Geography
Earthquakes
Earthquakes are natural phenomena characterized by the sudden release of energy in the Earth's crust, resulting in seismic waves. They can cause destruction and loss of life, and are often associated with tectonic plate boundaries. The magnitude of an earthquake is measured on the Richter scale, with larger magnitudes indicating more powerful seismic activity.
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10 Key excerpts on "Earthquakes"
eBook - PDF- Charles Fletcher, Dan Gibson, Kevin Ansdell(Authors)
- 2014(Publication Date)
- Wiley(Publisher)
An earthquake is a sudden shaking of the crust, usually caused by the rupture of a fault. Earthquakes produce seismic waves that scientists can use to improve our understanding of Earth’s inte- rior. Earthquakes are also dangerous geologic hazards that are responsible for enormous damage to many communities, such as that shown in the opening photo of Beichuan, China, dam- aged by the 2008 Sichuan earthquake. More than 380 major cit- ies lie on or near unstable regions of Earth’s crust, and, because of swelling urban populations, geologists fear that a devastating earthquake capable of killing 1 million people could occur this century. But crowded inner cities are not the only concern— global growth in human population in the past 100 years means that people now live in areas that were previously remote and thought to be too dangerous for communities. Based on knowl- edge of seismic risk, designing and constructing buildings that can withstand shaking and establishing effective earthquake re- sponse and disaster assistance networks have become important elements of managing these hazards in our communities. 303 12-1 An earthquake is a sudden shaking of the crust. LO 12-1 Describe why the risk from Earthquakes has increased in recent decades. 12-2 There are several types of earthquake hazards. LO 12-2 List and describe earthquake hazards. 12-3 The elastic rebound theory explains the origin of Earthquakes. LO 12-3 Define the elastic rebound theory. 12-4 Most Earthquakes occur at plate boundaries, but intraplate seismicity is also common. LO 12-4 Describe the relationship of Earthquakes to plate boundaries. 12-5 Divergent, convergent, and transform boundaries are the sites of frequent earthquake activity. LO 12-5 Compare and contrast seismicity at divergent, convergent, and transform plate boundaries. 12-6 Earthquakes produce four kinds of seismic waves. LO 12-6 List and describe the four kinds of seismic waves.
- Jonathan Hinks(Author)
- 2023(Publication Date)
- ICE Publishing(Publisher)
2.2. What is an earthquake? Earthquakes are the result of sudden movement along faults within the earth’s crust that cause the release of stored-up elastic strain energy in the form of seismic waves that propagate through the earth and cause the ground surface to shake. The ground shaking causes damage to surface structures because inertial forces cause the centre of gravity of buildings to move relative to their base or founda- tion. Such movement on faults is generally a response to long-term deformation and build-up of stress. Following the great San Francisco earthquake in 1906, Harry Fielding Reid put forward his elastic rebound theory, in which he suggested that the earthquake was the result of the sudden release of previ- ously stored elastic strain energy through the sudden movement on the fault (Reid, 1910). Reid proposed that distant forces acting in opposite directions result in the accumulation of strain energy along the fault over hundreds of years (Figure 2.1). For long periods of time the fault remains locked in place, but eventually the accumulated strain overcomes the friction between the rocks on either side of the fault and an earthquake occurs, as in 1906. During an earthquake, the rocks snap back into their original undeformed state, releasing the accumulated strain. The strain energy that has accumulated gradually over many years is released in just a few seconds. Reid’s theory is largely supported by precise global positioning system measurements, although it fails to answer questions such as how the Young’s modu- lus and shear modulus of the rock vary with depth. Earthquake Engineering for Dams and Reservoirs 4 The size of any earthquake depends on both the area of the fault that ruptures and the average amount of slip or displacement on the rupture plane. Larger rupture areas and larger displacements lead to larger Earthquakes.
