Isotropic and Deviatoric Moment Inversion of Regional Surface Waves from Nevada Test Site Explosions: Implications for Yield Estimation and Seismic Discrimination PDF Download
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Author: Bradley B. Woods
Publisher:
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Category :
Languages : en
Pages : 8
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Seismic moments of Nevada Test Site (NTS) explosions were determined from regional surface wave spectra. Two methods were used. In one the moment is solved for assuming only an explosive source, or average scalar moment; in the other a joint inversion for an isotropic (explosive) source plus a constrained double couple moment component representing tectonic strain release (TSR). Although the general moment tensor solution to this joint inversion problem is non-unique, if some assumptions are made concerning the non-isotropic moment components, then the remaining source parameters can be solved by a linear least-squares inversion scheme. We examined the errors in determining the isotropic moment component (M sub I) by this latter method of constrained linear inversion solutions in a canonical study using a theoretical network of long-period (6-60 sec.) surface wave data. The network azimuthal coverage was chosen to represent that of a long-period North American super-network of 55 stations used for the actual NTS events. We compared these errors in moment estimate to those obtained from surface wave magnitude (M sub s) and spectral scalar moment (M sub 0) measurements for the same surface wave observations. For a ratio of M sub expl/M sub eq less than 1.0 we found that the inverted M sub I solution is a much better estimate of the actual isotropic moment than either M sub s or M sub O, and the standard deviation in this estimate is substantially less than that using the other two methods for the great majority of isotropic source + double couple sources. Even when the inversion constraints are off in dip and rake each by 30 deg, the mis-estimate of the isotropic moment is less than 35 percent of the actual value.
Author: Bradley B. Woods
Publisher:
ISBN:
Category :
Languages : en
Pages : 8
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Book Description
Seismic moments of Nevada Test Site (NTS) explosions were determined from regional surface wave spectra. Two methods were used. In one the moment is solved for assuming only an explosive source, or average scalar moment; in the other a joint inversion for an isotropic (explosive) source plus a constrained double couple moment component representing tectonic strain release (TSR). Although the general moment tensor solution to this joint inversion problem is non-unique, if some assumptions are made concerning the non-isotropic moment components, then the remaining source parameters can be solved by a linear least-squares inversion scheme. We examined the errors in determining the isotropic moment component (M sub I) by this latter method of constrained linear inversion solutions in a canonical study using a theoretical network of long-period (6-60 sec.) surface wave data. The network azimuthal coverage was chosen to represent that of a long-period North American super-network of 55 stations used for the actual NTS events. We compared these errors in moment estimate to those obtained from surface wave magnitude (M sub s) and spectral scalar moment (M sub 0) measurements for the same surface wave observations. For a ratio of M sub expl/M sub eq less than 1.0 we found that the inverted M sub I solution is a much better estimate of the actual isotropic moment than either M sub s or M sub O, and the standard deviation in this estimate is substantially less than that using the other two methods for the great majority of isotropic source + double couple sources. Even when the inversion constraints are off in dip and rake each by 30 deg, the mis-estimate of the isotropic moment is less than 35 percent of the actual value.
Author: Bradley B. Woods
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ISBN:
Category : Earthquake hazard analysis
Languages : en
Pages : 214
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Author: Don V. Helmberger
Publisher:
ISBN:
Category :
Languages : en
Pages : 10
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With the installation of broadband, high dynamic range instruments, it has become possible to compare the regional waveforms of earthquakes and explosions at magnitudes 3 to 6. These waveforms are similar for event sequences in many situations and can be inverted for source mechanisms. We find that flat-layered models are sufficient for inverting seismograms at periods greater than a few seconds. This paper presents three studies aimed at determining crustal models, source finiteness and modeling complex structure near a receiver. We have conducted a set of sensitivity tests on the parameters of 1-D models to compare their impact on different segments of regional seismograms. We found that P sub nl waves (extended P-waves) are controlled in broadband character by the mid-crust while the top layer contributes to the long-period motions. The SV wave is mostly controlled by the shear wave velocity of the lower crust, especially the crustal layer just below the source depth. The top crustal layer controls the shape of the surface waves at ranges from 300 to 600 km, and the upper crust, especially the crustal layer just above the source depth, controls their timing. Applying these tests in modeling three earthquakes in the Basin-and-Range province, we found that a simple two-layer crustal model could effectively explain the data both in timing and in shape. The main crustal layer has P and S velocities of 6.1 km/sec and 3.6 km/sec, similar to those found by Langston and Helmberger (1974). A surface layer of thickness 2.5 to 3.5 km is required to fit the Rayleigh waves. Fast estimation of point-source parameters for earthquakes has witnessed much progress in recent years due to the development of broadband seismic networks.
