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A specialized boundary element algorithm developed to calculate the state of stress in the Anza Gap, San Jacinto Fault Zone, Southern, CA

Description: The widely-used algorithm of Crouch and Starfield is unstable when used to solve our mixed boundary equation problem of interest. Altering the boundary conditions and correspondingly rearranging the system of equations to utilize double-sided boundary elements overcomes this drawback. The new algorithm described here is more physically realistic as in that it allows for rotation of the fault segments in the strain field resulting from satisfying the fault static shear strength condition. Preliminary test results indicate that a fault trifurcation gap model may describe the non-strike slip components to some of the seismicity.
Date: June 1, 1995
Creator: Aster, R.; Flores, R. & Fehler, M.
Partner: UNT Libraries Government Documents Department

Preliminary results of microearthquake survey, Northern Adak Island, Alaska

Description: Nine MEQ-800 portable seismic systems were emplaced and recordings taken during the 30 day period between September 5 to October 4, 1982. During this interval 190 events were correlated on two or more stations by Mincomp. Twenty four of these, seen on four or more stations and considered to be local in origin, yielded, according to Mincomp, reasonable hypocenters and origin times using a homogeneous earth model having a velocity of 5 km/sec. A plot of these hypocenters showed much of the microearthquake activity recorded during the survey to be located beneath Mt. Adagdak. This is different from the events located by the Butler and Keller (1974) microearthquake survey which placed hypocenters beneath the sea in Andrew Bay north and northwest of Mt. Adagdak. Butler and Keller did project a fault plane to the surface which would project southwest through Mt. Adagdak and Andrew Bay Volcano. ESL hypocenter locations using the layered earth model show many of the identified events to occur on the northeast corner of the island at focal depths of 8-10 km. It is not obvious that the observed events are related to a single active fault. If so, the fault must be at a low dip angle as shown by the least-squares-fit to the data on Figure 3. Alternatively, the majority of the events occurring within a fairly restrictive range of focal depths may be more indicative of a magma chamber and the movement of magma. Further interpretation of the microearthquake data obtained during 1982 is, however, outside the scope of this report. The relatively small error ellipses for hypocenter locations, compared to the distribution of hypocenters shown on Plates V and VI lead us to question the validity of the projection of all hypocenters to define a single fault location and orientation. It is apparent ...
Date: January 1, 1982
Creator: Mackelprang, Claron E.
Partner: UNT Libraries Government Documents Department

Investigation of rifting processes in the Rio Grande Rift using data from unusually large earthquake swarms

Description: San Acacia Swarm in the Rio Grande Rift. Because the Rio Grande rift is one of the best seismically instrumented rift zones in the world, studying its seismicity provides an exceptional opportunity to explore the active tectonic processes within continental rifts. We have been studying earthquake swarms recorded near Socorro in an effort to link seismicity directly to the rifting process. For FY94, our research has focused on the San Acacia swarm, which occurred 25 km north of Socorro, New Mexico, along the accommodation zone between the Albuquerque-Belen and Socorro basins of the central Rio Grande rift. The swarm commenced on 25 February 1983, had a magnitude 4.2 main shock on 2 March and ended on 17 March, 1983.
Date: December 1, 1995
Creator: Sanford, A.; Balch, R.; House, L. & Hartse, H.
Partner: UNT Libraries Government Documents Department

Calculation and interpretation of crustal shortening along the Central Basin Platform, West Texas: A method to calculate basement motion for modeling input

Description: The analysis carried out in the Chemical Interaction of Rocks and Fluids Basin (CIRFB) model describes the chemical and physical evolution of the entire system. One aspect of this is the deformation of the rocks, and its treatment with a rigorous flow and rheological model. This type of analysis depends on knowing the state of the model domain`s boundaries as functions of time. In the Andrews and Ector County areas of the Central Basin Platform of West Texas, the authors calculate this shortening with a simple interpretation of the basic motion and a restoration of the Ellenburger formation. Despite its simplicity, this calculation reveals two distinct periods of shortening/extension, a relatively uniform directionality to all the deformation, and the localization of deformation effects to the immediate vicinities of the major faults in the area. Conclusions are drawn regarding the appropriate expressions of these boundary conditions in the CIRFB model and possible implications for exploration.
Date: December 31, 1998
Creator: Hoak, T. E.; Sundberg, K. R. & Ortoleva, P.
Partner: UNT Libraries Government Documents Department

