Physics
Seismic Waves
Seismic waves are vibrations that travel through the Earth as a result of sudden energy release, such as from an earthquake or explosion. They can be classified into two main types: body waves, which travel through the Earth's interior, and surface waves, which travel along the Earth's surface. These waves provide valuable information about the Earth's interior and are used in seismology to study earthquakes and the structure of the Earth.
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11 Key excerpts on "Seismic Waves"
- Tanjina Nur(Author)
- 2019(Publication Date)
- Arcler Press(Publisher)
• The major breakthrough in the field of seismology is made by E. Weichert by the development of seismometer with viscous damping which is capable of producing a useful record for the whole duration of ground shaking. 3.4. SEISMIC WAVE The waves of energy caused by the sudden breaking of rock within the earth or an explosion are called Seismic Waves. Seismic Waves are the vibrations caused by an earthquake, explosion, and movement of lava inside the earth or any other source of energy which travels inside the earth (Figure 3.3). To record the Seismic Waves which travel beneath and on the surface of the earth seismograph is made. It records the amplitude and the frequency of the Seismic Waves and the data generates the information regarding the earth and its subsurface structure. In oil and gas industry and in civil engineering, artificially produced Seismic Waves which are generated during the seismic survey are used. Engineering Seismology 59 Figure 3.3. Seismic Waves. Source: https://www.sciencelearn.org.nz/system/images/images/000/000/352/ embed/Seismic-waves20151005–11221-bfwvp3.jpg?1522294003 Seismic Waves are broadly divided into two types. They are known as body waves and surface waves. The waves which propagate inside the earth’s inner layer is called body waves whereas the waves which move along the surface of the earth just as ripples of water are called surface waves. Both body and surface waves are produced during an earthquake. 3.4.1. Body Waves The waves which propagate inside the earth are known as body waves. It arrives before the surface waves during an earthquake. The frequency of body waves is more than that of surface waves. There are two types of body wave’s namely primary waves or p waves and secondary or s waves. Primary waves are the fastest Seismic Waves and therefore it is first recorded too in a seismograph (Figure 3.4). Figure 3.4. Body waves.
eBook - PDF- William Lowrie, Andreas Fichtner(Authors)
- 2020(Publication Date)
- Cambridge University Press(Publisher)
6.2 Seismic Waves 6.2.1 Introduction The propagation of Seismic Waves through the Earth can be triggered by a wide range of sources, including earthquakes, man-made explosions, volcanic eruptions, landslides, and meteorite impacts. Close to the source, the medium is often permanently destroyed, which means that the deformation is non-recoverable. However, at some distance from the source, deformation amplitudes are small and the medium deforms (an)elastically. The particles of the medium carry out harmonic oscillations, and energy is transmitted over large distances in the form of waves. Seismic Waves can be classified into two main types. When energy is released near the surface (Fig. 6.2), part of it pro- pagates through the body of the medium as body waves. The remaining energy spreads out over the surface as surface waves, analogous to the ripples on the surface of a pool of water into which a stone has been thrown. 6.2.2 Seismic Body Waves When a body wave reaches a distance r from its source in a homogeneous medium, the wavefront (defined as the surface in which all particles vibrate with the same phase) has a spherical shape, and the wave is called a spherical wave. Fig. 6.1 Seismograms recorded at a seismological station in the Swiss Alps documenting a large, magnitude 7.3 earthquake that occurred on November 12, 2017 in the Zagros mountains in Iran. (a) Vertical component of ground displacement at a period of around 10 s. (b)–(d) North–south component of ground displacement at periods of around 10 s, 30 s, and 100 s. A selection of seismic wave types, explained in Section 6.2, is indicated. Each of these waves propagates through different parts of the Earth.