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Category : Dissertations, Academic
Languages : en
Pages : 912
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Author:
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Category :
Languages : en
Pages : 11
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In our previous work the deviatoric and isotropic source components for 17 explosions at the Nevada Test Site, as well as 12 earthquakes and 4 collapses in the surrounding region of the western US, were calculated using a regional time-domain full waveform inversion for the complete moment tensor (Dreger et al., 2008; Ford et al., 2008; Ford et al., 2009a). The events separate into specific populations according to their deviation from a pure double-couple and ratio of isotropic to deviatoric energy. The separation allows for anomalous event identification and discrimination between explosions, earthquakes, and collapses. Confidence regions of the model parameters are estimated from the data misfit by assuming normally distributed parameter values. We developed a new Network Sensitivity Solution (NSS) in which the fit of sources distributed over a source-type plot (Hudson et al., 1989) show the resolution of the source parameters. The NSS takes into account the unique station distribution, frequency band, and signal-to-noise ratio of a given event scenario. The NSS compares both a hypothetical pure source (for example an explosion or an earthquake) and the actual data with several thousand sets of synthetic data from a uniform distribution of all possible sources. The comparison with a hypothetical pure source provides the theoretically best-constrained source-type region for a given set of stations, and with it one can determine whether further analysis with the data is warranted. We apply the NSS to a NTS nuclear explosion, and earthquake, as well as the 2006 North Korean explosion, and a nearby earthquake. The results show that explosions and earthquakes are distinguishable, however the solution space depends strongly on the station coverage. Finally, on May 25, 2009 a second North Korean test took place. Our preliminary results show that the explosive nature of the event may be determined using the regional distance moment tensor method. Results indicate that the 2009 event is approximately 5-6 times larger than the earlier test, with an isotropic moment of about 1.8e+22 dyne cm. We perform a series of inversions for pure double-couple, pure explosion, combined double-couple and explosion, full moment tensor, and damped moment tensor inversions to assess the resolution of the isotropic moment of the event.
Author:
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ISBN:
Category : Earthquakes
Languages : en
Pages : 430
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Author: Steven R. Taylor
Publisher: American Geophysical Union
ISBN:
Category : Science
Languages : en
Pages : 290
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Published by the American Geophysical Union as part of the Geophysical Monograph Series, Volume 65. In March 1989, the Department of Energy (DOE) and the Lawrence Livermore National Laboratory (LLNL) sponsored a symposium on Explosion Source Phenomenology at Lake Tahoe, California. The purpose was to summarize the state of knowledge of the underground explosion source, based on U.S. experience at the Nevada Test Site (NTS). Specifically, the goals were to summarize knowledge of the explosion source, to identify limits of that knowledge and existing problems, and to propose directions of future research and data?]collection efforts.