Trench logs from a strand of the Rock Valley Fault System, Nevada Test Site, Nye County, Nevada

Description: The Rock Valley fault system trends northeasterly through the southeast corner of the Nevada Test Site. The system records left-lateral offset of Paleozoic and Tertiary rocks, although total offset amounts to only a few kilometers. Distinct scarps in alluvial deposits of Quaternary age and a concentration of seismicity, particularly at its north end, suggest that the Rock Valley fault system may be active. Two trenches were excavated by backhoe in 1978 across a 0.5-m-high scarp produced by a strand of the Rock Valley fault system. A detailed logging of the two Rock Valley fault trenches was undertaken during the spring of 1984. This report presents: (1) logs of both walls of the two trenches, (2) a general description of the lithologic units and the soils formed in these units that are exposed in and near the fault trenches, (3) observations of the clast fabric of unfaulted and faulted deposits exposed in the trench walls, and (4) a map of the surficial deposits in the vicinity of the trenches.
Date: December 31, 1987
Creator: Yount, J.C.; Shroba, R.R.; McMasters, C.R.; Huckins, H.E. & Rodriguez, E.A.
Partner: UNT Libraries Government Documents Department

Integrated reservoir management for the long term - the Carpinteria Offshore Field

Description: The Carpinteria Offshore Field, Santa Barbara, California, has produced more than 100 million barrels of oil to date. This mature field has continued operations in an economically and politically challenging environment that finally resulted in the abandonment of the field`s California State leases by the lease holder. The abandoned leases, together with adjoining federal leases are now operated by an independent producer. Los Alamos National Laboratory has joined with that independent operator, Pacific Operators Offshore, and with the State Lands Commission of California and the Minerals Management Service, in a unique collaborative effort to redevelop the mature field. This project is a part of a larger umbrella project, the Advanced Reservoir Management Project (ARM), that is designed to demonstrate the worth of advanced computational tools and state of the art methods for independent oil and gas producers. The Carpinteria Reservoir Redevelopment project takes a long-term view of reservoir management - as a result, our management plan includes a continuing investment in time and technology in order to better understand the reservoir. In particular, we have completed an extensive reservoir characterization and geological modeling effort that has created a self-consistent model, satisfying geophysical, geological, and engineering data constraints. We have begun the engineering-intensive flow simulation phase of the project using the current geological description of the reservoir, and are confident that our careful efforts in geological modeling will result in a reasonable reservoir flow model. Dynamic documents exist that are used by participants to stay abreast of developments on the project.
Date: May 1, 1997
Creator: Whitney, E.M.; Brickey, M.R. & Coombs, S.E.
Partner: UNT Libraries Government Documents Department

Analecta of structures formed during the 28 June 1992 Landers-Big Bear, California earthquake sequence (including maps of shear zones, belts of shear zones, tectonic ridge, duplex en echelon fault, fault elements, and thrusts in restraining steps)

Description: The June 28, 1992, M{sub s} 7.5 earthquake at Landers, California, which occurred about 10 km north of the community of Yucca Valley, California, produced spectacular ground rupturing more than 80 km in length (Hough and others, 1993). The ground rupturing, which was dominated by right-lateral shearing, extended along at least four distinct faults arranged broadly en echelon. The faults were connected through wide transfer zones by stepovers, consisting of right-lateral fault zones and tension cracks. The Landers earthquakes occurred in the desert of southeastern California, where details of ruptures were well preserved, and patterns of rupturing were generally unaffected by urbanization. The structures were varied and well-displayed and, because the differential displacements were so large, spectacular. The scarcity of vegetation, the aridity of the area, the compactness of the alluvium and bedrock, and the relative isotropy and brittleness of surficial materials collaborated to provide a marvelous visual record of the character of the deformation zones. The authors present a series of analecta -- that is, verbal clips or snippets -- dealing with a variety of structures, including belts of shear zones, segmentation of ruptures, rotating fault block, en echelon fault zones, releasing duplex structures, spines, and ramps. All of these structures are documented with detailed maps in text figures or in plates (in pocket). The purpose is to describe the structures and to present an understanding of the mechanics of their formation. Hence, most descriptions focus on structures where the authors have information on differential displacements as well as spatial data on the position and orientation of fractures.
Date: December 31, 1997
Creator: Johnson, A.M.; Johnson, N.A.; Johnson, K.M.; Wei, W.; Fleming, R.W.; Cruikshank, K.M. et al.
Partner: UNT Libraries Government Documents Department

