150Lec7-2

150Lec7-2 - Lecture #7: Introduction to Earthquakes...

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Lecture #7: Introduction to Earthquakes (Abbott, pp. 86-99, 152-153) Causes of Earthquakes and the Elastic Rebound Model Most, but not all, earthquakes are associated with active plate boundaries and hot spots (Figure 3.9, p. 56). The worst earthquake in American history occurred in Anchorage, Alaska in 1964 (Table 6.1, p. 133). Surprisingly, though, some of the worst US earthquakes occurred in 1811-1812 along the New Madrid fault (Table 7.1, p. 163), which is not associated with any known modern plate boundary. The fault is located in western Kentucky, western Tennessee, northeastern Arkansas, and southeastern Missouri (Figure 7.18, p. 174). Traditionally, the elastic rebound model (Figure 6.32, p. 152) has been used to describe the build up of energy and earthquake generation along faults, such as the San Andreas in California. Slow movements along the fault cause deformation and the buildup of stress (forces on the rocks; Lecture #6) along the fault (Figure 6.32a-b, p. 152). The stress builds up to a point where the stress overcomes the friction between the rocks in the fault. The fault ruptures and produces an earthquake. During the earthquake, the movement of the rocks on one or both sides of the fault releases some of the stress (Figure 6.32c, p. 152). Geologists now recognize that an earthquake may not release all of the stress on a fault, but the stress is often relieved by a series of earthquakes (p. 153) involving aftershocks (p. 95). In some cases, the slow movements associated with the build up of stress along a fault can be detected with satellite global positioning system (GPS) stations (p. 157). If the buildup of stress is detected, the information may be used to evaluate the future probabilities of an earthquake in an area. Elastic Rebound Model: After an earthquake, offsets or displacements along the fault may be very noticeable, especially on a strike-slip fault (Figure 4.2, p. 79). That is, fences, roads, walls, and other features may be separated on either side of the fault.
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Characteristics of Waves Earthquakes produce seismic waves on and in the Earth much like a pebble disturbing the surface of a pond sends waves through the pond. Like other waves, a seismic wave may be described by its wavelength, amplitude, frequency and period (p. 88). The crest is the top of the wave and the bottom is the trough . The wavelength is the lateral distance from the crest of one wave to the crest of the next wave (Figure 4.17, p. 88). The amplitude is the vertical distance between the top of the crest and the "equator" of the wave or the location half-way between the crest and trough (Figure 4-17, p. 88). Wavelengths and amplitudes are measured in millimeters, centimeters, meters or other units of length . Components of a Wave: Crest, Trough, Wavelength, Amplitude, Frequency and Period
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150Lec7-2 - Lecture #7: Introduction to Earthquakes...

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