to be large offset detachment faults associated with initial rifting. These faults have been reactivated as reverse faults with 3–5 km of throw. The Douglas Fault Zone is not directly connected with the Isle Royale Fault Zone. The seismic data has imaged two large basins filled with more than 22 km of middle Keweenawan pre-Portage Lake and Portage Lake volcanic rocks and up to 8 km of upper Keweenawan Oronto and Bayfield sedimentary rocks. These basins persisted throughout Keweenawan time and are separated by a ridge of Archean rocks and a narrow trough bounded by the Keweenaw Fault Zone to the south. Another fault zone, herein named the Ojibwa fault zone, previously interpreted as the northeastern extension of the Douglas Fault Zone, has been reinterpreted as a reverse fault that closely follows the ridge of Archean rocks. Previous researchers have stated that neighboring segments of the rift display alternating polarity of basins associated with large detachment faults. Accommodation zones have been previously interpreted to exist between rift segments; however, the seismic data do not image a clearly identifiable accommodation zone separating the two basins in western Lake Superior. Thus, the seismic profile may lie directly above the pivot of a scissors-type accommodation fault zone, there is no vertical offset associated with the zone, or the zone does not exist. Seismic data interpretations indicate that application of a simple alternating polarity basin – accommodation zone model is an oversimplification of the complex geological structures associated with the Midcontinent Rift System.
In the fall of 1986, the Geological Survey of Canada (GSC), the United States Geological Survey (USGS), two Canadian universities -- University of Western Ontario and University of Saskatchewan, and four American universities -- Northern Illinois University, Southern Illinois University, University of Wisconsin-Madison and University of Wisconsin-Oshkosh participated in a major deep seismic experiment in Lake Superior under the GLIMPCE (Great Lakes International Multidisciplinary Program on Crustal Evolution) umbrella. This Open-File Report presents the seismic sections for line A, which was shot specifically for refraction recording. The main target for study by this line was the Mid-Continent Rift System. All recording stations, 31 in total (26 land stations and 5 OBSs), recorded energy from shots fired every two minutes (333 m spacing) by a tuned airgun array towed by a contracted ship along line A in Lake Superior. These data are the densest such data ever recorded in the continental North America over such distances. It is also unique since coincident seismic reflection and refraction are available.
Other| November 01, 2005 Constraints on the Location of the Late Quaternary Reelfoot and New Madrid North Faults in the Northern New Madrid Seismic Zone, Central United States J. N. Baldwin; J. N. Baldwin 1William Lettis & Associates, Inc. Search for other works by this author on: GSW Google Scholar J. B. Harris; J. B. Harris 2Millsaps College Search for other works by this author on: GSW Google Scholar R. B. Van Arsdale; R. B. Van Arsdale 3University of Memphis Search for other works by this author on: GSW Google Scholar R. Givler; R. Givler 1William Lettis & Associates, Inc. Search for other works by this author on: GSW Google Scholar K. I. Kelson; K. I. Kelson 1William Lettis & Associates, Inc. Search for other works by this author on: GSW Google Scholar J. L. Sexton; J. L. Sexton 4Southern Illinois University Search for other works by this author on: GSW Google Scholar M. Lake M. Lake 4Southern Illinois University Search for other works by this author on: GSW Google Scholar Seismological Research Letters (2005) 76 (6): 772–789. https://doi.org/10.1785/gssrl.76.6.772 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 J. N. Baldwin, J. B. Harris, R. B. Van Arsdale, R. Givler, K. I. Kelson, J. L. Sexton, M. Lake; Constraints on the Location of the Late Quaternary Reelfoot and New Madrid North Faults in the Northern New Madrid Seismic Zone, Central United States. Seismological Research Letters 2005;; 76 (6): 772–789. doi: https://doi.org/10.1785/gssrl.76.6.772 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 Abstract The New Madrid North and Reelfoot Faults are believed to have ruptured during large-magnitude earthquakes on 23 January and 7 February 1812, respectively, based on the regional distribution of liquefaction, contemporary New Madrid seismic zone microseismicity, and historical accounts of earthquake-related damage. Although the location of the Reelfoot Fault generally is well constrained between the Kentucky Bend of the Mississippi River in Kentucky and the southeastern margin of the Mississippi Valley in Tennessee, the northern extent of the fault remains enigmatic in southeastern Missouri. This is equally true of the New Madrid North Fault, which has a postulated location based primarily on contemporary microseismicity and rupture scenario