Download or read book The Use of Propagation Path Corrections to Improve Regional Seismic Event Location in Western China written by and published by . This book was released on 1999 with total page 11 pages. Available in PDF, EPUB and Kindle. Book excerpt: In an effort to improve the ability to locate seismic events in western China using only regional data, the authors have developed empirical propagation path corrections (PPCs) and applied such corrections using both traditional location routines as well as a nonlinear grid search method. Thus far, the authors have concentrated on corrections to observed P arrival times for shallow events using travel-time observations available from the USGS EDRs, the ISC catalogs, their own travel-tim picks from regional data, and data from other catalogs. They relocate events with the algorithm of Bratt and Bache (1988) from a region encompassing China. For individual stations having sufficient data, they produce a map of the regional travel-time residuals from all well-located teleseismic events. From these maps, interpolated PPC surfaces have been constructed using both surface fitting under tension and modified Bayesian kriging. The latter method offers the advantage of providing well-behaved interpolants, but requires that the authors have adequate error estimates associated with the travel-time residuals. To improve error estimates for kriging and event location, they separate measurement error from modeling error. The modeling error is defined as the travel-time variance of a particular model as a function of distance, while the measurement error is defined as the picking error associated with each phase. They estimate measurement errors for arrivals from the EDRs based on roundoff or truncation, and use signal-to-noise for the travel-time picks from the waveform data set.

Download or read book The Use of Propagation Path Corrections to Improve Seismic Event Location in Western China written by and published by . This book was released on 1998 with total page 47 pages. Available in PDF, EPUB and Kindle. Book excerpt: In an effort to improve ability to locate events in western China using only regional data, the authors have developed propagation path corrections to seismic travel times, and applied such corrections using both traditional location routines as well as a nonlinear grid search method. Thus far, they have concentrated on corrections to observed P arrival times. They have constructed such corrections by using travel time observations available from the USGS Earthquake Data Reports, as well as data reported by the ISC. They have also constructed corrections for six stations that are a part of the International monitoring System. For each station having sufficient data, they produce a map of the travel-time residuals from all located events. Large-amplitude residuals are removed by median filtering, and the resulting data are gridded. For a given source location, the correction at a particular station is then interpolated from the correction grid associated with the station. They have constrained the magnitude of the corrections to be (less-than or equal to) 3 s. They have evaluated the utility of the calculated corrections by applying the corrections to the regional relocation of 10 well-located Chinese nuclear tests, as well as a single, well-located aftershock in nearby Kyrgyzstan. The use of corrections having magnitudes> 2 s is troubling when using traditional location codes, as the corrections amount to a nonlinear perturbation correction, and when large may destabilize the location algorithm. Partly for this reason, the authors have begun using grid search methods to relocate regional events. Such methods are easy to implement and fully nonlinear. Moreover, the misfit function used to locate the event can very easily be changed; they have used L1- and L2-norm misfit functions, for example. Instances in which multiple local minima occur in a location problem are easily recognized by simply contouring or otherwise displaying the misfit function.

Download or read book Monitoring the Comprehensive Nuclear Test Ban Treaty Source Location written by Frode Ringdal and published by Birkhäuser. This book was released on 2013-03-07 with total page 414 pages. Available in PDF, EPUB and Kindle. Book excerpt: In September 1996, the United Nations General Assembly adopted the Comprehensive Nuclear-Test-Ban Treaty (CTBT), prohibiting nuclear explosions worldwide, in all environments. The treaty calls for a global verification system, including a network of 321 monitoring stations distributed around the globe, a data communications network, an international data centre (IDC), and on-site inspections, to verify compliance. This volume contains research papers focusing on seismic ecent location in the CTBT context. The on-site inspection protocol of the treaty specifies a search area not to exceed 1000 square km. Much of the current research effort is therefore directed towards refining the accuracy of event location by including allowances for three-dimensional structure within the Earth. The aim is that the true location of each event will lie within the specified source zone regarding postulated location. The papers in this volume cover many aspects of seismic event location, including the development of algorithms suitable for use with three-dimensional models, allowances for regional structure, use of calibration events and source-specific station corrections. They provide a broad overview of the current international effort to improve seismic event location accuracy, and the editors hope that it will stimulate increased interest and further advances in this important field.