Geologic map of Paleozoic rocks in the Calico Hills, Nevada Test Site, southern Nevada

Description: The Calico Hills area in the southwestern part of the Nevada Test Site, Nye County, Nevada, exposes a core of pre-Tertiary rocks surrounded by middle Miocene volcanic strata. This map portrays the very complex relationships among the pre-Tertiary stratigraphic units of the region. The Devonian and Mississippian rocks of the Calico Hills are distinct from age-equivalent carbonate-shelf or submarine-fan strata in other parts of the Nevada Test Site. The Calico Hills strata are interpreted to have been deposited beyond the continental shelf edge from alternating silicic and carbonate clastic sources. Structures of the Calico Hills area record the compounded effects of: (1) eastward-directed, foreland-vergent thrusting; (2) younger folds, kink zones, and thrusts formed by hinterland-vergent deformation toward northwesterly and northerly directions; and (3) low-angle normal faults that displaced blocks of Middle Paleozoic carbonate strata across the contractionally deformed terrane. All of these structures are older than any of the middle Miocene volcanic rocks that were erupted across the Calico Hills.
Date: November 1, 1998
Creator: Cole, J.C. & Cashman, P.H.
Partner: UNT Libraries Government Documents Department

Probabilistic Approach to Site Characterization: MIU site, Tono Region, Japan

Description: Geostatistical simulation is used to extrapolate data derived from site characterization activities at the MIU site into information describing the three-dimensional distribution of hydraulic conductivity at the site and the uncertainty in the estimates of hydraulic conductivity. This process is demonstrated for six different data sets representing incrementally increasing amounts of characterization data. Short horizontal ranges characterize the spatial variability of both the rock types (facies) and the hydraulic conductivity measurements. For each of the six data sets, 50 geostatistical realizations of the facies and 50 realizations of the hydraulic conductivity are combined to produce 50 final realizations of the hydraulic conductivity distribution. Analysis of these final realizations indicates that the mean hydraulic conductivity value increases with the addition of site characterization data. The average hydraulic conductivity as a function of elevation changes from a uniform profile to a profile showing relatively high hydraulic conductivity values near the top and bottom of the simulation domain. Three-dimensional uncertainty maps show the highest amount of uncertainty in the hydraulic conductivity distribution near the top and bottom of the model. These upper and lower areas of high uncertainty are interpreted to be due to the unconformity at the top of the granitic rocks and the Tsukyoshi fault respectively.
Date: February 1, 2001
Creator: MCKENNA, SEAN A.
Partner: UNT Libraries Government Documents Department

A PHYSICAL MODEL OF THE EFFECT OF A SHALLOW WEAK LAYER ON STRONG GROUND MOTION FOR STRIKE-SLIP RUPTURES

Description: We report results of foam-rubber modeling of the effect of a shallow weak layer on ground motion from strike-slip ruptures. Computer modeling of strong ground motion from strike-slip earthquakes has involved somewhat arbitrary assumptions about the nature of slip along the shallow part of the fault (e.g., fixing the slip to be zero along the upper 2 kilometers of the fault plane) in order to match certain strong motion accelerograms. Most modeling studies of earthquake strong ground motion have used what is termed kinematic dislocation modeling. In kinematic modeling the time function for slip on the fault is prescribed, and the response of the layered medium is calculated. Unfortunately, there is no guarantee that the model and the prescribed slip are physically reasonable unless the true nature of the medium and its motions are known ahead of time. There is good reason to believe that in many cases faults are weak along the upper few kilometers of the fault zone and may not be able to maintain high levels of shear strain required for high dynamic energy release during earthquakes. Physical models of faulting, as distinct from numerical or mathematical models, are guaranteed to obey static and dynamic mechanical laws. Foam-rubber modeling studies have been reported in a number of publications. The object of this paper is to present results of physical modeling using a shallow weak layer, in order to verify the physical basis for assuming a long rise time and a reduced high frequency pulse for the slip on the shallow part of faults. It appears a 2-kilometer deep, weak zone along strike-slip faults could indeed reduce the high frequency energy radiated from shallow slip, and that this effect can best be represented by superimposing a small amplitude, short rise-time pulse at the onset of a much longer ...
Date: February 23, 1998
Creator: Brune, James N. & Anooshehpoor, Abdolrasool
Partner: UNT Libraries Government Documents Department

