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Seismic Event Location Using Levenberg-Marquardt Least Squares Inversion

Description: The most widely used algorithm for estimating seismic event hypocenters and origin times is iterative linear least squares inversion. In this paper we review the mathematical basis of the algorithm and discuss the major assumptions made during its derivation. We go on to explore the utility of using Levenberg-Marquardt damping to improve the performance of the algorithm in cases where some of these assumptions are violated. We also describe how location parameter uncertainties are calculated. A technique to estimate an initial seismic event location is described in an appendix.
Date: October 2002
Creator: Ballard, Sanford
Partner: UNT Libraries Government Documents Department

Evidence for back scattering of near-podal seismic P'P' waves from the 150-220 km zone in Earth's upper mantle

Description: The deepest and most inaccessible parts of Earth's interior--the core and core-mantle boundary regions can be studied from compressional waves that turn in the core and are routinely observed following large earthquakes at epicentral distances between 145{sup o} and 180{sup o} (also called P', PKIKP or PKP waves). P'P' (PKPPKP) are P' waves that travel from a hypocenter through the Earth's core, reflect from the free surface and travel back through the core to a recording station on the surface. P'P' waves are sometimes accompanied by precursors, which were reported first in the 1960s as small-amplitude arrivals on seismograms at epicentral distances of about 50{sup o}-70{sup o}. Most prominent of these observed precursors were explained by P'P' waves generated by earthquakes or explosions that did not reach the Earth's surface but were reflected from the underside of first order velocity discontinuities at 410 and 660 km in the upper mantle mantle. Here we report the discovery of hitherto unobserved near-podal P'P' waves (at epicentral distance less than 10{sup o}) and very prominent precursors preceding the main energy by as much as 55 seconds. We interpret these precursors as a back scattered energy from undocumented structure in the upper mantle, in a zone between 150 and 220 km depth beneath Earth's surface. From these observations, we identify a frequency dependence of Q (attenuation quality factor) in the lithosphere that can be modeled by a flat relaxation spectrum below about 0.05-0.1 Hz and increasing with as the first power of frequency above this value, confirming pioneering work by B. Gutenberg.
Date: July 15, 2005
Creator: Tkalcic, H; Flanagan, M P & Cormier, V F
Partner: UNT Libraries Government Documents Department

FY05 LDRD Final Report The Innermost Inner Core: Fact or Artifact?

Description: P'P' (PKPPKP) are P waves that travel from a hypocenter through the Earth's core, reflect from the free surface and travel back through the core to a recording station on the surface. Here we report the observations of hitherto unobserved near-podal P'P' waves (at epicentral distance < 10{sup o}) and very prominent precursors preceding the main energy by as much as 60 s. We interpret these precursors as a back-scattered energy from previously undocumented horizontally connected small-scale heterogeneity in the upper mantle beneath the oceans in a zone between 150 and 220 km depth beneath the Earth's surface. From these observations, we identify a frequency dependence of attenuation quality factor Q in the lithosphere through forward modeling of the observed amplitude spectra of the main and back-scattered P'P' waves. In addition, we did not find that travel times corresponding to very polar paths through the centermost inner core with respect to the rotation axis of Earth are anomalously advanced, which argues for isotropic or at best --weakly-anisotropic center of Earth in the direction parallel with the rotation axis. More systematic sampling near Earth's center and characterization of anisotropy in Earth's center will be a subject of future research efforts.
Date: January 9, 2006
Creator: Tkalcic, H; Flanagan, M P & Mogri, H
Partner: UNT Libraries Government Documents Department