Download or read book Monitoring the Comprehensive Nuclear Test Ban Treaty written by Frode Ringdal and published by Springer Science & Business Media. This book was released on 2001-04 with total page 432 pages. Available in PDF, EPUB and Kindle. Book excerpt: In September 1996, the United Nations General Assembly adopted the Comprehensive Nuclear-Test-Ban Treaty (CTBT), prohibiting nuclear explosions worldwide, in all environments. The treaty calls for a global verification system, including a network of 321 monitoring stations distributed around the globe, a data communications network, an international data centre (IDC), and on-site inspections, to verify compliance. This volume contains research papers focusing on seismic ecent location in the CTBT context. The on-site inspection protocol of the treaty specifies a search area not to exceed 1000 square km. Much of the current research effort is therefore directed towards refining the accuracy of event location by including allowances for three-dimensional structure within the Earth. The aim is that the true location of each event will lie within the specified source zone regarding postulated location. The papers in this volume cover many aspects of seismic event location, including the development of algorithms suitable for use with three-dimensional models, allowances for regional structure, use of calibration events and source-specific station corrections. They provide a broad overview of the current international effort to improve seismic event location accuracy, and the editors hope that it will stimulate increased interest and further advances in this important field.

Download or read book Regional Location in Western China written by and published by . This book was released on 1996 with total page 12 pages. Available in PDF, EPUB and Kindle. Book excerpt: Accurately locating seismic events in western China using only regional seismic stations is a challenge. Not only is the number of seismic stations available for locating events small, but most stations available to researchers are often over 10° distant. Here the authors describe the relocation, using regional stations, of both nuclear and earthquake sources near the Lop Nor test site in western China. For such relocations, they used the Earthquake Data Reports provided by the US Geological Survey (USGS) for the reported travel times. Such reports provide a listing of all phases reported to the USGS from stations throughout the world, including many stations in the People's Republic of China. LocSAT was used as the location code. The authors systematically relocated each event int his study several times, using fewer and fewer stations at reach relocation, with the farther stations being eliminated at each step. They found that location accuracy, judged by comparing solutions from few stations to the solution provided using all available stations, remained good typically until fewer than seven stations remained. With a good station distribution, location accuracy remained surprisingly good (within 7 km) using as few as 3 stations. Because these relocations were computed without good station corrections and without source-specific station corrections (that is, path corrections), they believe that such regional locations can be substantially improved, largely using static station corrections and source-specific station corrections, at least in the Lop nor area, where sources have known locations. Elsewhere in China, one must rely upon known locations of regionally-recorded explosions. Locating such sources is clearly one of the major problems to be overcome before one can provide event locations with any assurance from regional stations.

Download or read book Applying Source and Path Corrections to Improve Discrimination in China written by and published by . This book was released on 1997 with total page 14 pages. Available in PDF, EPUB and Kindle. Book excerpt: Monitoring the Comprehensive Test Ban Treaty (CTBT) to magnitude levels below 4.0 will require use of regional seismic data recorded at distances of less than 2000 km. To improve regional discriminant performance we tested three different methods of correcting for path effects, and the third method includes a correction for source-scaling. We used regional recordings of broadband from stations in and near China. Our first method removes trends between phase ratios and physical parameters associated with each event-station path. This approach requires knowledge of the physical parameters along an event-station path, such as topography, basin thickness, and crustal thickness. Our second approach is somewhat more empirical. We examine spatial distributions of phase amplitudes after subtracting event magnitude and correcting for path distance. For a given station, phase, and frequency band, we grid and then smooth the magnitude-corrected and distance-corrected amplitudes to create a map representing a correction surface. We reference these maps to correct phase amplitudes prior to forming discrimination ratios. Our third approach is the most complicated, but also the most rigorous. For a given station and phase, we invert the spectra of a number of well-recorded earthquakes for source and path parameters. We then use the values obtained from the inversion to correct phase amplitudes for the effects of source size, distance, and attenuation. Finally, the amplitude residuals are gridded and smoothed to create a correction surface representing secondary path effects. We find that simple ratio- parameter corrections can improve discrimination performance along some paths (such as Kazakh Test Site (KTS) to WMQ), but for other paths (such as Lop Nor to AAK) the corrections are not beneficial. Our second method, the empirical path correction surfaces, improves discrimination performance for Lop Nor to AAK paths. Our third method, combined source and path corrections, has only been tested at WMQ (primarily using KTS explosions and northwest China earthquakes), but the method successfully corrects for source-scaling and propagation effects. We favor this combined source and path correction technique over the other correction methods, and we plan to test this method at other Asian stations. 11 refs., 7 figs.