models of the 1811-1812 earthquake sequence. In this paper, we synthesize existing unpublished and published data with recently acquired subsurface and geomorphic information to clarify the locations of the Reelfoot and New Madrid North Faults in southeastern Missouri.On the basis of this data synthesis, we interpret that the Reelfoot Fault trends northwest across the Kentucky Bend of the Mississippi River as a northwest-facing scarp coincident with Des Cyprie Slough near New Madrid, Missouri, and anomalous elevated topography on southern Sikeston Ridge northwest of New Madrid. Furthermore, Quaternary faulting and folding imaged from seismic-reflection profiles across Des Cyprie Slough coincide with bedrock structural lineaments, a northeast-trending band of contemporary microseismicity, and a distinct northwest-trending post-Tertiary change in alluvial thickness. We trace the Reelfoot Fault as much as several kilometers northwest of the Mississippi River, where it either dies out or steps north-northeast to merge with the New Madrid North Fault. The New Madrid North Fault appears to be expressed geomorphically as left-stepping, en echelon northeast-trending fractures preserved in Pleistocene glacial outwash material comprising Sikeston Ridge. The fractures coincide with Quaternary faults and folds, as well as deeper Cretaceous and Paleozoic faults and flexures, imaged in geophysical profiles. In summary, the surface locations of the Reelfoot and New Madrid North Faults directly west-northwest of New Madrid, Missouri are constrained by geomorphic, geologic, geophysical, and historical seismological data sets and reflect transfer of strain from the northeast-verging Reelfoot reverse fault to the northeast-striking, dextral New Madrid North Fault. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.
Abstract Results of geological and geophysical research conducted in the New Madrid seismic zone since the early 1970’s indicate that much of the seismicity of the area is associated with late Precambrian age rift-related geological structures that have been reactivated by contemporary stresses. Deep seismic reflection surveys have been used to detect and delineate deeply buried geological structures thought to be associated with the seismicity. Satellite imagery and aerial photographs have recently been used to detect a linear feature named the Bootheel lineament inferred to be the surface expression of one of the faults responsible for the 1811–1812 earthquakes. To assess the seismogenic potential of these deep structures and linear features, high resolution seismic reflection and geomorphic studies are required. In July and August, 1990, Mini-Sosie high resolution reflection surveys were conducted in the New Madrid seismic zone. A total of 23 line-kilometers of high resolution reflection data were collected at nine locations. Specific targets for the new surveys include several locations on the Bootheel lineament in the New Madrid area, its northern projection near Sikeston, Missouri, and its southern projection near Blytheville, Arkansas at locations related to the Blytheville arch. A location several kilometers south of Charleston, Missouri, was also selected. Data presented in this paper consist of 7 line-kilometers recorded at locations on or close to the Bootheel lineament near New Madrid, Missouri, Hayti, Missouri, and Blytheville, Arkansas. Numerous small-offset faults, channels and other structures in Tertiary, Cretaceous and Paleozoic age rocks have been interpreted from the Mini-Sosie seismic sections. These structures, although generally not major features themselves, may be associated with deep seated rift-related reactivated structures. Many of the small-offset faults appear to deform or offset Quaternary age sediments. The spatial correlation of the observed faulting with sandblows and lineaments identified from aerial photographs, suggests the possibility that the observed faulting, sandblows, and linear features may be genetically related. If this is the case, then, because the origin of the sandblows has generally been attributed to the 1811–1812 seismic activity, the observed faulting may have been active at that time. It is not possible to directly link a single correlatable seismic signature with the Bootheel lineament, and thus we cannot state unequivocally that the lineament is continuous from Blytheville, Arkansas to New Madrid, Missouri. However, each seismic line has imaged similar small-offset faulting and gentle folding. If the faults and deformation observed are directly caused by reactivated deep structures associated with the Bootheel lineament, then, due to its great length, the total of which is yet undefined, this structure may be a source zone for major earthquakes, and therefore requires further investigations. The possibility exists, however, that the small scale faulting and deformation are ubiquitous throughout the New Madrid seismic zone. Additional high resolution seismic data are required to resolve this question.