Long-term seismic monitoring of the Roosevelt - Cove Fort KGRA's

Description: Earthquake monitoring of the Roosevelt-Cove Fort Hot Springs KGRA's was implemented by installation of three RF telemetered, vertical component, seismograph stations: CFU, MNU and RHU. These station sites were selected on the basis of proximity to the KGRA's, with respect to known earthquake activity determined in the microearthquake surveys. The three permanent stations form the basic long-term monitoring capability of the Roosevelt-Cove Fort KGRA's. The signals are FM transmitted to a collecting site near Milford then they are telephone-transmitted to the University of Utah campus for recording. The limitations of only three-stations precludes accurate hypocenter determinations but allows detection to a minimum threshold of about M-0.5 for close-in events. Locations can be determined for earthquakes of about M-0.7 or greater. Regional coverage of the south-central Utah KGRA's is supplemented by the use of other existing University of Utah stations to the east: MSU, PUU, and RFU. Together the six stations allow long-term detection of this geothermally active region.
Date: December 1, 1977
Creator: Smith, Robert B.
Partner: UNT Libraries Government Documents Department

Correlation Processing Of Local Seismic Data: Applications for Autonomous Sensor Deployments

Description: Excavation and operation of an underground facility is likely to produce an extensive suite of seismic signals observable at the surface for perhaps several km. Probably a large fraction of such signals will be correlated, so the design of a monitoring framework should include consideration of a correlation processing capability. Correlation detectors have been shown to be significantly more sensitive than beam-forming power detectors. Although correlation detectors have a limited detection footprint, they can be generalized into multi-rank subspace detectors which are sensitive over a much larger range of source mechanisms and positions. Production of subspace detectors can be automated, so their use in an autonomous framework may be contemplated. Waveform correlation also can be used to produce very high precision phase picks which may be jointly inverted to simultaneously relocate groups of events. The relative precision of the resulting hypocenters is sufficient to visualize structural detail at a scale of less than a few tens of meters. Three possible correlation processor systems are presented. All use a subspace signal detection framework. The simplest system uses a single-component sensor and is capable of detection and classification of signals. The most complicated system uses many sensors deployed around the facility, and is capable of detection, classification, and high-precision source location. Data from a deep underground mine are presented to demonstrate the applicability of correlation processing to monitoring an underground facility. Although the source region covers an area of about 600m by 580m, all but two of the events form clusters at a threshold of 0.7. All the events could have been detected and classified by the subspace detection framework, and high-precision picks can be computed for all cluster members.
Date: November 16, 2010
Creator: Dodge, D A
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

Investigation of rifting processes in the Rio Grande Rift using data from an unusually large earthquake swarm. Final report, October 1, 1992--September 30, 1993

Description: 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 elucidate the active tectonic processes within continental rifts. Beginning on 29 November 1989, a 15 square km region near Bernardo, NM, produced the strongest and longest lasting sequence of earthquakes in the rift in 54 years. Our research focuses on the Bernardo swarm which occurred 40 km north of Socorro, New Mexico in the axial region of the central Rio Grande rift. Important characteristics concerning hypocenters, fault mechanisms, and seismogenic zones are discussed.
Date: March 1, 1995
Creator: Sanford, A.; Balch, R.; Hartse, H. & House, L.
Partner: UNT Libraries Government Documents Department

2001 - 2002 Upper Three Runs Sequence of Earthquakes at the SRS, South Carolina

Description: On October 08, 2001 a small felt earthquake occurred near Upper Three Runs Creek in the north central area of the Savannah River Site, South Carolina. Seven very small aftershocks followed the main event with the last one occurring March 06, 2002. All activity occurred within a small area. Further analysis of collected data indicates a correlation of this low level seismic activity with a small northwest trending structure observed in detailed gravity and magnetic data. Both single event and composite focal mechanisms were derived using local and regional stations. Results indicated predominantly dip-slip motion along a fault striking NNW at 335 degrees and dipping 41 degrees to the southwest. A 3D plot of the eight hypocenters clearly defines a fault plane nearly analogous to that obtained from the focal solutions. The Upper Three Runs series of events is another example of a separate class of earthquakes that occur within the central Piedmont and upper Coastal Plain of South Carolina. The Upper Three Runs sequence of events demonstrates that shallow intersections of structures interpreted from potential field data can be the foci for localized stress concentrations where microearthquake activity can occur. These earthquakes are attributable to small scale faults associated with pockets of relatively high stress concentrations and are generally accompanied by loud noises. Their shallow depth and small aerial extent suggest that these earthquakes are extremely localized and are not attributable to any large scale regional features.
Date: October 16, 2003
Creator: Stevenson, D.A.
Partner: UNT Libraries Government Documents Department


