Investigating Source Processes of Isotropic Events PDF Download

Author: Andrea Chiang
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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.