CHAPTER 1 2 Seismic Wave Propagation and Earth models Peter Bormann, Bob Engdahl and Rainer Kind 2.1 Introduction The key data to be recorded by means of seismic sensors (Chapter 5) and recorders (Chapter 6) at seismological observatories (stations – Chapter 7, networks – Chapter 8, arrays – Chapter 9) are seismic waves, radiated by seismic sources (Chapter 3). Weak signals may be masked or significantly distorted by seismic noise (Chapter 4), which is usually considered disturbing and unwanted. Only in some special engineering-seismological applications is seismic noise also appreciated as a useful signal, from which some information on the structure, velocity and fundamental resonance frequency of the uppermost sedimentary layers can be derived (e.g. Bard, 1999). But most of what we know today of the structure and physical properties of our planet Earth, from its uppermost crust down to its center, results from the analysis of seismic waves generated by more or less localized natural or man-made sources such as earthquakes or explosions (Figs. 3.1 to 3.4). Either (repeatedly) solving the so-called forward (direct) or the inverse problem of data analysis (Fig. 1.1) achieves this. It is not the task of the New Manual of Seismological Observatory Practice (NMSOP), to provide an in-depth understanding of the theoretical tools for this kind of analysis. There exist quite a number of good introductory (Lillie, 1999; Shearer, 1999) and more advanced textbooks (e.g., Aki and Richards, 1980 and 2002; Ben-Menahem and Singh,1981; Bullen and Bolt, 1985; Dahlen and Tromp, 1998; Lay and Wallace, 1995; Kennett, 2001), and a variety of special papers and monographs related to specific methods (e.g. Fuchs and Müller, 1971; Červený et al., 1977; Kennett, 1983; Müller, 1985; Červený, 2001), types of seismic waves (e.g., Malischewsky, 1987; Lapwood and Usami, 1981) or applications (e.g., Gilbert and Dziewonski, 1975; Sherif and Geldart, 1995). Rather, we will take here a more phenomenological approach and refer to related fundamentals in physics and mathematical theory only as far as they are indispensable for understanding the most essential features of seismic waves and their appearance in seismic records and as far as they are required for: • identifying and discriminating the various types of seismic waves; • understanding how the onset-times of these phases, as observed at different distances from the source, form so-called travel-time curves; • understanding how these curves and some of their characteristic features are related to the velocity-structure of the Earth and to the observed (relative) amplitudes of these phases in seismic records; • using travel-time and amplitude-distance curves for seismic source location and magnitude estimation; • understanding how much these source-parameter estimates depend on the precision and accuracy of the commonly used 1-D Earth models (see IS 11.1);
Seismic Wave Propagation and Earth models
Jun 24, 2023
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