Description: The northern Tien Shan of Central Asia is an area of active mid-continent deformation. Although far from a plate boundary, this region has experienced 5 earthquakes larger than magnitude 7 in the past century and includes one event that may as be as large as Mw 8.0. Previous studies based on GPS measurements indicate on the order of 23 mm/yr of shortening across the entire Tien Shan and up to 15 mm/year in the northern Tien Shan (Figure 1). The seismic moment release rate appears comparable with the geodetic measured slip, at least to first order, suggesting that geodetic rates can be considered a proxy for accumulation rates of stress for seismic hazard estimation. Interferometric synthetic aperture radar may provide a means to make detailed spatial measurements and hence in identifying block boundaries and assisting in seismic hazard. Therefore, we hoped to define block boundaries by direct measurement and by identifying and resolving earthquake slip. Due to political instability in Kyrgzystan, the existing seismic network has not performed as well as required to precisely determine earthquake hypocenters in remote areas and hence InSAR is highly useful. In this paper we present the result of three earthquake studies and show that InSAR is useful for refining locations of teleseismically located earthquakes. ALOS PALSAR data is used to investigate crustal motion in the Tien Shan mountains of Central Asia. As part of the work, considerable software development was undertaken to process PALSAR data. This software has been made freely available. Two damaging earthquakes have been imaged in the Tien Shan and the locations provided by ALOS InSAR have helped to refine seismological velocity models. A third earthquake south of Kyrgyzstan was also imaged. The use of InSAR data and especially L band is therefore very useful in providing groundtruth for earthquake locations.
Date: February 25, 2011
Creator: Mellors, R J
Partner: UNT Libraries Government Documents Department

Double Difference Earthquake Locations at the Salton Sea Geothermal Reservoir

Description: The purpose of this paper is to report on processing of raw waveform data from 4547 events recorded at 12 stations between 2001 and 2005 by the Salton Sea Geothermal Field (SSGF) seismic network. We identified a central region of the network where vertically elongated distributions of hypocenters have previously been located from regional network analysis. We process the data from the local network by first autopicking first P and S arrivals; second, improving these with hand picks when necessary; then, using cross-correlation to provide very precise P and S relative arrival times. We used the HypoDD earthquake location algorithm to locate the events. We found that the originally elongated distributions of hypocenters became more tightly clustered and extend down the extent of the study volume at 10 Km. However, we found the shapes to depend on choices of location parameters. We speculate that these narrow elongated zones of seismicity may be due to stress release caused by fluid flow.
Date: August 8, 2007
Creator: Boyle, K L; Hutchings, L J; Bonner, B P; Foxall, W & Kasameyer, P W
Partner: UNT Libraries Government Documents Department

Earthquake Swarm Activity Beneath the Tokaanu-Waihi Geothermal System, Lake Taupo, New Zealand

Description: The hypocenters of 4 earthquake swarms (total of 54 events), recorded with a local network between 1986 April and 1987 January, occur within upper crustal rocks of the deeper Tokaanu-Waihi geothermal reservoir; all the events had a magnitude M{sub L} {le} 3.2. Most foci are aligned along two NW-trending basement fault structures along which young rhyodacitic extrusions can be found. The swarm activity has been interpreted in terms of injections into basement fractures of magma from deeper chambers (dyke injection swarm activity).
Date: January 1, 1995
Creator: Hochstein, M. P.; Sherburn, S. & Tikku, J.
Partner: UNT Libraries Government Documents Department

Using ancillary information to improve hypocenter estimation: Bayesian single event location (BSEL)