Structure of the Espanola Basin, Rio Grande Rift, New Mexico, from SAGE seismic and gravity data

Description: Seismic and gravity data, acquired by the SAGE program over the past twelve years, are used to define the geometry of the Espanola basin and the extent of pre-Tertiary sedimentary rocks. The Paleozoic and Mesozoic units have been thinned and removed during Laramide uplift in an area now obscured by the younger rift basin. The Espanola basin is generally a shallow, asymmetric transitional structure between deeper, better developed basins to the northeast and southwest. The gravity data indicate the presence of three narrow, but deep, structural lows arrayed along the Embudo/Pajarito fault system. These sub-basins seem to be younger than the faults on the basin margins. This apparent focussing of deformation in the later history of the basin may be a response to changes in regional stress or more local accommodation of the rift extension. Future work is planned to develop seismic data over one of these sub-basins, the Velarde graben, and to better define the gravity map in order to facilitate three-dimensional modeling.
Date: April 1, 1995
Creator: Ferguson, J.F.; Baldridge, W.S.; Braile, L.W.; Biehler, S.; Gilpin, B. & Jiracek, G.R.
Partner: UNT Libraries Government Documents Department

Broad belts of shear zones: The common form of surface rupture produced by the 28 June 1992 Landers, California, earthquake

Description: Surface rupturing during the 28 June 1992, Landers, California earthquake, east of Los Angeles, accommodated right-lateral offsets up to about 6 m along segments of distinct, en echelon fault zones with a total length of about 80 km. The offsets were accommodated generally not by faults -- distinct slip surfaces -- but rather by shear zones, tabular bands of localized shearing. In long, straight stretches of fault zones at Landers the rupture is characterized by telescoping of shear zones and intensification of shearing: broad shear zones of mild shearing, containing narrow shear zones of more intense shearing, containing even-narrower shear zones of very intense shearing, which may contain a fault. Thus the ground ruptured across broad belts of shearing with subparallel walls, oriented NW. Each broad belt consists of a broad zone of mild shearing, extending across its entire width (50 to 200 m), and much narrower (a few m wide) shear zones that accommodate most of the offset of the belt and are portrayed by en echelon tension cracks. In response to right-lateral shearing, the slices of ground bounded by the tension cracks rotated in a clockwise sense, producing left lateral shearing, and the slices were forced against the walls of the shear zone, producing thrusting. Even narrower shear zones formed within the narrow shear zones, and some of these were faults. Although the narrower shear zones probably are indicators to right-lateral fault segments at depth, the surface rupturing during the earthquake is characterized not by faulting, but by zones of shearing at various scales. Furthermore, understanding of the formation of the shear zones may be critical to understanding of earthquake faulting because, where faulting is associated with the formation of a shear zone, the faulting occurs late in the development of the shear zone. The faulting occurs after ...
Date: December 31, 1993
Creator: Johnson, A.M.; Cruikshank, K.M. & Fleming, R.W.
Partner: UNT Libraries Government Documents Department

Gravity and magnetic data across the Ghost Dance Fault in WT-2 Wash, Yucca Mountain, Nevada

Description: Detailed gravity and ground magnetic data were obtained in September 1993 along a 4,650 ft-long profile across the Ghost Dance Fault system in WT-2 Wash. Gravity stations were established every 150 feet along the profile. Total-field magnetic measurements made initially every 50 ft along the profile, then remade every 20 ft through the fault zone. These new data are part of a geologic and geophysical study of the Ghost Dance Fault (GDF) which includes detailed geologic mapping, seismic reflection, and some drilling including geologic and geophysical logging. The Ghost Dance Fault is the only through-going fault that has been identified within the potential repository for high-level radioactive waste at Yucca Mountain, Nevada. Preliminary gravity results show a distinct decrease of 0.1 to 0.2 mGal over a 600-ft-wide zone to the east of and including the mapped fault. The gravity decrease probably marks a zone of brecciation. Another fault-offset located about 2,000 ft to the east of the GDF was detected by seismic reflection data and is also marked by a distinct gravity low. The ground magnetic data show a 200-ft-wide magnetic low of about 400 nT centered about 100 ft east of the Ghost Dance Fault. The magnetic low probably marks a zone of brecciation within the normally polarized Topopah Spring Tuff, the top of which is about 170 ft below the surface, and which is known from drilling to extend to a depth of about 1,700 ft. Three-component magnetometer logging in drill hole WT-2 located about 2,700 ft east of the Ghost Dance Fault shows that the Topopah Spring Tuff is strongly polarized magnetically in this area, so that fault brecciation of a vertical zone within the Tuff could provide an average negative magnetic contrast of the 4 Am{sup {minus}1} needed to produce the 400 nT low observed at ...
Date: December 31, 1994
Creator: Oliver, H.W. & Sikora, R.F.
Partner: UNT Libraries Government Documents Department

