150Lec20-2 - Lecture#20 Earthquakes on Strike-Slip...

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Lecture #20: Earthquakes on Strike-Slip Faults (Abbott, pp. 71-73, 84-86, 132-133, 138-160) Note: Know the names and years of the three major California earthquakes mentioned in this lecture. Also, review Lectures 6-10 and make sure that you understand the various concepts associated with faults and earthquakes, such as focus, epicenter, the different types of faults, P waves, S waves, etc. San Andreas Fault, California As mentioned earlier, the San Andreas Fault is a 1,200 kilometer long right-lateral strike-slip (transform) located on a boundary between the North American and Pacific plates (p. 139; Figure 3.3, p. 52; Figure 6.10, p. 139; Figures 6.11 and 6.12, p. 140; Figure 6.13, p. 141). Although the San Andreas and other strike-slip faults appear very sharp and linear (Figure 6.13, p. 141), they actually consist of very complex zones of fractured and interlocking rocks that extend to depths of several kilometers (p. 84-85). During earthquakes, the rocks fracture, slide and grind past each other ( metamorphism , Lecture #3). The San Andreas also has numerous branching and subparallel faults (Figure 6.10, p. 139; Figure 6.36, p. 156). The San Andreas Fault passes through the Gulf of California and northward into western California. The fault eventually moves off the northern California coast near San Francisco and merges with the right-lateral Mendocino transform fault, which forms the southern border of the oceanic Gorda Plate (Figure 5.12, p. 120). The Gorda Plate is located to the south of the Juan de Fuca Plate (Figure 9.5, p. 220). The continental section of the Pacific Plate is located on the western side of the San Andreas Fault and includes Baja California and much of western California. The "continental" sections of western California also include oceanic crust that has been uplfited above sea level and combined with the continental materials. Western California and Baja California are moving northward. If this northward movement continues, San Francisco will reach Alaska several tens of millions of years from now (p. 133). As shown in Figure 6.1 (p. 132), the San Andreas Fault did not exist 30 million years ago. A subduction zone extended along the entire western coast of North America. The Juan de Fuca , Explorer, Gorda, and Cocos plates were all part of the oceanic Farallon Plate (Figure 6.1, p. 132). The San Andreas formed about 28 million years ago once the spreading zone between the Pacific and Farallon plates subducted (p. 132; Figure 6.1, p. 132). San Andreas Fault of Southern California:
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The San Andreas Fault passes through San Francisco (Figure 6.10, p. 139; Figure 6.27, p. 149; Figure 6.20, p. 145) and about 50 kilometers east of Los Angeles (Figure 6.19, p.
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