Description: We have developed and tested an algorithm, Bayesian Single Event Location (BSEL), for estimating the location of a seismic event. The main driver for our research is the inadequate representation of ancillary information in the hypocenter estimation procedure. The added benefit is that we have also addressed instability issues often encountered with historical NLR solvers (e.g., non-convergence or seismically infeasible results). BSEL differs from established nonlinear regression techniques by using a Bayesian prior probability density function (prior PDF) to incorporate ancillary physical basis constraints about event location. P-wave arrival times from seismic events are used in the development. Depth, a focus of this paper, may be modeled with a prior PDF (potentially skewed) that captures physical basis bounds from surface wave observations. This PDF is constructed from a Rayleigh wave depth excitation eigenfunction that is based on the observed minimum period from a spectrogram analysis and estimated near-source elastic parameters. For example, if the surface wave is an Rg phase, it potentially provides a strong constraint for depth, which has important implications for remote monitoring of nuclear explosions. The proposed Bayesian algorithm is illustrated with events that demonstrate its congruity with established hypocenter estimation methods and its application potential. The BSEL method is applied to three events: (1) A shallow Mw 4 earthquake that occurred near Bardwell, KY on June 6, 2003, (2) the Mw 5.6 earthquake of July 26, 2005 that occurred near Dillon, MT, and (3) a deep Mw 5.7 earthquake that occurred off the coast of Japan on April 22, 1980. A strong Rg was observed from the Bardwell, KY earthquake that places very strong constraints on depth and origin time. No Rg was observed for the Dillon, MT earthquake, but we used the minimum observed period of a Rayleigh wave (7 seconds) to reduce the depth ...
Date: January 1, 2008
Creator: Anderson, Dale N
Partner: UNT Libraries Government Documents Department

Microearthquake monitoring at the Southeast Geysers using a high-resolution digital array

Description: Microearthquake activity at the Southeast Geysers, California, geothermal field is monitored with a high-resolution digital seismic network. Hypocenters are spatially clustered in both injection and production areas, but also occur in more diffuse patterns, mostly at depths from 1 to 2.8 km. Hypocenters near the injection well DV-11 exhibit a striking correlation with movement of injectate and injectate-derived steam. Preliminary moment tensor results show promise to provide information on the differing source mechanisms resulting from fluid injection and steam extraction.
Date: January 26, 1995
Creator: Kirkpatrick, Ann; Peterson, John E., Jr. & Majer, Ernie L.
Partner: UNT Libraries Government Documents Department

Microearthquake Study of the Salton Sea Geothermal Field, California: Evidence of Stress Triggering - Masters Thesis

Description: A digital network of 24 seismograph stations was operated from September 15, 1987 to September 30, 1988, by Lawrence Livermore National Laboratory and Unocal as part of the Salton Sea Scientific Drilling Project to study seismicity related to tectonics and geothermal activity near the drilling site. More than 2001 microearthquakes were relocated in this study in order to image any pervasive structures that may exist within the Salton Sea geothermal field. First, detailed velocity models were obtained through standard 1-D inversion techniques. These velocity models were then used to relocate events using both single event methods and Double-Differencing, a joint hypocenter location method. An anisotropic velocity model was built from anisotropy estimates obtained from well logs within the study area. During the study period, the Superstition wills sequence occurred with two moderate earthquakes of MS 6.2 and MS 6.6. These moderate earthquakes caused a rotation of the stress field as observed from the inversion of first motion data from microearthquakes at the Salton Sea geothermal field. Coulomb failure analysis also indicates that microearthquakes occurring after the Superstition Hills sequence are located within a region of stress increase suggesting stress triggering caused by the moderate earthquakes.
Date: February 1, 2002
Creator: Holland, Austin Adams
Partner: UNT Libraries Government Documents Department

Ground motion modeling of Hayward fault scenario earthquakes II:Simulation of long-period and broadband ground motions

