Effects of Near-source Heterogeneity on Wave Fields Emanating from Crustal Sources Observed at Regional and Teleseismic Distances PDF Download

Author: Megan S. Avants
Publisher:
ISBN: 9781303842283
Category :
Languages : en
Pages : 142

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Book Description
Near-source path effects imprint the wave field emanating from a seismic source and, if not well resolved, can obscure the details of source characteristics determined from observations of the seismic waves at regional and teleseismic distances (& ge;200 km). These effects are particularly strong for crustal sources such as shallow earthquakes and underground nuclear explosions. First, I explore 2D effects of random seismic P-wave velocity heterogeneity resulting from volumetric heterogeneity in the upper mantle and variability of the Moho on the amplitude decay of the regional phase Pn. Results indicate that the pattern of amplitude decay due to geometric spreading for a simple Earth model is more complex than that for an Earth model containing strong heterogeneity in the mantle lid. Next, I implement the representation theorem in a method which collects displacement and strain components output from a 3D finite difference program capable of including realistic surface topography and geologic structure in a 3D velocity model, and calculates teleseismic 3D Green functions (3DGFs) to specified receiver locations. Green functions produced from a 3D source model match Green functions produced from a 1D source model for theoretical source-receiver geometries. This new method is then applied to the problem of constraining the source depth and location of the three nuclear tests conducted by North Korea, by using a realistic topography model for the mountainous test region to calculate 3DGFs for several possible locations of each event. Amplitude ratios of P and pP from 3DGFs are correlated to those in observed stacked traces. Results show a sensitivity of this method to source depth and location across the test site region with source depths slightly greater than published estimates, but relative locations consistent with other studies. Finally, I determine a rupture model of the 2008 Wenchuan earthquake using 3DGFs calculated in a velocity model containing the dramatic topographic contrast in the fault area, and compare results with a rupture model produced using 1DGFs. Rupture models derived from 1D and 3D synthetic source regions are very similar to each other, and both are consistent with other studies.

Author: Yong-Gang Li
Publisher: Springer
ISBN: 9811380155
Category : Nature
Languages : en
Pages : 245

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Book Description
This book presents review papers and research articles focusing on the 2008 Wenchuan earthquake in Sichuan, China, discussing cross-disciplinary and multiple thematic aspects of modern seismological, geophysical, geological and stochastic methodology and technology. Resulting from international and regional earthquake research and disaster mitigation collaborations, and written by international authors from multiple institutions and disciplines, it describes methods and techniques in earthquake science based on investigations of the Wenchuan earthquake. It also includes extensive reference lists to aid further research. The book helps both senior researchers and graduate students in earthquake science to broaden their horizons in data analysis, numerical modeling and structural retrieval for the tectonic, geological, geophysical and mechanical interpretation of the 2008 M8 Wenchuan earthquake to support a global and regional cooperation for preparedness, and the mitigation and management of seismic risk.

Author:
Publisher:
ISBN:
Category :
Languages : en
Pages : 11

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Book Description
In spite of extensive prior research on the generation of seismic waves by underground nuclear explosions, it is still not possible to provide a complete explanation for the observed wavefields, particularly at regional distances. Spherically symmetric explosion models embedded in layered elastic media effectively model the P phases generated by explosions and the major characteristics of some reflected and transmitted phases. Nonlinear axisymmetric finite difference calculations of explosions, including gravity and the effect of the free surface, can model a more realistic explosion source that directly generated shear waves. These models explain more characteristics of explosion-generated seismic waves, including some aspects of regional shear phases. However, it is clear that linear and nonlinear near-source 3D effects are important in many cases. SH waves are commonly observed within a few kilometers of explosions, too close to have been generated by (simple) conversion of vertical and radial components, and often larger than those components. Furthermore, it has not been established what impact 3D effects have on discriminants and on explosion yield estimates. It is important, therefore, to be able to model and understand how 3D source and source region heterogeneity affect the seismic wavefield and what impact this has on parameters used for nuclear monitoring. In this new project, we are implementing a technique that allows us to propagate the results of near-source 3D finite difference calculations to regional and teleseismic distances, and to use the results to investigate the impact of 3D near-source effects on regional and teleseismic phases, focusing in particular on the generation of SH phases by explosion sources. Our approach is to perform 3D explosion source region calculations, and then to propagate the wavefield to local, regional and teleseismic distances using layered earth Green's functions.