eBook - PDF- Victor Gioncu, Federico Mazzolani(Authors)
- 2010(Publication Date)
- CRC Press(Publisher)
These movements of Earth masses produce a division of the crust in some portions called tectonic plates, moving in different directions and with different velocities (Fig. 2.2). Sometimes equilibrium among plates exists due to the shear forces along the plate boundaries, named faults. But when the strain energy is exceeded in some fault portions, a sudden slipping occurs, being the source of the earthquake. The comparison between the distribution of the epicenters of strong Earthquakes and the tectonic plate borders shows very clearly that the main cause of Earthquakes is the relative movements of tectonic plates. The majority of Earthquakes in the world occur along these borders, named interplate Earthquakes, but there are also Earthquakes shaking the zones within the plate itself, far from the plate borders, named intra-plate Earthquakes. Earthquakes occur everywhere through the world. Any minute of a day, the Internet site for the European-Mediterranean Seismological Centre (EMSC) presents a map with the European Earthquakes produced in the last few hours, days or weeks. The US Geological Survey gives the same information about the world Earthquakes (Fig. 2.3 shows an example of this, with the strong South Taiwan earthquake). The continuous movement of the Earth can be noticed by the fact that every day thousands of small ground movements are recorded in the world, every week a moderate earthquake is recorded in some place. At least one significant earthquake causing damage and injuries occurs every month, while every year two or three strong Earthquakes produce important economic losses, killing thousands of people. Statistically, the frequency of earthquake occurrence is given in Table 2.1.
eBook - PDFThe Blue Planet
An Introduction to Earth System Science
- Brian J. Skinner, Barbara W. Murck(Authors)
- 2011(Publication Date)
- Wiley(Publisher)
Earthquakes: WHEN ROCKS SHIFT They can be devastating in terms of human impacts, but Earthquakes are of enormous scientific importance. They are by far the most powerful way to study Earth’s interior and the workings of the geosphere. The way Earth vibrates after a large quake is controlled by the properties of the rocks inside. Earthquake vibrations can be used to study Earth’s interior, in the same way that a doctor uses X-rays or CT scans to study the inside of a human body—they are remote sensing probes that allow scientists to sense and measure the characteristics of the materials beneath our feet. Earthquake activity, or seismicity, also plays an essential role in outlining the boundaries of tectonic plates. The study of Earthquakes, together with the information derived from volcanism, has allowed scientists to decipher the mechanisms by which plates are formed at spreading centers and con- sumed in subduction zones. What Is an Earthquake? Place a thin stick of wood across your knees. Press both ends downward, and the wood bends. Stop pressing and the wood springs back to its original shape. A change in shape or size of a body is called deformation, and any change of shape or size that reverses when the deforming force is removed is called elastic deformation. The muscle energy you used to bend the wood doesn’t disappear—it is stored as elastic energy in the wood. When the bending force is removed, it is this elastic energy that restores the wood to its original shape. Consider what happens, how- ever, when the pressure is so great that the elastic limit of the wood is exceeded: The stick breaks with a sudden snap, and the stored elastic energy is released all at once (FIG. 6.1). The elastic energy is converted partly to heat at the breaking point in the wood, partly to sound waves that make the snapping noise, and partly to vibrations in the wood. Earthquake vibrations are the same kind of vibrations that you feel when the wood breaks.