Download or read book Seismological Research Letters written by and published by . This book was released on 2002 with total page 464 pages. Available in PDF, EPUB and Kindle. Book excerpt:

Download or read book Monitoring the Comprehensive Nuclear Test Ban Treaty written by H.J. Patton and published by Springer Science & Business Media. This book was released on 2001-09-01 with total page 232 pages. Available in PDF, EPUB and Kindle. Book excerpt: Regional seismograms are dominated by the phases Pn, Pg, Sn, and Lg. More often Sn and Lg are used to infer the attenuation structure of the lithosphere. The seismic phase Sn is a high-frequency shear-wave (typically from 1 to 4 Hz and occasionally higher) that travels in the lithospheric mantle above the negative velocity gradient which usually marks the lithosphere-asthenosphere boundary. Sn has been reported out to distances of 35° (e. g. , MOLNAR and OLIVER, 1969; HUESTIS et aI. , 1973). Sn arrives as a high-frequency wave train lasting tens of seconds and up to 1 to 2 minutes. Sn velocities are typically 4. 7 km/s in stable continental and oceanic lithosphere (HUESTIS et al. , 1973) and as low as 4. 3 km/s (KADINSKY-CADE et al. , 1981) in more tectonically active regions. Lg is a complex short period guided wave consisting of high-frequency P and S energy which travels primarily in the earth's crust at frequencies typically between 0. 5 and 5 Hz. It has been modeled as higher-mode Love and Rayleigh waves as well as a sequence of multiply reflected post-critical S waves trapped in a crustal guide (BOUCHON, 1982; KENNETT, 1986; BOSTOCK and KENNETT, 1990). Lg has been observed not to propagate in oceanic or very thin continental crust (PRESS and EWING, 1952; SEARLE, 1975; ZHANG and LAY, 1995).

Download or read book Source and Path Effects on Regional Phases in China written by and published by . This book was released on 1997 with total page 13 pages. Available in PDF, EPUB and Kindle. Book excerpt: As part of the CTBT Research and Development regional characterization effort, we are assembling, organizing and analyzing geological, geophysical, and seismic data for inclusion in a knowledge base for China. We have collected seismic data from 11 Chinese Digital Seismic Network (CDSN) stations as well as IRIS stations AAK, TLY, ULN and NIL from adjoining regions. Using the published event locations and origin times, we identify Pn, Pg, Sn, and Lg phases, construct travel time curves, and estimate apparent velocities from broadband and short period seismograms. Following this, we collect amplitudes of regional seismic phases and associated noise levels using bandpassed waveforms. Studies of path specific propagation of the seismic phases have mapped blockages and have generated corrections useful in reducing scatter in magnitude estimates and in discriminant ratios. Such path corrections reduce RMS distance and mb- corrected Lg amplitude to as much as 60% of its original level (log10 domain). Path corrections are less effective with Pn data. We also study source scaling effects on these data which will allow us to refine path corrections further.

Download or read book Amplitude Path Corrections for Regional Phases in China written by and published by . This book was released on 1998 with total page 9 pages. Available in PDF, EPUB and Kindle. Book excerpt: The authors investigate the effectiveness of amplitude path corrections for regional phases on seismic event discrimination and magnitude estimation. Waveform data from digital stations in China for regional, shallow (