Author: K. Smith
Publisher:
ISBN:
Category :
Languages : en
Pages : 12

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Book Description
Regional seismic discriminants based on high-frequency P/S ratios reliably distinguish between earthquakes and explosions. However, P/S discriminants in the 0.5 to 3 Hz band (where SNR can be highest) rarely perform well, with similar ratios for earthquake and explosion populations. Variability in discriminant performance has spawned numerous investigations into the generation of S-waves from explosions. Several viable mechanisms for the generation of S-waves from explosions have been forwarded, but most of these mechanisms do not explain observations of frequency-dependant S-wave generation. Recent studies have focused on the affect of near-source scattering to explain the frequency-dependence of both S-wave generation and P/S discriminant performance. In this study we investigate near-source scatter through numerical simulation with a realistic geological model We have constructed a realistic, 3-dimensional earth model of the southern Basin and Range. This regional model includes detailed constraints at the Nevada Test Site (NTS) based on extensive geologic and geophysical studies. Gross structure of the crust and upper mantle is taken from regional surface-wave studies. Variations in crustal thickness are based on receiver function analysis and a compilation of reflection/refraction studies. Upper-crustal constraints are derived from geologic maps and detailed studies of sedimentary basin geometry throughout the study area. The free surface is based on a 10-meter digital elevation model (DEM) at NTS, and a 60-meter DEM elsewhere. The model extends to a depth of 150km, making it suitable for simulations at local and regional distances. Our simulation source is based on the 1993 Non-Proliferation Experiment explosion at the NTS. This shot was well recorded, offering ample validation data. Our validation tests include measures of long-period waveform fit and relative amplitude measurements for P and S phases. Our primary conclusion is that near-source topography and geologic complexity in the upper crust strongly contributed to the generation of S-waves from the NPE shot. When either geologic heterogeneity or topography is removed from the model, simulated amplitudes of regional S-waves are diminished. We also find that deeper sources scatter less energy off of topography and upper-crustal structures, resulting in diminished S-wave amplitudes with increasing source depth.

Author:
Publisher:
ISBN:
Category :
Languages : en
Pages : 140

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Book Description
This report presents the results of an investigation of near-source effects on teleseismic P-wave and early P-wave coda spectra for underground explosions. The underlying objective of this research has been to determine whether the differential behavior of P and P coda can be exploited to remotely determine near-source properties such as overburden velocity, burial depth, and degree of saturation. The report also contains a review paper on the pP phase, focussed on assessing the various methodologies that have been used to determine pP parameters and trying to reconcile the existing discrepancies in various estimates. The final section of the report is an encyclopedia article on the role of seismological monitoring of nuclear testing treaties. Energy radiated upward from underground nuclear explosions has a complex interaction with the free surface that strongly influences the seismic wavefields recorded at teleseismic and regional distances. This interaction, differing from that for earthquakes primarily due to the much higher strains and strain rates involved, is essential to understand for both explosion yield estimation and event discrimination. Reflection of explosion P wave energy from the free surface, which produces the pP phase, involves frequency-dependent, non-linear processes that are intimately linked to surface spallation.

Author: Glenn Eli Baker
Publisher:
ISBN:
Category : Seismic waves
Languages : en
Pages : 448

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Author: William Alan Peppin
Publisher:
ISBN:
Category :
Languages : en
Pages : 442

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Author: Thompson McCaskill
Publisher:
ISBN:
Category :
Languages : en
Pages : 182