No longer available |Learn more- (Author)
- 2014(Publication Date)
- Academic Studio(Publisher)
________________________ WORLD TECHNOLOGIES ________________________ Chapter 5 Seismic Wave Body waves and surface waves ________________________ WORLD TECHNOLOGIES ________________________ p-wave and s-wave from seismograph Seismic Waves are waves of energy that travel through the earth or other elastic bodies, for example as a result of an earthquake, explosion, or some other process that imparts low-frequency acoustic energy. Seismic Waves are studied by seismologists and geophy-sicists. Seismic wavefields are measured by a seismograph, geophone, hydrophone (in water), or accelerometer. The propagation velocity of the waves depends on density and elasticity of the medium. Velocity tends to increase with depth, and ranges from approximately 2 to 8 km/s in the Earth's crust up to 13 km/s in the deep mantle. Earthquakes create various types of waves with different velocities; when reaching seismic observatories, their different travel time enables the scientists to locate the epicenter. In geophysics the refraction or reflection of Seismic Waves is used for research of the Earth's interior, and artificial vibrations to investigate subsurface structures. Types of Seismic Waves There are two types of Seismic Waves, body waves and surface waves . Other modes of wave propagation exist than those described in this article, but they are of comparatively minor importance for earth-borne waves, although they are important in the case of asteroseismology, especially helioseismology. ________________________ WORLD TECHNOLOGIES ________________________ Body waves Body waves travel through the interior of the Earth. They follow raypaths refracted by the varying density and modulus (stiffness) of the Earth's interior. The density and modulus, in turn, vary according to temperature, composition, and phase. This effect is similar to the refraction of light waves.
eBook - PDF- Dentcho Ivanov(Author)
- 2015(Publication Date)
- CRC Press(Publisher)
1 Origin of Earthquakes and Seismic Waves During a disturbance deep in the Earth’s crust or mantle such as faults, tectonic plate shifts, volcano eruptions or underground explosions, a large amount of energy is released as a result of which seismic longitudinal (P-waves), shear (S-waves), and surface waves propagate, causing an earthquake. This cause-and-effect process seems simple and easy to understand. However for describing step by step the generation and propagation of various types of Seismic Waves, many factors should be taken into consideration. Deep seismic disturbances cannot be directly studied, but observations of man-caused explosions causing tremors point to the generation of short, powerful, pressure pulses reaching Mega or even Giga Pascal ranges in the focus of the earthquake. However, a pressure pulse is not a wave. A wave is a periodic transient motion in a medium of propagation that requires a continuous driving force. The question is how a pressure pulse becomes the origin of a seismic wave? Of course, in an earthquake’s focus not one, but a raw of pressure pulses can occur. They follow each other at random intervals of time and the question of the origin of Seismic Waves is still relevant. In this chapter we will discuss the mechanism of the generation of various kinds of transient Seismic Waves from a high energy pressure pulse that are able to travel long distances in a heterogeneous, dispersive, and dissipative media causing what we call an earthquake. 1.1 Statement of the problem Seismic Waves are usually assumed to be elastic waves that propagate in the Earth’s body. When an elastic wave propagates in some material it applies 8 Seismic Resistant Design and Technology periodic mechanical force per unit of surface F 1 (stress) resulting in a periodic deformation (strain). The material reacts to the deformation with a restoring force – F 2 trying to bring the material’s initial shape back to its equilibrium state.