Abstract An important aspect of seismogenesis concerns the role of preexisting faults and other structural features as preferred zones of weakness in determining the pattern of strain accumulation and seismicity. Reactivation of zones of weakness by present day stress fields may be the cause of many intraplate earthquakes. To understand the relation between reactivated structures and seismicity, it is necessary to identify structures which are properly oriented with respect to the present-day stress field so that reactivation can occur. The seismic reflection method is very useful for identifying and delineating structures, particularly in areas where the structures are buried as in the New Madrid seismic zone. Application of the seismic reflection method in widely separated locations within the New Madrid rift complex has resulted in successful detection and delineation of reactivated rift-related structures which are believed to be associated with earthquake activity. The purpose of this paper is to discuss results from seismic reflection profiling in the New Madrid rift complex. Reflection data from several surveys including USGS Vibroseis* surveys in the Reelfoot rift area reveal reactivated faults and other deep rift-related structures which appear to be associated with seismicity. High-resolution explosive and Mini-Sosie** reflection surveys on Reelfoot scarp and through the town of Cottonwood Grove, Tennessee, clearly show reverse faults in Paleozoic and younger rocks which have been reactivated to offset younger rocks. A Vibroseis survey in the Wabash Valley area of the New Madrid rift complex provides direct evidence for a few hundred feet of post-Pennsylvanian age reactivation of large-offset normal faults in Precambrian-age basement rocks. Several earthquake epicenters have been located in the vicinity of these structures. In the Rough Creek graben, Vibroseis reflection data provide clear evidence for reactivation of basement faults. The success of these reflection surveys shows that well-planned seismic reflection surveys must be included in any program seeking to determine the relationship between preexisting zones of weakness and seismicity of an area.
An integrated gravity, magnetic, crustal seismic refraction, and basement geology study is being conducted of the northeastern extension of the New Madrid Fault Zone in the vicinity of the 38th Parallel Lineament. Gravity and magnetic anomaly maps prepared of this area plus regional seismicity suggest that the basement structural feature associated with the New Madrid seismicity extends northeasterly into southern Indiana to at least 39/sup 0/N latitude. Gravity and subsurface data indicate that the Rough Creek Fault Zone, a major element of the 38th Parallel Lineament, is the northern boundary of a complex graben which formed in late Precambrian-early Paleozoic time and since has been reactivated. Surface wave studies indicate that the crustal thickness of the northern Mississippi Embayment is probably in the range of 50 to 55 km, and the structure of the crust obtained from these studies is highly suggestive of a failed rift. 40 figures, 3 tables.
A Mini‐Sosie™ high‐resolution seismic reflection survey was conducted on Reelfoot scarp in the northwestern Tennessee portion of the New Madrid seismic zone. Interpretation of the Mini‐Sosie data revealed the need to reinterpret previously collected reflection data obtained from explosive source and Vibroseis® surveys. Interpretation and integration of the three data sets have resulted in a new model for the subsurface of Reelfoot scarp and provide evidence for recurrent movement along Reelfoot fault, the major reverse fault associated with Reelfoot scarp. Estimated displacements on Reelfoot fault vary from 60 m (60 ms) for late Paleozoic rocks to 15 m (20 ms) for late Eocene sedimentary units. No clear offsets are observed on this particular fault for units younger than late Eocene age; however, uplift, folding, and related structures are observed in younger sediments. An observed variation of offset with depth (age) and the presence of the younger structures are evidence of reactivation of Reelfoot fault. Small‐offset (10 to 20 m) faults were also detected and have been interpreted to have constant displacement with depth, and therefore, to have occurred as a single faulting event rather than as recurrent movement on a fault plane. Two of these faults are interpreted to have been formed in the middle to late Eocene. A small reverse fault located a few hundred feet east of Reelfoot fault appears to be a single faulting event which extends into sediments of Holocene age. There is a small displacement graben structure which probably extends into Holocene age sediments near the apex of the folded sediments of Reelfoot scarp. The location of the graben structure coincides with a zone of small‐offset nomal faulting with 2 to 3 m of total offset within Holocene sediments observed in a trench excavated over Reelfoot scarp. This small zone of faulting has previously been interpreted to be of tectonic origin. The close association of the faults observed in the trench and the graben structure observed on the seismic data suggests that the two features are directly related, and that both structures were formed by Holocene‐era reactivation of Reelfoot fault. Additional evidence supporting our interpretation is provided by synthetic seismograms for models derived from the various data sets and paleosections of the high‐resolution reflection data. A fault map based on all the reflection data shows that our interpretation is consistent with the data sets.