Ground fracturing at the southern end of Summit Ridge caused by October 17, 1989 Loma Prieta, California, earthquake sequence (maps of Summit Ridge Shear Zones, en echelon tension cracks, complex and compound fractures, and small faults that formed coactively with the earthquake sequence)

Description: The Loma Prieta earthquake of 17 October 1989 was the first of three large earthquakes that occurred in California in less than 5 years. The main shock of the Loma Prieta earthquake was deep-seated, the rupture zones of the main shock did not reach the surface, and the earthquake produced enigmatic surface ruptures along the frontal faults of the Coast Range and in the epicentral area that were explained in several quite different ways. The Landers earthquake of 28 June 1992 was near surface and produced more than 80 km of spectacular surface rupture of many different kinematic expressions. Detailed study of fractures at Landers has provided a basis for re-evaluating earlier work on fractures produced by the Loma Prieta earthquake. This paper is a description of some of the fractures produced by the Loma Prieta earthquake and a discussion of their causes. Detailed mapping (scale of 1:250) in an area on either side of Summit Road and between Morrell Cutoff Road in the northwest and the intersection of Summit Road and San Jose-Soquel Road in the southeast has provided documentation of fracture orientations and differential displacements required to decipher the ground deformation in that area during the Loma Prieta earthquake.
Date: December 31, 1997
Creator: Martosudarmo, S.Y.; Johnson, A.M. & Fleming, R.W.
Partner: UNT Libraries Government Documents Department

Geologic map of the Paintbrush Canyon Area, Yucca Mountain, Nevada

Description: This geologic map is produced to support site characterization studies of Yucca Mountain, Nevada, site of a potential nuclear waste storage facility. The area encompassed by this map lies between Yucca Wash and Fortymile Canyon, northeast of Yucca Mountain. It is on the southern flank of the Timber Mountain caldera complex within the southwest Nevada volcanic field. Miocene tuffs and lavas of the Calico Hills Formation, the Paintbrush Group, and the Timber Mountain Group crop out in the area of this map. The source vents of the tuff cones and lava domes commonly are located beneath the thickest deposits of pyroclastic ejecta and lava flows. The rocks within the mapped area have been deformed by north- and northwest-striking, dominantly west-dipping normal faults and a few east-dipping normal faults. Faults commonly are characterized by well developed fault scarps, thick breccia zones, and hanging-wall grabens. Latest movement as preserved by slickensides on west-dipping fault scarps is oblique down towards the southwest. Two of these faults, the Paintbrush Canyon fault and the Bow Ridge fault, are major block-bounding faults here and to the south at Yucca Mountain. Offset of stratigraphic units across faults indicates that faulting occurred throughout the time these volcanic units were deposited.
Date: November 1, 1998
Creator: Dickerson, R.P. & Drake, R.M. II
Partner: UNT Libraries Government Documents Department

Petrochemical variation of Topopah Spring tuff matrix with depth (stratigraphic level), drill hole USW G-4, Yucca Mountain, Nevada

Description: This study describes and interprets petrochemical variation of the matrix (excluding fractures and large gas cavities) of the Topopah Spring Member of the Paintbrush Tuff. This tuff includes the candidate host rock for a high-level nuclear waste repository at Yucca Mountain on the Nevada Test Site. Cored hole USW G-4, near the site of a potential exploratory shaft at Yucca Mountain, penetrated 359.4 m (1179 ft) of the member within the unsaturated zone. This study shows that petrographic textures and chemistry of the matrix vary systematically within recognizable lithologic subunits related to crystallization (cooling) zones, welding (compaction) zones, and compositional zones (rhyolite versus quartz latite). The methods used for this study include petrographic modal thin section analysis using an automated counter and electron microprobe analysis of the groundmass. Distinctive textural categories are defined, and they can be ranked from finest to coarsest as vitrophyre (glass), cryptocrystalline groundmass, spherulites, granophyre, lithic fragments, and phenocrysts. The two main groundmass compositions are also defined: rhyolite high silica) and quartz latite. The value of these petrochemical studies lies in providing microscopic criteria for recognizing the zonal subunits where they may have greatly limited exposure, as in mined drifts and in core from horizontal drill holes. For example, the lower nonlithophysal zone can be distinguished microscopically from the middle nonlithophysal zone by (1) degree of compaction, (2) amount of quartz, and (3) amount of lithic fragments. The variability between these textural categories should also be considered in designing physical and chemical tests of the Topopah Spring.
Date: December 1, 1985
Creator: Byers, F.M. Jr.
Partner: UNT Libraries Government Documents Department