eBook - PDFThe Solid Earth
An Introduction to Global Geophysics
- C. M. R. Fowler(Author)
- 2004(Publication Date)
- Cambridge University Press(Publisher)
Chapter 4 Seismology Measuring the interior 4.1 Waves through the Earth 4.1.1 Introduction Seismology is the study of the passage of elastic waves through the Earth. It is arguably the most powerful method available for studying the structure of the interior of the Earth, especially the crust and mantle. There are various other geo-physical techniques, including the study of gravity, magnetism and the electrical properties of the Earth, which can be applied on scales ranging from the planet as a whole to large regions or small areas or even individual rock samples (Telford et al . 1990; Dobrin and Savit 1988); but seismology is probably the most widely used and the most informative. This chapter discusses the methods by which we obtain information about the interior of the planet from the study of elastic waves passing through the Earth. Earthquake seismology is perhaps the best technique for investigating the Earth’s interior. The study of earthquakes was of major significance in giving us our understanding of plate tectonics: earthquake foci have delineated the bound-aries of the tectonic plates very accurately. It has also helped us to map the internal structure of our planet. The distribution of earthquakes shows us where the Earth is active (mostly near the surface), and the passage of Seismic Waves through the Earth allows us, as it were, to CAT-scan its interior. When an earthquake or an explosion occurs within the Earth, part of the energy released takes the form of elastic waves that are transmitted through the Earth. These waves can be detected by an instrument called a seismograph , 1 consisting of a seismometer , which measures and amplifies the motion of the ground on which is rests, and a recorder, which transfers the data onto paper, magnetic tape or disc. The speed with which these elastic waves travel depends on the density and elastic moduli of the rocks through which the waves pass.
eBook - ePubIntroduction to Earth Sciences
A Physics Approach
- Luc Thomas Ikelle(Author)
- 2017(Publication Date)
- WSPC(Publisher)
Part A
Earthquake and Volcanoes
Passage contains an image
Chapter 2: Seismic Waves and Evidences of Earth’s Interior Structure
1.Seismic Waves 1.1.Wave propagation and seismic data 1.2.Body waves: P- and S-waves 1.3.Reflection, refraction, transmission, and diffraction 1.4.Surface waves: Love and Raleigh waves 2.EARTH’S INTERIOR: EVIDENCE 2.1.The crust/mantle boundary 2.2.The mantle/core boundary: the P-wave and S-wave shadowed zones 2.3.The outer-core/inner-core boundary 2.4.Scientific drilling programsHow can scientists — more precisely geophysicists and geochemists — find out what is happening deep inside the earth? The temperatures are too high, the pressures are extreme, and the distances are too vast for drilling (see Figure 1.8 ). How did geophysicists determine that the mantle is solid, the outer core is liquid, and the inner core is solid? To do so, scientists relied on Seismic Waves (elastic waves) — waves generated by earthquakes and explosions that travel through the earth and across its surface to reveal the structure of the interior of the planet. Thousands of earthquakes occur every year, and each one provides a glimpse of the earth’s interior.Seismic signals associated with these earthquakes consist of several kinds of waves. Those important for understanding the earth’s interior are P-waves (also known as primary waves, compressional waves, or longitudinal waves) and S-waves (also known as secondary or shear waves), which travel through solid, liquid, and gaseous materials in different ways. There are also surface waves. Our objective in this chapter is to introduce Seismic Waves and describe how these waves allow us to understand the solid earth’s interior.1Seismic Waves
1.1Wave propagation and seismic data
There is nothing more important in the education of geophysicists than developing their understanding of and intuition about how Seismic Waves propagate in the ground. Suppose that you are in a dark room in the library, surrounded by a multitude of books. You are looking for a particular book. You will need a flashlight to guide you to the specific row and column where you can locate this desired book. The problem of exploring solid earth is quite similar. We are trying to see through a dark and compact Earth, and Seismic Waves are one of the flashlights which help us to see beneath the earth’s surface. So we start this section by describing wave propagation.
