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Summary

As we have seen in Chapter 2, earthquakes produce motions in the Earth due to the propagation of seismic waves that result in destruction and damage to buildings and other structures with large loss of human life. These effects are the first type of observations about earthquakes, and are often called macroseismic data because they are produced by large motions. This is the only information we have for historical earthquakes. From the end of the nineteenth century, ground motion itself began to be measured and recorded by instruments called seismographs at different locations on the Earth's surface; which constitute the second type of observations. These observations represent the quantitative measurement of the ground displacement u(t), velocity v(t), and acceleration a(t) as a function of time, which are present in equations of the application of the principles of mechanics that we will consider in the following chapters. For this reason, we first need to know basic ideas about seismographs and of the processing and analysis of their recordings or seismograms. The discipline that studies the principles and characteristics of seismographs is called seismometry.We will present firstly the basic theory of the seismograph and some examples of analog instruments, to focus later on digital data from broad-band seismographs, which are the instruments most in use at present. An important part of this chapter is dedicated to conversion of the different data formats and the use of SAC software (Seismic Analysis Code) to process digital data (Tapley and Tull, 1992). The SAC macros used in this chapter to process seismograms may be found in the online resources, together with the digital seismograms, in order for students to be able to reproduce the examples shown in this chapter.

The historical evolution of seismographs

The oldest instrument used to detect the occurrence of an earthquake was probably constructed in China during the second century AD and is attributed to the philosopher Chian-hen. This instrument consisted of a bronze figure of a dragon with eight heads whose mouths each contained a ball. Inside the figure there was some kind of pendular device that pushed the balls and made them fall when it was shaken by an earthquake. The figure was oriented in the geographic directions so that, upon arrival of the seismic waves, the corresponding ball would fall and show the occurrence and orientation of the shock (Bolt, 1978).

Summary

In all previous chapters we have considered the mechanical behavior of perfectly elastic bodies to approximate the motion produced by earthquakes in the Earth. Many problems in seismology can be solved assuming for the Earth an approximation of perfect elasticity and isotropy. We know, however, that the Earth cannot behave in this way and in some cases is important to take in account the anelasticity and anisotropy of its material.

Perfectly elastic bodies do not exist since according to the second law of thermodynamics, in all physical deformable media, there is dissipation of energy in the form of heat through mechanisms known as internal friction (Section 4.4). As we have seen, in a perfectly elastic medium, the amplitudes of waves only decrease with distance due to geometric spreading. The lack of perfect elasticity adds a further decrease in wave amplitude due to anelastic attenuation. From the very first studies (Jeffreys, 1957; Lomnitz, 1957), anelastic attenuation of seismic waves in the Earth has been an important subject of seismology (Jackson and Anderson, 1970; Minster, 1980; Romanowicz and Mitchell, 2009).

In addition to perfect elasticity, we have also assumed isotropy for the material of the Earth. Now we will consider how this material is not only perfectly elastic, but also it is not perfectly isotropic. Consideration of the lack of isotropy or anisotropy in the Earth is an important subject in seismology and it is related to geodynamic processes that influence its material properties. Although deviations from the conditions of isotropy are small, their effect on the propagation of seismic waves can be observed and provide important information about the Earth's structure and dynamics (Crampin, 1977; Babuska and Cara, 1991; Maupin and Park, 2009).

Anelasticity and damping

Loss of energy in the Earth due to internal friction is responsible for the attenuation and damping of the amplitudes of seismic waves with distance and time. Concrete mechanisms of internal friction are complex and depend on the atomic and molecular structure of crystals in minerals, the presence of small cracks and fractures, and the inclusion of liquids in rocks. Since these mechanisms depend on the nature of the materials through which waves propagate, their effect is called intrinsic attenuation.

Summary

In contrast to monologues or institutionally regulated interaction, it is the local management and negotiation of turn exchange that is crucial for the organization of conversation. In conversation the issue of who takes a turn at what point in time is not predetermined, but is negotiated between the participants at the moment when it becomes relevant in the interaction. In general, there is a normative orientation to a next speaker taking the turn “on time”, i.e., not too early and not too late, with minimal overlap and minimal gap, after the prior speaker has ended his or her turn. Taking over the floor too early or too late can give rise to inferences concerning possible reasons for noticeably deviating from normative expectations. But how is it possible for turn taking to work so smoothly?

The basic mechanism of turn taking was reconstructed in the seminal paper by Sacks et al. (1974). According to their model, turn taking is organized in two components:

Summary

The first approach to the study of earthquake occurrence is from a descriptive point of view. Following this approach, we present in this chapter a first view of the basic ideas about earthquake occurrence and damage, seismic intensity as a measure of the size of earthquakes, earthquakes in relation to geological faults, their geographical and time distribution, some basic ideas of the Earth's structure and dynamics related to earthquake occurrence, and of seismic hazard and risk. This short presentation serves as an introduction to and shows the need for a physical mathematical approach to the quantified study of earthquakes, which is the subject of subsequent chapters.