Description: We simulate long-period (T > 1.0-2.0 s) and broadband (T > 0.1 s) ground motions for 39 scenarios earthquakes (Mw 6.7-7.2) involving the Hayward, Calaveras, and Rodgers Creek faults. For rupture on the Hayward fault we consider the effects of creep on coseismic slip using two different approaches, both of which reduce the ground motions compared with neglecting the influence of creep. Nevertheless, the scenario earthquakes generate strong shaking throughout the San Francisco Bay area with about 50% of the urban area experiencing MMI VII or greater for the magnitude 7.0 scenario events. Long-period simulations of the 2007 Mw 4.18 Oakland and 2007 Mw 4.5 Alum Rock earthquakes show that the USGS Bay Area Velocity Model version 08.3.0 permits simulation of the amplitude and duration of shaking throughout the San Francisco Bay area, with the greatest accuracy in the Santa Clara Valley (San Jose area). The ground motions exhibit a strong sensitivity to the rupture length (or magnitude), hypocenter (or rupture directivity), and slip distribution. The ground motions display a much weaker sensitivity to the rise time and rupture speed. Peak velocities, peak accelerations, and spectral accelerations from the synthetic broadband ground motions are, on average, slightly higher than the Next Generation Attenuation (NGA) ground-motion prediction equations. We attribute at least some of this difference to the relatively narrow width of the Hayward fault ruptures. The simulations suggest that the Spudich and Chiou (2008) directivity corrections to the NGA relations could be improved by including a dependence on the rupture speed and increasing the areal extent of rupture directivity with period. The simulations also indicate that the NGA relations may under-predict amplification in shallow sedimentary basins.
Date: November 4, 2009
Creator: Aagaard, B T; Graves, R W; Rodgers, A; Brocher, T M; Simpson, R W; Dreger, D et al.
Partner: UNT Libraries Government Documents Department

Integrated modeling and field study of potential mechanisms forinduced seismicity at The Geysers Goethermal Field, California

Description: In this paper, we present progress made in a study aimed atincreasing the understanding of the relative contributions of differentmechanisms that may be causing the seismicity occurring at The Geysersgeothermal field, California. The approach we take is to integrate: (1)coupled reservoir geomechanical numerical modeling, (2) data fromrecently upgraded and expanded NCPA/Calpine/LBNL seismic arrays, and (3)tens of years of archival InSAR data from monthly satellite passes. Wehave conducted a coupled reservoir geomechanical analysis to studypotential mechanisms induced by steam production. Our simulation resultscorroborate co-locations of hypocenter field observations of inducedseismicity and their correlation with steam production as reported in theliterature. Seismic and InSAR data are being collected and processed foruse in constraining the coupled reservoir geomechanicalmodel.
Date: June 7, 2006
Creator: Rutqvist, Jonny; Majer, Ernie; Oldenburg, Curt; Peterson, John & Vasco, Don
Partner: UNT Libraries Government Documents Department

Accuracy of teleseismic event locations in the Middle East and North Africa

Description: Seismic characterization at the regional level requires accurate determination of phases and travel times for many combinations of stations and events. An important consideration in the process is the accuracy of event locations. The LLNL Comprehensive Test Ban Treaty Research Program is currently working on data from the Middle East and North Africa, where seismic station coverage is relatively sparse and ``ground truth`` seismic source information is practically nonexistent. In this report the investigator use after shock studies as a source of local ground truth. He evaluates teleseismic location accuracy by comparing hypocenters determined by local networks with those determined teleseismically [e.g. the International Seismological Center (ISC) and the National Earthquake Information Center (NEIC)]. Epicentral locations, origin times, and depth determinations of events from three aftershocks studies (Algeria, Armenia, and Iran) and one local network study (Iran) are compared with ISC and NEIC locations for the same events. The key parameter for the ISC locations is the number of observations used in the location determination. For more than 40-50 observations, the agreement rapidly diminishes and ISC locations can differ from local determinations by as much as 80 km or more. Events in Iran show a distinct bias of ISC location errors toward the northeast; events in Armenia and Algeria show no directional bias. This study shows that only events with ISC M{sub b} {gt} 4.4-4.5 or NEIS M{sub b} {gt} 4.7-4. should be used for compiling travel time information from teleseismic bulletins in the Middle East/North Africa region when locations from the NEIC and ISC bulletins are used.
Date: December 4, 1996
Creator: Sweeney, J.J.
Partner: UNT Libraries Government Documents Department

Preliminary seismicity and focal mechanisms for the southern Great Basin of Nevada and California: January 1992 through September 1992