No longer available |Learn more- Donald Hyndman, David Hyndman(Authors)
- 2016(Publication Date)
- Cengage Learning EMEA(Publisher)
Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it. Earthquakes and Their Causes 61 Key Points Faults and Earthquakes ■ Faults move in both vertical and lateral motions controlled by earth stresses in different orientations. FIGURE 3-1 and By the Numbers 3-1. ■ Earthquakes are caused when stresses in Earth deform or strain rocks until they finally snap. Small strains may be elastic, where the stress is relieved and the rocks return to their original shape; plastic, where the shape change is permanent; or brittle,where the rocks break in an earthquake. FIGURES 3-3 and 3-4. ■ The size of an earthquake is related to the surface rupture length, and the offset indicates how much the fault moved. Faults can move continuously, through creep, or a sudden snap. FIGURES 3-6 and 3-7. Tectonic Environments of Faults ■ Earthquakes typically occur along plate boundaries. Strike-slip faults move along transform boundaries such as along the San Andreas Fault in western California; thrust faults are typically associated with subduction zones such as in the Pacific Northwest and continent–continent collision boundaries; and normal faults move in spreading zones such as in the Basin and Range area. FIGURES 3-8 to 3-13. ■ Intraplate Earthquakes, though less frequent, can also be quite large as in the case of the New Madrid, Missouri, events of 1811–1812. FIGURES 3-14 and 3-15. ■ Large Earthquakes along the eastern fringe of North America are less frequent but can be significant and highly damaging. FIGURES 3-15 and 3-16. Earthquake Waves ■ When a fault first slips in an earthquake, energy travels outward from the focus of the earthquake in the form of seismic waves.
eBook - PDFGeology
Earth in Perspective
- Reed Wicander, James Monroe, Reed Wicander(Authors)
- 2019(Publication Date)
- Cengage Learning EMEA(Publisher)
Copyright 2021 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it. 164 CHAPTER 8 Earthquakes and Earth’s Interior THE DESTRUCTIVE EFFECTS OF Earthquakes LO19 Discuss the factors that determine the destructive- ness of an earthquake LO20 List the four major destructive effects caused by Earthquakes LO21 Discuss what the outcomes are for each type of destructive event The number of deaths and injuries, as well as the amount of property damage resulting from an earthquake, depends on several factors. Generally speaking, Earthquakes that occur during working and school hours in densely populated urban areas are the most destructive and cause the most fatalities and injuries. However, magnitude, duration of shaking, distance from the epicenter, geology of the affected region, and the type of structures are also important consid- erations. Given these variables, it should not be surprising that a comparatively small earthquake can have disastrous effects whereas a much larger one might go largely unno- ticed, except perhaps by seismologists. The destructive effects of Earthquakes include ground shaking, fire, seismic sea waves, and landslides, as well as panic, disruption of vital services, and psychological shock. Ground Shaking Ground shaking, the most obvious and immediate effect of an earthquake, varies depending on the earthquake’s mag- nitude, distance from the epicenter, and type of underlying materials in the area—unconsolidated sediment or fill ver- sus bedrock, for instance.
eBook - PDF- Barbara W. Murck, Brian J. Skinner(Authors)
- 2016(Publication Date)
- Wiley(Publisher)
It was the seven- teenth giant earthquake of magnitude 8.5 or higher that has occurred, worldwide, since 1900 (Figure 5.1). Almost all of these giant quakes were in subduction zones and are referred to as megathrust Earthquakes. The one exception is the Assam, Tibet, quake of 1950, which was located in a conti- nental collision zone. REMEMBER THIS! Do you remember how the distribu- tion of earthquake locations is related to plate boundaries? You can remind yourself by reviewing Figures 4.7 and 4.8. The close association between subduction zones and giant Earthquakes suggests that the convergent motion of two plates must be responsible for the very largest quakes. But plate motion is very gradual, typically on the order of a few centimeters per year. Why, then, should Earthquakes be so sudden and the big ones so cat- astrophic? And why, after long time intervals, do they recur in the same places? Through the science of seismology we seek to answer these and related questions. Earthquakes and Plate Motion Most Earthquakes are caused by the sudden movement of stressed blocks of Earth’s crust along a fault. If the rocks could slide past one another smoothly, like the parts of a well-oiled engine, big quakes such as the Tōhoku quake would not happen. In the real world, smooth sliding is rare; friction between the huge blocks of rock causes them to seize up, bringing the motion along the locked part of the fault to a temporary stop. While the fault remains locked by friction, energy continues to build up as a result of the plate motion, causing rocks adjacent to the jammed section to bend and buckle. Finally, the stress becomes great enough to over- come the friction along the fault. All at once, the blocks slip, and the pent-up energy in the rocks is released as the vio- lent tremors of an earthquake. This cycle of slow buildup of energy followed by abrupt movement along a fault repeats itself many times.