Scientific drilling into the San Andreas fault and site characterization research: Planning and coordination efforts. Final technical report

Description: The fundamental scientific issue addressed in this proposal, obtaining an improved understanding of the physical and chemical processes responsible for earthquakes along major fault zones, is clearly of global scientific interest. By sampling the San Andreas fault zone and making direct measurements of fault zone properties to 4.0 km at Parkfield they will be studying an active plate-boundary fault at a depth where aseismic creep and small earthquakes occur and where a number of the scientific questions associated with deeper fault zone drilling can begin to be addressed. Also, the technological challenges associated with drilling, coring, downhole measurements and borehole instrumentation that may eventually have to be faced in deeper drilling can first be addressed at moderate depth and temperature in the Parkfield hole. Throughout the planning process leading to the development of this proposal they have invited participation by scientists from around the world. As a result, the workshops and meetings they have held for this project have involved about 350 scientists and engineers from about a dozen countries.
Date: August 30, 1998
Creator: Zoback, M. D.
Partner: UNT Libraries Government Documents Department

Reexamination of fault angles predicted by shear localization theory

Description: This paper reexamines orientations of shear bands (fault angles) predicted by a theory of shear localization as a bifurcation from homogeneous deformation. In contrast to the Coulomb prediction, which does not depend on deviatoric stress state, the angle between the band normal and the least (most compressive) principal stress increases as the deviatoric stress state varies from axisymmetric compression to axisymmetric extension. This variation is consistent with the data of Mogi (1967) on Dunham dolomite for axisymmetric compression, extension and biaxial compression, but the predicted angles are generally less than observed. This discrepancy may be due to anisotropy that develops due to crack growth in preferred orientations. Results from specialized constitutive relations for axisymmetric compression and plane strain that include this anisotropy indicate that it tends to increase the predicted angles. Measurements for a weak, porous sandstone (Castlegate) indicate that the band angle decreases with increasing inelastic compaction that accompanies increasing mean stress. This trend is consistent with the predictions of the theory but, for this rock, the observed angles are less than predicted.
Date: February 1, 1998
Creator: Rudnicki, J.W. & Olsson, W.A.
Partner: UNT Libraries Government Documents Department

Stresses and fractures in the Frontier Formation, Green River Basin, predicted from basin-margin tectonic element interactions

Description: Natural fractures and in situ stresses commonly dictate subsurface reservoir permeability and permeability anisotropy, as well as the effectiveness of stimulation techniques in low-permeability, natural gas reservoirs. This paper offers an initial prediction for the orientations of the fracture and stress systems in the tight gas reservoirs of the Frontier Formation, in the Green River basin of southwestern Wyoming. It builds on a previous report that addressed fractures and stresses in the western part of the basin and on ideas developed for the rest of the basin, using the principle that thrust faults are capable of affecting the stress magnitudes and orientations in little-deformed strata several hundreds of kilometers in front of a thrust. The prediction of subsurface stresses and natural fracture orientations is an undertaking that requires the willingness to revise models as definitive data are acquired during drilling. The predictions made in this paper are offered with the caveat that geology in the subsurface is always full of surprises.
Date: January 1, 1996
Creator: Lorenz, J.C.
Partner: UNT Libraries Government Documents Department

The dominant processes responsible for subsidence of coastal wetlands in south Louisiana