Description: The telemetered southern Great Basin seismic network (SGBSN) is operated for the Department of Energy`s Yucca Mountain Project (YMP). The US Geological Survey, Branch of Earthquake and Landslide Hazards, maintained this network until September 30, 1992, at which time all operational and analysis responsibilities were transferred to the University of Nevada at Reno Seismological Laboratory (UNRSL). This report contains preliminary earthquake and chemical explosion hypocenter listings and preliminary earthquake focal mechanism solutions for USGS/SGBSN data for the period January 1, 1992 through September 30, 1992, 15:00 UTC.
Date: June 1, 1994
Creator: Harmsen, S.C.
Partner: UNT Libraries Government Documents Department

Fault zone structure determined through the analysis of earthquake arrival times

Description: This thesis develops and applies a technique for the simultaneous determination of P and S wave velocity models and hypocenters from a set of arrival times. The velocity models are parameterized in terms of cubic B-splines basis functions which permit the retrieval of smooth models that can be used directly for generation of synthetic seismograms using the ray method. In addition, this type of smoothing limits the rise of instabilities related to the poor resolving power of the data. V{sub P}/V{sub S} ratios calculated from P and S models display generally instabilities related to the different ray-coverages of compressional and shear waves. However, V{sub P}/V{sub S} ratios are important for correct identification of rock types and this study introduces a new methodology based on adding some coupling (i.e., proportionality) between P and S models which stabilizes the V{sub P}/V{sub S} models around some average preset value determined from the data. Tests of the technique with synthetic data show that this additional coupling regularizes effectively the resulting models.
Date: October 1, 1991
Creator: Michelini, A.
Partner: UNT Libraries Government Documents Department

Seismic monitoring of EGS tests at the Coso Geothermal area, California, using accurate MEQ locations and full moment tensors

Description: We studied high-resolution relative locations and full moment tensors of microearthquakes (MEQs) occurring before, during and following Enhanced Geothermal Systems (EGS) experiments in two wells at the Coso geothermal area, California. The objective was to map new fractures, determine the mode and sense of failure, and characterize the stress cycle associated with injection. New software developed for this work combines waveform crosscorrelation measurement of arrival times with relative relocation methods, and assesses confidence regions for moment tensors derived using linearprogramming methods. For moment tensor determination we also developed a convenient Graphical User Interface (GUI), to streamline the work. We used data from the U.S. Navy’s permanent network of three-component digital borehole seismometers and from 14 portable three-component digital instruments. The latter supplemented the permanent network during injection experiments in well 34A-9 in 2004 and well 34-9RD2 in 2005. In the experiment in well 34A-9, the co-injection earthquakes were more numerous, smaller, more explosive and had more horizontal motion, compared with the pre-injection earthquakes. In the experiment in well 34-9RD2 the relocated hypocenters reveal a well-defined planar structure, 700 m long and 600 m high in the depth range 0.8 to 1.4 km below sea level, striking N 20° E and dipping at 75° to the WNW. The moment tensors show that it corresponds to a mode I (opening) crack. For both wells, the perturbed stress state near the bottom of the well persisted for at least two months following the injection.
Date: April 1, 2008
Creator: Foulger, G.R.; B.R. Julian, B.R. & Monastero, F.
Partner: UNT Libraries Government Documents Department

Adak Island, Alaska, Microearthquake survey: Preliminary Hypocenter Determinations