eBook - PDF- François Henri Cornet(Author)
- 2015(Publication Date)
- Cambridge University Press(Publisher)
Longer periods are usually investigated in geodetics and higher frequencies in rock physics. Because wave propagation depends on the mechanical characteristics of the material through which the waves propagate, the analysis of Seismic Waves may be used to investi- gate the various structural elements that have influenced the wave train on its travel from the source to the receiver. For example, analysis of the seismic signals generated by large earth- quakes has helped refine our understanding of the deeper structure of the earth. Analysis of the mechanical waves observed in other planets, and even stars, is one of the few means available to understand the structure of these celestial objects. In order to exploit the information that may be retrieved from mechanical waves for characterizing the medium through which the waves have propagated, artificial sources have been developed for specific needs, such as the exploration of hydrocarbon reservoirs or the monitoring of the development of unstable zones in underground workings. The words “seismics” and “seismic engineering” often refer to the analysis of waves generated by engineered vibratory sources. The objective of this chapter is to introduce elementary concepts of wave propaga- tion and natural sources of vibration. These elements of seismology are more and more 283 284 Elements of seismology often applied to various engineering problems such as mitigating man-made seismicity. They also help us to understand present-day natural deformation processes, as will be discussed in chapter 14, and have proved to be helpfull in gaining an efficient under- standing of deep fluid–solid interactions; they also shed light on the relationship between seismic and non-seismic deformation processes. These concepts may be used to vali- date, or invalidate, numerical models of rock mass perturbations induced by engineering undertakings.
eBook - ePub- Thorne Lay, Terry C. Wallace(Authors)
- 1995(Publication Date)
- Academic Press(Publisher)
International Geophysics , Vol. 58, Suppl. (C), 1995ISSN: 0074-6142doi: 10.1016/S0074-6142(05)80003-8Chapter 2 Elasticity and Seismic WavesSeismology involves analysis of ground motions produced by energy sources within the Earth, such as earthquake faulting or explosions. Except in the immediate vicinity of the source, most of the ground motion is ephemeral; the ground returns to its initial position after the transient motions have subsided. Vibrations of this type involve small elastic deformations, or strains , in response to internal forces in the rock, or stresses. The theory of elasticity provides mathematical relationships between the stresses and strains in the medium, and it has spawned a vast literature filled with theory and empirical documentation of elastic behavior. Here we develop only the basics of the theory of elasticity required for seismological applications, including the concepts of strain and stress, the equations of equilibrium and motion, and the fundamental nature of solutions to the equations of motion: Seismic Waves. Chapters 3 and 4 characterize wave interactions relevant to Seismic Waves in the Earth, and subsequent chapters apply these basic ideas to describe how seismologists study the Earth’s interior and the sources of Seismic Waves.Our development of elasticity follows that typical of texts on solid mechanics, and many more detailed discussions are available, some being listed in the References. In the study of solids, a useful, idealized concept for dealing with macroscopic phenomena is that of a continuum , in which matter is viewed as being continuously distributed in space. Within this continuous material we can define mathematical functions for displacement, strain, or stress fields, which have well-defined continuous spatial derivatives. We will see that applying simple laws of physics to a continuum (continuum mechanics
eBook - PDFPhysical Geology
The Science of Earth
- Charles Fletcher(Author)
- 2017(Publication Date)
- Wiley(Publisher)
Earthquake magnitude is a way to measure the amount of destruction it causes. Earthquake intensity is a measure of the physical effects of shaking and how much damage it caused. Expand Your Thinking 24. What method is typically used by the media to characterize an earthquake? Why? LO 11.9 Describe how seismic wave characteristics result in P-wave and S-wave shadow zones. One of the great discoveries in the science of geophysics was that Seismic Waves from earthquakes can be used to improve our understanding of Earth’s interior. Seismology Is the Study of Seismic Waves to Improve Our Understanding of Earth’s Interior 341 FIGURE 11.25 Seismic Waves both reflect and refract when they encounter the boundary between two materials of differing density. Earthquake focus Earthquake focus Reflected wave Less dense rock More dense rock Refracted wave bent up Reflected wave More dense rock Less dense rock Refracted wave bent down Sunlight refracts when it passes from the air into water. Which way does a light ray refract: upward or downward? Within Earth’s interior, seismic wave velocities generally increase with depth because rock density usually increases with depth and waves travel faster in denser material. As a result, waves are continually refracted along curved paths (called ray paths) that arc gently back toward Earth’s sur- face (Figure 11.26a). However, should there be a sudden change in rock density, the wave velocity will change sud- denly in response; such an interface is called a discontinuity. Discontinuities found at well-mapped depths in the interior of the planet help us mark boundaries between various layers, such as core, mantle, and crust. P waves change the volume of material that they encounter. Hence, any material that resists changing volume, including most liquids and solids, will transmit P waves rather than absorb them.