Author: Andrea Chiang
Publisher:
ISBN:
Category :
Languages : en
Pages : 125
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This dissertation demonstrates the utility of the complete waveform regional moment tensor inversion (Dreger and Woods, 2002; Dreger, 2003, Minson and Dreger, 2008) for nuclear event discrimination. I explore the source processes and associated uncertainties for explosions and earthquakes under the effects of limited station coverage, compound seismic sources, assumptions in velocity models and the corresponding Green's functions, and the effects of shallow source depth and free-surface conditions. The motivation to develop better techniques to obtain reliable source mechanism and assess uncertainties is not limited to nuclear monitoring, but they also provide quantitative information about the characteristics of seismic hazards (e.g. Petersen et al., 2014), local and regional tectonics and in-situ stress fields of the region (Hardebeck and Hauksson, 2001; Hardebeck and Michael, 2006). This dissertation begins with the analysis of three sparsely recorded events: the 14 September 1988 US-Soviet Joint Verification Experiment (JVE) nuclear test at the Semipalatinsk test site in Eastern Kazakhstan, and two nuclear explosions at the Chinese Lop Nor test site. We utilize a regional distance seismic waveform method fitting long-period, complete, three-component waveforms jointly with first-motion observations from regional stations and teleseismic arrays. The combination of long period waveforms and first motion observations provides unique discrimination of these sparsely recorded events in the context of the Hudson et al. (1989) source-type diagram. We demonstrate through a series of Jackknife tests and sensitivity analyses that the source-type of the explosions is well constrained. One event, a 1996 Lop Nor shaft explosion, displays large Love waves and possibly reversed Rayleigh waves at one station, indicative of a large tectonic release. We demonstrate the behavior of Network Sensitivity Solutions [NSS] (Ford et al., 2010) for models of tectonic release (Toksöz et al., 1965) and spall-based tensile damage (Patton and Taylor, 2008) over a range of F-factors and K-factors. A potential issue for moment tensor inversion of explosions is that Green's functions have vanishing amplitudes at the free surface. Because explosions are detonated at very shallow depths, this can result in bias in the moment tensor solution (Stevens and Murphy, 2001). It is important to understand these free surface effects on discriminating shallow explosive sources for nuclear monitoring purposes. It may also be important in natural systems that have shallow seismicity such as volcanoes and geothermal systems. To tackle this problem, we examine the effects of the free surface on the moment tensor via synthetic testing, and apply the moment tensor based discrimination method to well-recorded chemical explosions. These shallow chemical explosions represent rather severe source-station geometry in terms of the vanishing traction issues. We show that the combined waveform and first motion method enables the unique discrimination of these events, even though the data include unmodeled single force components resulting from the collapse and blowout of the quarry face immediately following the initial explosion. In contrast, recovering the announced explosive yield using seismic moment estimates from moment tensor inversion remains challenging but we can begin to put error bounds on our moment estimates using the NSS technique. The estimation of seismic source parameters is dependent upon having a well-calibrated velocity model to compute the Green's functions for the inverse problem. Ideally, seismic velocity models are calibrated through broadband waveform modeling (e.g. Dreger and Helmberger, 1990; Bhattacharyya et al., 1999), however in regions of low seismicity velocity models derived from body or surface wave tomography may be employed (e.g. Tape et al., 2010; Shen et al., 2013; Porritt et al. 2014). Whether a velocity model is 1D or 3D, or based on broadband seismic waveform modeling or the various tomographic techniques, the uncertainty in the velocity model can be the greatest source of error in moment tensor inversion. These errors have not been fully investigated for the nuclear discrimination problem. To study the effects of unmodeled structures on the moment tensor inversion, we set up a synthetic experiment where we produce synthetic seismograms for a 3D model (Moschetti et al., 2010) and invert these data using Green's functions computed with a 1D velocity mode (Song et al., 1996) to evaluate the recoverability of input solutions, paying particular attention to biases in the isotropic component. We then evaluate source inversions for real data using Green's functions for 1D and 3D velocity models in which the Green's functions were computed by utilizing the principle of source-receiver reciprocity (Aki and Richards, 2002; Dahlen and Tromp, 1998), and the finite-difference method (Appelo and Petersson, 2008; Eisner and Clayton, 2001; Graves and Wald, 2001). Using the full waveform moment tensor inversion method we analyze earthquakes and explosions at NTS using 1D and 3D Earth models and compare the solutions and associated uncertainties at different frequency bands. The synthetic experiment results indicate that the 1D model assumption is valid for moment tensor inversions at periods as short as 10 seconds for the 1D western U.S. model (Song et al., 1996). The correct earthquake mechanisms and source depth are recovered with statistically insignificant isotropic components as determined by the F-test. Shallow explosions are biased by the theoretical ISO-CLVD tradeoff but the tectonic release component remains low, and the tradeoff can be eliminated with constraints from P wave first motion. Path-calibration to the 1D model can reduce non-double-couple components in earthquakes, non-isotropic components in explosions and composite sources and improve the fit to the data. When we apply the 3D model to real data, at long periods (20-50 seconds), we see good agreement in the solutions between the 1D and 3D models and slight improvement in waveform fits when using the 3D velocity model Green's functions. At high frequencies the advantage of the 3D model is limited except for paths from NTS to the San Francisco Bay, where we see a marked improvement in waveform fit. However, we do not see a clear reduction in source uncertainties when using a 3D model. A larger sample size is required to make useful interpretations about the use of 3D models in estimating source uncertainties. Our results indicate that the 3D model for the western U.S. (Moschetti et al., 2010) still needs further refinement to adequately model wave propagation at high frequencies and that path-averaged 1D models derived from the 3D model may be a more attractive approach than the more costly 3D simulation for short period inversions.