Description: Microearthquakes, defined as shocks having magnitudes less than 4, are commonly recorded in the vicinity of geothermal manifestations and volcanism. They have been mapped from producing geothermal fields as well as those not yet developed, in such places as Iceland, El Salvador, Japan, Kenya and the US. Microearthquakes have been recorded at several geothermal sites in the Imperial Valley and Coso Hot Springs, California; Kilbourne Hole, New Mexico; Yellowstone National Park, Wyoming; and The Geysers, California, where there is debate over whether or not the seismicity is induced by steam production. Seismicity occurs around active volcanoes, but appears reduced directly over zones of high temperature or magma, where the depth of the brittle fracture zone is shallow, as over Yellowstone caldera. In areas of active hydrothermalism, regional stress is likely to be relieved by low-level seismicity rather than occasional large ruptures, owing to the high temperatures, presence of fluids, and crustal weakening due to alteration and fracturing. Active faulting maintains the permeability of the system, which in its absence, might otherwise seal. on the microscopic scale, pore-fluid pressures rise as a result of heating, resulting in the decrease of effective pressure at the pore-mineral boundary. When this effective pressure becomes less than the rock's tensile strength, the pore ruptures; and if it intersects a through-going fracture under hydrostatic pressure can result in a shock detectable on seismographs at the surface. Such a mechanism might also account for the swarms of very small events seen in a number of geothermal areas. A microearthquake survey was conducted on Adak Island, Alaska for the purpose of identifying seismicity associated with a possible geothermal reservoir. During 30 days of recording in September and October 1982, 190 seismic events were recorded on two or more stations of a nine-station network. Of the total, 33 were ...
Date: November 5, 1982
Creator: Lange, Arthur L. & Avramenko, Walter
Partner: UNT Libraries Government Documents Department

Spatial analysis of hypocenter to fault relationships for determining fault process zone width in Japan.

Description: Preliminary investigation areas (PIA) for a potential repository of high-level radioactive waste must be evaluated by NUMO with regard to a number of qualifying factors. One of these factors is related to earthquakes and fault activity. This study develops a spatial statistical assessment method that can be applied to the active faults in Japan to perform such screening evaluations. This analysis uses the distribution of seismicity near faults to define the width of the associated process zone. This concept is based on previous observations of aftershock earthquakes clustered near active faults and on the assumption that such seismic activity is indicative of fracturing and associated impacts on bedrock integrity. Preliminary analyses of aggregate data for all of Japan confirmed that the frequency of earthquakes is higher near active faults. Data used in the analysis were obtained from NUMO and consist of three primary sources: (1) active fault attributes compiled in a spreadsheet, (2) earthquake hypocenter data, and (3) active fault locations. Examination of these data revealed several limitations with regard to the ability to associate fault attributes from the spreadsheet to locations of individual fault trace segments. In particular, there was no direct link between attributes of the active faults in the spreadsheet and the active fault locations in the GIS database. In addition, the hypocenter location resolution in the pre-1983 data was less accurate than for later data. These pre-1983 hypocenters were eliminated from further analysis.
Date: September 1, 2004
Creator: Arnold, Bill Walter; Roberts, Barry L.; McKenna, Sean Andrew & Coburn, Timothy C. (Abilene Christian University, Abilene, TX)
Partner: UNT Libraries Government Documents Department

Seismicity and focal mechanisms for the southern Great Basin of Nevada and California: 1987 through 1989

Description: For the calendar year 1987, the southern Great basin seismic network (SGBSN) recorded about 820 earthquakes in the southern Great Basin (SGB). Local magnitudes ranged from 0.2 to 4.2 (December 30, 1987, 22:50:42 UTC at Hot Creek Valley). Five earthquakes epicenters in 1987 within the detection threshold of the seismic network are at Yucca Mountain, the site of a potential national, high-level nuclear waste repository. The maximum magnitude of those five earthquakes is 1.1, and their estimated depths of focus ranged from 3.1 to 7.6 km below sea level. For the calendar year 1988, about 1280 SGB earthquakes were catalogued, with maximum magnitude-4.4 for an Owens Valley, California, earthquake on July 5, 1988. Eight earthquake epicenters in 1988 are at Yucca Mountain, with depths ranging from three to 12 km below sea level, and maximum magnitude 2.1. For the calendar year 1989, about 1190 SGB earthquakes were located and catalogued, with maximum magnitude equal to 3.5 for earthquake about ten miles north of Las Vegas, Nevada, on January 9. No Yucca Mountain earthquakes were recorded in 1989. An earthquake having a well-constrained depth of about 30 km below sea level was observed on August 21, 1989, in eastern Nevada Test Site (NTS).
Date: December 1991
Creator: Harmsen, S. C. & Bufe, C. G.
Partner: UNT Libraries Government Documents Department