Description: Wetland loss in coastal areas of Terrebonne and Lafourche Parishes, Louisiana, largely results from two subsurface processes: (1) consolidation of recently deposited Holocene deltaic sediments and (2) active growth faulting. Locally, settlement is high where the thickness of valley fill is great and in broad interdistributary basins where the thickness of consolidation-prone, peaty soils is great. The delta cycle is identified as the fundamental sedimentologic unit that constitutes the lower delta plain. Peaty soils from the waning phase of the delta cycle are identified as the deltaic facies most subject to consolidation settlement. Data indicate direct relationships between the thickness of deltaic sediments in individual delta cycles, and the thickness of peaty soils capping these cycles, with present patterns of coastal tract land loss. In addition, active growth faulting is correlated with new areas of interior tract wetland loss. Consolidation and faulting largely explain the curious nature of wetland loss patterns in south Louisiana. Subsidence in The Netherlands has been attributed to similar causes, i.e. thick deposits of consolidation-prone sediments that accumulate on the downthrown sides of basin margin faults.
Date: December 31, 1995
Creator: Kuecher, G.J.
Partner: UNT Libraries Government Documents Department

Characterization and application of microearthquake clusters to problems of scaling, fault zone dynamics, and seismic monitoring at Parkfield, California

Description: This document contains information about the characterization and application of microearthquake clusters and fault zone dynamics. Topics discussed include: Seismological studies; fault-zone dynamics; periodic recurrence; scaling of microearthquakes to large earthquakes; implications of fault mechanics and seismic hazards; and wave propagation and temporal changes.
Date: October 1, 1995
Creator: Nadeau, R. M.
Partner: UNT Libraries Government Documents Department

Characterization of Quaternary and suspected Quaternary faults, regional studies, Nevada and California

Description: This report presents the results of geologic studies that help define the Quaternary history of selected faults in the region around Yucca Mountain, Nevada. These results are relevant to the seismic-design basis of a potential nuclear waste repository at Yucca Mountain. The relevancy is based, in part, on a need for additional geologic data that became apparent in ongoing studies that resulted in the identification of 51 relevant and potentially relevant individual and compound faults and fault zones in the 100-km-radius region around the Yucca Mountain site. Geologic data used to characterize the regional faults and fault zones as relevant or potentially relevant seismic sources includes age and displacement information, maximum fault lengths, and minimum distances between the fault and the Yucca Mountain site. For many of the regional faults, no paleoseismic field studies have previously been conducted, and age and displacement data are sparse to nonexistent. In November 1994, the Branch of Earthquake and Landslide Hazards entered into two Memoranda of Agreement with the Yucca Mountain Project Branch to conduct field reconnaissance, analysis, and interpretation of six relevant and six potentially relevant regional faults. This report describes the results of study of those faults exclusive of those in the Pahrump-Stewart Valley-Ash Meadows-Amargosa Valley areas. We also include results of a cursory study of faults on the west flank of the Specter Range and in the northern part of the Last Chance Range. A four-phase strategy was implemented for the field study.
Date: December 31, 1995
Creator: Anderson, R.E.; Bucknam, R.C.; Crone, A.J.; Haller, K.M.; Machette, M.N.; Personius, S.F. et al.
Partner: UNT Libraries Government Documents Department

Isostatic uplift, crustal attenuation, and the evolution of an extensional detachment system in southwestern Nevada

Description: Geological and geophysical evidence supports the existence of extensional detachments, between the Sheep Range and Death Valley. It is proposed that geographically separated pieces of detachments between Death Valley and the Sheep Range are parts of a regional detachment system that has evolved since the Miocene, and that the system consists of lenses of strata separated by an anastomosing network of low- and high-angle normal faults. This manuscript emphasizes the probability that isostatic uplift within the region of greatest crustal attenuation in this system, the Bullfrog Hills core complex, controlled the evolution of the detachment system between the breakaway zone a the Sheep Range and the core complex. Features in this system are described from east to west, which is the apparent direction of tectonic transport.
Date: December 31, 1987
Creator: Scott, R.B.
Partner: UNT Libraries Government Documents Department

Joint orientation and characteristics as observed in a trench excavated near TA-3 and a basement excavated at TA-55

Description: Walls of excavations in the Bandelier Tuff for pipelines and foundations for structures provide excellent areas to determine the orientation (strike and dip) and characteristics of the joints (frequency, width, and type of material filling the joint). Joints or fractures are commonly associated with structural adjustments such as faulting; however, joints formed in the tuff mainly result from the shrinkage of the ash-flow tuff as it cools. The presence of faults can restrict the siting of buildings or structures. In waste disposal operations, open joints can be pathways for the transport of contaminants.
Date: October 1, 1995
Creator: Purtymun, W.D.; Koenig, E.; Morgan, T. & Sagon, E.
Partner: UNT Libraries Government Documents Department