eBook - PDFEarth Science
An Introduction
- Mark Hendrix, Graham Thompson, Mark Hendrix(Authors)
- 2020(Publication Date)
- Cengage Learning EMEA(Publisher)
LO8 Discuss the occurrence of Earthquakes in the interior portion of a tectonic plate. LO9 Indicate the effect various soil types have on structures during an earthquake. LO10 Explain how wealth can affect the mortality rate during an earthquake. LO11 Describe the formation and warning signs of a tsunami. LO12 Give three examples of how seismic waves have helped geologists understand the structure of Earth’s interior. LO13 Explain how having a liquid outer core contributes to Earth’s magnetic field. INTRODUCTION As we learned in Chapter 6, the Earth’s tectonic plates slide slowly over the soft asthenosphere, about as fast as your fingernails grow. Friction often holds the two sides of a plate boundary locked together. Rocks stretch and compress across the boundary as forces build, but other- wise nothing big happens for long periods of time that can range from decades to multiple millennia. Then, suddenly, the two sides of the boundary snap free, releasing all at once the energy that had built up over time and causing an earthquake. An earthquake is a classic example of a threshold effect. The interiors of plates move at a steady rate, but motion is constrained at plate boundaries, causing rocks there to slowly accumulate energy from ongoing plate movements. Once the threshold level of energy has accu- mulated, it is suddenly and cataclysmically released as an earthquake. ANATOMY OF AN EARTHQUAKE Rock appears rigid, but if you apply enough stress, rock will deform. When stress is applied to a rock, the rock can deform in one of three ways: (1) elastically, (2) by fracturing, or (3) plastically. Under small amounts of stress, the rock deforms elasti- cally. If the stress is removed, the rock returns to its original size and shape. A rubber band deforms elastically when you stretch it. The energy used to stretch the rubber band is stored in the elongated rubber. When the stress is removed, it springs back and releases the stored energy.
eBook - PDF- Tom Freeman(Author)
- 2012(Publication Date)
- Wiley(Publisher)
Earthquakes 129 Figure 8.1 Some 21 feet of plastic strain across the San Andreas fault—exhibited here by distorted utility lines and such—accumulated before the correction of 1906. 8 Earthquakes Topics A. What is the physical explanation for why Earthquakes occur? B. What are the three types of earthquake waves, and how do they differ one from another? How are Earthquakes recorded, and how is the distance between a seismograph station and an earthquake determined? C. How is the location of an earthquake determined? D. What are the two common scales of earthquake severity, and how do they differ one from another? Why aren’t fields of Mercalli intensity values bounded by perfect circles? E. How is the Richter magnitude of a distant earthquake determined? How is the moment magnitude of an earthquake measured? Why are there no Earthquakes of magnitude 11.0? How does the geology of the Northridge are explain such extreme ground acceleration? F. Why the greater number of fatalities from Earthquakes in underdeveloped countries? What is the ShakeMap project? What is the “Did you feel it?” project? G. What is the methodology in determining the location of a quake in southeast Missouri? H. What geologic feature caused the Sumatra tsunami? What geologic feature saved countless lives in Bangladesh? Earthquakes are produced by abrupt motion along a fault when friction that resists such motion is overcome by stress (Fig. 8.1). This is called elastic rebound. It’s quite analogous to the A. What causes Earthquakes? bending and breaking of a green twig. Some 21 feet of abrupt adjustment along the San Andreas Fault generated the fateful San Francisco earthquake of 1906. Most Earthquakes are produced by movement along a fault. But movement is not along the entire fault during any one event. Instead, movement involves a region of the fault measured over a few kilometers. The place within that region where movement first occurs is called the focus.
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.
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