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Category :
Languages : en
Pages : 7
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We use regional waveform data from the Nevada Test Site (NTS) to investigate phenomenological relationships between recorded amplitude and explosion yield as well as test regional depth estimation procedures. Our goal is to better understand the performance of seismic observables in other regions of monitoring interest, especially at small magnitudes (m{sub b}“4.5). Some of the topics we are studying include: stable yield estimation, depth estimation, and M{sub g}:m{sub b} performance. We use Lawrence Livermore National Laboratory's NTS explosion database, which consists of several hundred events ranging from (almost equal to)200 to (almost equal to)1500-m depth and yields ranging from a few tenths of a kiloton to the megaton range. In addition to the broadband explosion data, we have a large dataset of well-located earthquakes on the test site with depths ranging from 2 to 17 km and magnitudes ranging between M{sub w}1.5 and 5.7. For yield estimation the relation between teleseismic body wave magnitude (mb) and nuclear explosion yield has been studied extensively over the past several decades for a number of test sites for large (>1 kt) explosions. In this paper we will look at broadband coda, P{sub g, } and L{sub g} from over 260 nuclear explosions to study yield estimation capability by comparing F-factors. For monitoring compliance with a CTBT, small events that are recorded only at regional distances will be used to estimate magnitude and equivalent yield, Past coda studies show that coda-derived magnitudes of earthquakes and explosions are more stable than any direct phase method, including mb(Lg). In fact, single-station coda measurements can be equivalent to a network average of at least ten direct phase measurements over a broad range of frequencies. In regions where the depth estimate is poorly constrained, other regional methods have been proposed to estimate depth. These include time-domain measures of P-wave complexity, cepstral peaking, and more recently spectral peaking from R{sub g}-to-S scattering. Myers et al. (1999) and Mayeda and Walter (1996) showed that strong spectral peaking in the S-wave and coda were likely due to R{sub g}-to-S scattering in the near-source region. We propose a side-by-side comparison of these techniques in a region with excellent ground truth, namely NTS. We will investigate to what extent cepstral peaking, coda spectra peaking, and complexity provide a reliable depth estimate. Finally, a number of large regional studies computing surface wave dispersion curves throughout the globe will be used to push the M{sub g} measurements to smaller magnitude by the use of phase-matched filters. For larger teleseismically recorded events, we will test to see if the M{sub g}:m{sub b} trends for explosions and earthquakes continue to separate at small magnitudes at regional distances. Although NTS is unique from other test sites in its geologic characteristics, this dataset of explosions and earthquakes is ideal for a number of reasons: (I) continuous recordings from high-quality broadband stations, (2) ground truth information that far exceeds any other area, (3) path and site effects that are virtually common for all events, and (4) wide range in depth, source size, and material properties. Because our goal is phenomenological in scope, we will use these results to guide our interpretations and assess our capability in other areas of monitoring interest.
Author:
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ISBN:
Category : Science
Languages : en
Pages : 960
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