Download or read book Monitoring the Comprehensive Nuclear Test Ban Treaty Seismic Event Discrimination and Identification written by William R. Walter and published by Springer. This book was released on 2013-04-18 with total page 285 pages. Available in PDF, EPUB and Kindle. Book excerpt: In September 1996, the United Nations General Assembly adopted the Comprehensive Nuclear-Test-Ban Treaty (CTBT), prohibiting nuclear explosions worldwide, in all environments. The treaty calls for a global verification system, including a network of 321 monitoring stations distributed around the globe, a data communications network, an international data center, and onsite inspections, to verify compliance. The problem of identifying small-magnitude banned nuclear tests and discriminating between such tests and the background of earthquakes and mining-related seismic events, is a challenging research problem. Because they emphasize CTBT verification research, the 12 papers in this special volume primarily addresses regional data recorded by a variety of arrays, broadband stations, and temporarily deployed stations. Nuclear explosions, earthquakes, mining-related explosions, mine collapses, single-charge and ripple-fired chemical explosions from Europe, Asia, North Africa, and North America are all studied. While the primary emphasis is on short-period, body-wave discriminants and associated source and path corrections, research that focuses on long-period data recorded at regional and teleseismic distances is also presented Hence, these papers demonstrate how event identification research in support of CTBT monitoring has expanded in recent years to include a wide variety of event types, data types, geographic regions and statistical techniques.

Download or read book Modeling Propagation Effects from Explosions in Western China and India written by and published by . This book was released on 1998 with total page 10 pages. Available in PDF, EPUB and Kindle. Book excerpt: The authors report on the results of finite-difference simulations of regional seismic wave propagation from a 1995 explosion at the Chinese test site at Lop Nor and from the recent Indian test. These simulations provide insight into variations in explosion seismograms recorded for these events. Previous modeling efforts by App et al., 1996, and Jones et al., 1998, have shown that many features in seismograms from explosions and earthquakes received at various stations surrounding Lop Nor can be reproduced with simple descriptions of the geologic structure along each path. However, differences in detail between simulations and the earthquake-like seismograms recorded at Station TLY in the Baikal Rift for the Lop Nor explosion have motivated further study of propagation effects. Differences between the TLY data and the simulations indicate that the use of homogeneous materials within the basins, crust, and mantle is too simplistic to capture some of the important features of the observed seismograms. Current efforts involve simulations in which 1-D and 2-D lithosphere models are merged to mimic gradients and 2-D structure, features which produce Rayleigh-wave dispersion and an improved agreement in group velocity curves. Source models have been modified to include a component of tectonic release during the Lop Nor explosion. The recent nuclear tests in India have been modeled in a similar fashion. The propagation path crosses the Moho step beneath the southern edge of the Tibetan Plateau before being received at station NIL. Similarly, the path from Lop Nor to NIL crosses a Moho step beneath the northern edge of the Tibetan Plateau and, in both cases, the effect of a step when combined with surface basin attenuation effectively block the Lg phase. Simulations show that both attenuation and crustal thickening are important when modeling propagation near the Tibetan Plateau.

Download or read book Cross Region Analysis Path Corrections and the Transportability of Regional Seismic Discriminants written by Douglas R. Baumgardt and published by . This book was released on 1995 with total page 9 pages. Available in PDF, EPUB and Kindle. Book excerpt: This study has investigated the problem of transporting regional discriminants, principally the regional P/S amplitude ratio and Lg spectral discriminants, between regions of differing tectonic type. The effects of differential regional phase attenuation and propagation blockages need to be taken into account and corrected before decision criteria for discriminants developed in one tectonic region can be transported to a different tectonic region. We have developed a set of frequency-dependent distance correction curves to correct the Pn/Sn and Pn/Lg amplitude ratio to a standard distance. Similar corrections have been developed for the Lg spectral ratio using a standard Lg attenuation model. Also, we have developed a method, called cross-region seismic event characterization, to characterize and identify new events which occur in aseismic regions or regions lacking known source types. Discrimination processing results of the Intelligent Seismic Event Identification System (ISEIS) for many different regions are stored in an Oracle database and can be called up on a regionalized basis. When a new event occurs, the method of cross-region seismic event characterization tries to find reference events from different regions, probably recorded at different stations than the new event, but at comparable distance for comparison which can be used as reference or training events to identify the new event. We construct crustal cross sections for the different propagation paths and determine if the new event has similar cross sections as the reference events. If the paths are not too different, the discriminants can be directly compared, after correcting for differences in distance.