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Book Description
Geotechnical analyses for earthquake engineering and other applications are often predicated on the accurate determination of shear wave velocity (VS) profiles. Surface wave methods (SWM) are a noninvasive approach to developing VS profiles that involve measurement of Rayleigh wave propagation between a wave-generating source and a receiver array placed on the ground surface. There are several variations of SWM, but all utilize the same three-step process for developing a VS profile, namely: (1) data collection -- measuring ground surface vibrations emanating from a source; (2) data processing -- developing an experimental dispersion curve relating wave velocity to wavelength or frequency; and (3) inversion -- finding the VS profile that produces a theoretical dispersion curve matching the experimental dispersion curve. In current practice, the theoretical model used to fit the experimental data is a far-field model that only simulates motions from planar Rayleigh waves. Therefore, the receiver array used to collect the data in step 1 must be located far from the source (or "far-field"), where body waves have largely dissipated (due to greater damping) and Rayleigh wavefronts are nearly planar. Closer to the source -- in the so-called "nearfield" -- the ground motion includes coupled interactions of body waves and non-planar Rayleigh waves and is inconsistent with a far-field theoretical model. The primary objective of this study was to investigate the effectiveness and potential benefits of including near-field contributions in both the surface wave data collection and modeling. First, it was hypothesized that source offset distance criteria currently used to mitigate near-field effects could be greatly reduced without affecting the quality of surface wave results.Second, it was hypothesized that additional information about the soil profile could be determined if the near-field portion of the dispersion curve was included in both the data collection and theoretical modeling. Three different studies were performed for this research, namely: (1) a preliminary sensitivity study, to study the sensitivity of the near-field portion of the dispersion curve to changes in various profile parameters, (2) surface wave analysis using simulated experimental data, to assess both profile recovery effectiveness and the possibility of inferring additional profile parameters (specifically, Poisson's ratio), and (3) surface wave analysis with real data, to validate the profile findings from the study using simulated data. Experimental data were collected and/or simulated using both the Spectral-Analysis-of-Surface-Waves (SASW) method and multi-channel surface wave methods, but the primary focus of this study was on the SASW method. The results from the study showed that surface wave analyses that included near-field contributions in both data collection and theoretical modeling were as effective or more effective at recovering the VS profile as conventional far-field approaches, with the benefit of shorter arrays and smaller sources. This study also showed that surface wave measurements that included near-field data were sensitive to changes in Poisson's ratio of the profile, as compared with the known insensitivity of conventional far-field surface wave methods. The results from the limited experimental study were less conclusive, but generally confirmed the findings from the study performed using simulated data.

Author: Barbara Romanowicz
Publisher: Elsevier
ISBN: 0444535756
Category : Science
Languages : en
Pages : 873

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Book Description
Treatise on Geophysics: Seismology and Structure of the Earth, Volume 1, provides a comprehensive review of the state of knowledge on the Earths structure and earthquakes. It addresses various aspects of structural seismology and its applications to other fields of Earth sciences. The book is organized into four parts. The first part principally covers theoretical developments and seismic data analysis techniques from the end of the nineteenth century until the present, with the main emphasis on the development of instrumentation and its deployment. The second part reviews the status of knowledge on the structure of the Earths shallow layers, starting with a global review of the Earth's crustal structure. The third part focuses on the Earth's deep structure, divided into its main units: the upper mantle, the transition zone and upper-mantle discontinuities, the D region at the base of the mantle, and the Earth's core. The fourth part comprises two chapters which discuss constraints on Earth structure from fields other than seismology: mineral physics and geodynamics. Self-contained volume starts with an overview of the subject then explores each topic with in depth detail Extensive reference lists and cross references with other volumes to facilitate further research Full-color figures and tables support the text and aid in understanding Content suited for both the expert and non-expert

Author: Brian Kennett
Publisher: ANU E Press
ISBN: 192153673X
Category : Reference
Languages : en
Pages : 298

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Book Description
Seismic Wave Propagation in Stratified Media presents a systematic treatment of the interaction of seismic waves with Earth structure. The theoretical development is physically based and is closely tied to the nature of the seismograms observed across a wide range of distance scales - from a few kilometres as in shallow reflection work for geophysical prospecting, to many thousands of kilometres for major earthquakes. A unified framework is presented for all classes of seismic phenomena, for both body waves and surface waves. Since its first publication in 1983 this book has been an important resource for understanding the way in which seismic waves can be understood in terms of reflection and transmission properties of Earth models, and how complete theoretical seismograms can be calculated. The methods allow the development of specific approximations that allow concentration on different seismic arrivals and hence provide a direct tie to seismic observations.