Chapter 8: A Violent Pulse: Earthquakes
Study Plan
An oblique air view, generated by a computer, of the San Francisco region. The red lines are faults on which earthquakes have occurred and will in the future. The complex topography of this region results from fault movement.
Credit: Department of the Interior, USGS
Guide to Reading
The topic of this chapter, earthquakes, is often a favorite of readers. After all, quakes are dramatic events, potentially dangerous, and many people live in or visit areas where a quake might happen at any moment. Those who have experienced major quakes aren't always so fond of them. Many testify that it's profoundly disturbing when the good old solid dependable Earth shakes and crashes around you.
Nevertheless, with such widespread interest, everybody basically knows what an earthquake is. But there's more to defining an earthquake than most people realize, and the chapter begins with more to say about the essentials (the what, why, how, where, and when of quakes) than you'd expect. There's seismicity, hypocenter (focus), epicenter, foreshocks, aftershocks, elastic-rebound theory, and stick-slip behavior. Most significant quakes are caused by movement along faults, so you'll read about hanging walls, footwalls, normal faults, reverse faults, thrust faults, strike-slip faults, displacement, and fault scarps and traces.
Keeping track of seismic activity is a worldwide concern. You read about the instrument that detects and records quakes (the seismograph) and how to interpret the seismograms it produces (using arrival times and travel-time curves). Earthquakes come in all sizes, from too small to be detected by humans to real monsters. Measuring their sizes and strengths began with the use of few and comparatively simple scales (Mercalli for quake intensity, Richter for magnitude), but improved technology and increased understanding has led to the complex measurement of numerous aspects of seismic activity (local magnitude, ML; surface-wave magnitude, MS; body-wave magnitude,MB; and moment magnitude, MW).
When the question arises, "Why do quakes occur where they do," guess what the answer is? By now you shouldn't be surprised: plate tectonics. The information in this section isn't new; it's just presented in a way to point out the seismic connections.
The last part of the chapter deals with how quakes affect society. Types of damage from earthquakes vary. Naturally, there's ground shaking and displacement, and depending on the location, there can also be landslides, avalanches, liquefaction, fire, tsunamis, and even widespread disease.
What can people do about earthquakes? We can't stop them. We've had limited success in predicting them (short-term and long-term prediction, recurrence intervals, and seismic gaps). Earthquake zoning and engineering seem to be the best ways to protect human life and property. The chapter ends with a discussion of what society as a whole and you as an individual can do to protect against quake dangers. It also reminds us that no matter what we do, plate tectonics will continue to shift the world and earthquakes will continue to shake it.
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