Earthquakes: natural disasters

Earthquakes, in some cases followed by tsunamis (Japanese for bay waves), landslides, and fires, are, first of all, natural disasters that take the greatest number of human lives and produce the heaviest material damage (Appendix 7). Reports about large earthquakes and their catastrophic consequences have come to us since ancient times. Historians have left us vivid descriptions of the occurrence of large earthquakes throughout human history. Some of the oldest earthquakes for which we have historical evidence are from as far back as a thousand years BC Some large earthquakes have had important consequences in human history; for example, an earthquake in 464 BC ended the hegemony of Sparta over Greece (Zeilinga de Boer and Sanders, 2005). The great Lisbon earthquake of 1755, the largest ever felt in Europe, which was followed by a large tsunami and caused some 15,000 casualties, gave origin to an intense philosophical and theological debate and is considered to mark the origin of seismology as a science, as we have seen in Chapter 1. Disasters caused by earthquakes are a continuous reminder of the vulnerability of human constructions. Even over the last ten years there have been a considerable number of very damaging large earthquakes with extremely high loss of human life. The Sumatra earthquake of 2004 produced about 228,000 deaths, most of these in the following large tsunami that struck the coasts of the Indian Ocean. In 2010 an earthquake destroyed Port-au-Prince, the capital of Haiti, causing some 316,000 deaths due to the low quality of its buildings (Fig. 2.1). Reconstruction after five years is only progressing at a very slow pace.

Summary

The term seismology is derived from two Greek words, seismos, shaking, and logos, science or treatise. Earthquakes were called seismos gês in Greek, literally, shaking of the Earth; the Latin term is terrae motus, and from the equivalents of these two terms come the words used in most occidental languages. Seismology means, then, the science of the shaking of the Earth or the science of earthquakes. The term seismology and similar ones in other occidental languages (séismologie, sismología, Seismologie, etc.) began to be used at around the middle of the nineteenth century. In this chapter we present a very short overview of the history of seismology (brief information about pertinent historical developments can also be found in each chapter), a discussion of seismology considered as a multidisciplinary science, its theoretical and observational aspects, international cooperation, and a summary of books, journals, and websites.

The historical development

In antiquity, the first rational explanations of earthquakes, beyond mythical stories, are from Greek natural philosophers beginning with Thales of Miletos in the sixth century BC. Aristotle (in the fourth century BC) discussed the nature and origin of earthquakes in the second book of his treatise on meteors (Meteorologica). The term meteors was used by the ancient Greeks for a variety of phenomena believed to take place somewhere above the Earth's surface and below the orbit of the Moon, such as rain, wind, thunder, lightning, comets, but also earthquakes and volcanic eruptions inside the Earth. The term meteorology derives from this word, but in modern use it refers only to atmospheric phenomena. Aristotle, following other Greek authors, such as Anaxagoras, Empedocles, and Democritus, proposed that the cause of earthquakes consists in the shaking of the Earth due to underground dry heated vapors or winds trapped in its interior and trying to leave toward the exterior. This explanation was part of his general theory for all meteors caused by various types of exhalations of gas or vapor (anathymiaseis) that extend from inside the Earth to the Lunar orbit. This theory was spread more widely by the encyclopedic Roman authors Lucius Anneus Seneca and Gaius Plinius (Pliny the Elder). It was commented upon by medieval philosophers such as Albert the Great and Thomas of Aquinus, and, with small changes, was accepted in the West until the seventeenth century.

Summary

In the previous chapter we have considered a simple isotropic point model for the earthquake source. This model was enough to determine its origin time, location, and size. We have already mentioned in Chapter 2, in a descriptive way, that most earthquakes are caused by breaking on faults due to acting tectonic stresses. In this chapter we will present basic ideas about the source mechanism. The problem with the source mechanism is in relating observed seismic waves to the parameters that describe the source. In the direct problem, theoretical seismic wave displacements are determined from source models, whereas in the inverse problem, the parameters of source models are derived from observed wave displacements. The first step in both problems is to define the seismic source in terms of a mechanical model that represents the physical fracture and derive from this the elastic wave displacement field. These models or representations of the source are defined by parameters whose number depends on their complexity. Simple models are defined by a few parameters, whereas more complex ones require a larger number of parameters (Madariaga, 1983; Udías, 1991; Koyama, 1997; Udías et al., 2014). We begin with formulation in terms of equivalent forces to proceed to those of fractures and dislocations. For mathematical formulation of the problem, a Green's function for an infinite medium is presented and applied to the problem of radiation of seismic waves from a point source shear dislocation.

Earthquakes and faults

The causes of earthquakes have interested man since antiquity. As was mentioned in Section 1.1, various ideas have been proposed from the time of the ancient Greek natural philosophers up to the present day. During the nineteenth century, systematic field studies were begun following earthquakes and the first attempts to relate these to tectonic processes were made by Mallet (Naples, Italy, 1857), Koto (Neo, Japan, 1891), and Oldham (Assan, India, 1897), among others. With increasing numbers of field observations and improving precision of localization of epicenters, the correlation between earthquakes and faults became clearer. Authors such as Suess, Koto, Montessus de Ballore, and Sieberg assigned the cause of earthquakes to stress accumulated in the Earth's crust by tectonic processes and their release by its fracture.

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3 - Instrumentation and Digital Data Processing

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27 - Characteristics of Laser Beams

15 - Anelasticity and Anisotropy

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9 - Implications for Language Theory

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8 - One-word Constructions: Particles

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Chapter 2 - Turn Construction and Turn Taking

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2 - Earthquakes, Earth Structure, and Dynamics

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13 - Fresnel Diffraction

Appendix 4 - Fourier and Laplace Transforms

Part II - How are Linguistic Resources Deployed in Interaction?

1 - Seismology, the Science of Earthquakes

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5 - Superposition of Waves

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17 - Basic Theory of Earthquake Mechanism

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