eBook - PDF- Frank D. Stacey, Paul M. Davis(Authors)
- 2008(Publication Date)
- Cambridge University Press(Publisher)
16.5 Surface waves On seismic records of distant earthquakes (tele-seisms), the waves of greatest amplitudes are generally surface waves that have followed the Earth’s surface and not penetrated the interior. The exceptions are seismograms of deep focus earthquakes, which are not effective generators of surface waves, so that the body waves are more prominent. The dominance of surface waves on teleseismic records is due to the geo-metrical effect of wave spreading. Body waves spread out on wavefronts that are essentially spherical. Thus, the wave energy passing through any element of area diminishes as 1/ r 2 , where r is the distance of travel from the focus. On the other hand, surface waves spread as an expanding circle across the surface. Thus, at near points the energy per unit length of wavefront falls off only as 1/ r , where r is now the radius of the circle. Moreover, this radius does not increase indefinitely but reaches a maximum when the waves have travelled 90 8 and beyond that it decreases again. Body waves are almost non-dispersive and since wave energy density is proportional to the square of wave amplitude, body wave ampli-tudes diminish as 1/ r . The spreading of surface wave energy does not translate as directly into wave amplitudes, because surface waves are strongly dispersive. The waveform changes, becoming spread out in time, or equivalently, distance in the direction of travel. But, in spite of the dispersion, surface wave amplitudes decrease less with distance than do body wave amplitudes. In this circumstance it is convenient that body waves are faster than surface waves and so are not obscured by them on seismic records (Fig. 16.6 ). There are two principal kinds of surface wave, both named after the originators of the theories describing them. Rayleigh waves appear as SV-waves, with a coupled P-wave component, as considered in Section 16.4 , and Love waves propagate in the manner of SH-waves.
eBook - PDFThe Seismic Cycle
From Observation to Modeling
- Frederique Rolandone(Author)
- 2022(Publication Date)
- Wiley-ISTE(Publisher)
1 Determining the Main Characteristics of Earthquakes from Seismological Data Martin V ALLÉE Institut de physique du globe de Paris, Paris Cité University, France 1.1. Introduction This chapter aims to illustrate how waveform modeling of seismograms makes it possible to determine the first-order characteristics of seismic processes. These first-order characteristics are typically the mechanism, magnitude, and location, as well as other information, such as the source duration or the average rupture velocity. Thus, the finer analysis of the process, and in particular the space–time description of the slip on the fault, will not be discussed here. The reader will find this topic addressed in Chapter 2. Section 1.2 shows the typical seismological observations, at far and close distances, which make up the data that we wish to model. Section 1.3 describes how the information from the seismic source is physically transmitted by the waves to the receiver. This section is not a complete theoretical guide to the propagation of elastic waves, but it should help the reader to better understand some fundamental concepts, especially in the case of distant body waves. These concepts are then used in section 1.4, where we present several procedures aiming to characterize seismic sources. The Seismic Cycle, coordinated by Frédérique ROLANDONE. © ISTE Ltd. 2022. The Seismic Cycle: From Observation to Modeling, First Edition. Frédérique Rolandone. © ISTE Ltd 2022. Published by ISTE Ltd and John Wiley & Sons, Inc. 2 The Seismic Cycle 1.2. Observation of the elastic waves generated by earthquakes For over a century now, earthquakes have been regularly detected and located using the elastic waves that they generate. These waves are conventionally recorded by seismometers of different kinds (velocimeters or accelerometers) and, more recently, by high-frequency GPS, when strong earthquakes occur close to the receivers.
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