Download or read book Source and Path Calibration in Regions of Poor Crustal Propagation Using Temporary Large Aperture High Resolution Seismic Arrays written by and published by . This book was released on 2009 with total page 9 pages. Available in PDF, EPUB and Kindle. Book excerpt: Broadband seismic data acquired during the Hi-CLIMB field experiment are used to study seismic events and path propagation in the Nepal Himalaya and south-central Tibetan Plateau. Similarities in regional propagation between Tibet and Iran motivate this new study. The 2002-2005 Hi-CLIMB experiment consisted of 233 stations distributed along a dense 800 km linear north-south array extending from the Himalayan foreland into the central Tibetan Plateau. The main array was flanked by a 350 km x 350 km sub-array in southern Tibet and central and eastern Nepal. Our dataset provides an opportunity to obtain seismic event locations for ground truth (GT) evaluation, with Q emphasis on depth, to determine source parameters, and to study distance evolution of seismic coda for yield estimation in low Q regions. Event detection and preliminary automatic location analysis show tens of thousands, otherwise undetected, local seismic events. We will obtain high-quality event locations from manual P- and S-wave picks by joint inversion for location and 2D and 3D velocity structure. We will also perform relative locations to resolve spatial relations of several highly active event clusters. Besides GT-compatibility, high-quality locations are essential for the source parameter and coda evolution portions of the study. We will perform moment tensor Q inversions in a wide magnitude range (1.5

Download or read book Grid Search Location Methods for Ground Truth Collection from Local and Regional Seismic Networks written by and published by . This book was released on 2003 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: The objective of this project is to develop improved seismic event location techniques that can be used to generate more and better quality reference events using data from local and regional seismic networks. Their approach is to extend existing methods of multiple-event location with more general models of the errors affecting seismic arrival time data, including picking errors and errors in model-based travel-times (path corrections). Toward this end, they are integrating a grid-search based algorithm for multiple-event location (GMEL) with a new parameterization of travel-time corrections and new kriging method for estimating the correction parameters from observed travel-time residuals. Like several other multiple-event location algorithms, GMEL currently assumes event-independent path corrections and is thus restricted to small event clusters. The new parameterization assumes that travel-time corrections are a function of both the event and station location, and builds in source-receiver reciprocity and correlation between the corrections from proximate paths as constraints. The new kriging method simultaneously interpolates travel-time residuals from multiple stations and events to estimate the correction parameters as functions of position. They are currently developing the algorithmic extensions to GMEL needed to combine the new parameterization and kriging method with the simultaneous location of events. The result will be a multiple-event location method which is applicable to non-clustered, spatially well-distributed events. They are applying the existing components of the new multiple-event location method to a data set of regional and local arrival times from Nevada Test Site (NTS) explosions with known origin parameters. Preliminary results show the feasibility and potential benefits of combining the location and kriging techniques. They also show some preliminary work on generalizing of the error model used in GMEL with the use of mixture-of-Gaussians probability distributions fit to observed travel-time residuals.

Download or read book Path Corrections For Regional Phase Discriminants written by and published by . This book was released on 2000 with total page 9 pages. Available in PDF, EPUB and Kindle. Book excerpt: In order to utilize regional seismic phases for reliable source identification, it is essential to account for wave propagation effects that may obscure the subtle differences in source radiation for distinct source types. Recent work has established three main approaches to the problem of characterizing and correcting regional phases for propagation effects: empirical methods based on parametric dependence on path properties such as pathlength or crustal waveguide parameters, empirical interpolation methods such as cap-averaging and Kriging for station dependent source region correction surfaces, and waveform modeling methods based on complete waveform synthesis for one-, two-, or three-dimensional crustal models. Empirically based methods remain the most viable general approach, as model based methods are still greatly limited by our wave propagation techniques and by lack of independent constraints on three-dimensional waveguide structure. Kriging and waveguide parameter regressions for frequency dependent regional phase amplitude ratio measurements for International Seismic Monitoring Stations, NIL and ZAL were conducted in the first year of this study. The data have a large amount of scatter, much of which is attributed to wave propagation effects; however, propagation corrections are limited by our understanding of source induced scatter and by effects of partial or total blockage of regional phases. We have addressed the issue of source induced scatter by analysis of a well-controlled data set of regional recordings of shallow earthquakes with reasonably well-determined focal mechanisms.

Download or read book Abstracts for the AGU Western Pacific Geophysics Meeting written by and published by . This book was released on 2008 with total page 444 pages. Available in PDF, EPUB and Kindle. Book excerpt: