Chapter 7 - Chapter 7 General Review mechanism 5 Ocean...

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Chapter 7 General Review mechanism 5 Ocean floor vs. continental lithosphere o In fact, most transform faults are found on ocean floors in exactly the role shown in the figure: offsetting spreading ridges. The earthquakes that result from motions on those breaks are small simply because the ocean lithosphere is thin and little stress can be built up. So, this is not the location of dangerous transform fault seismic activity. Also, because the motion is mostly in a horizontal sense, there s no significant vertical displacement of water, thus no tsunami generation. Creep o Sections that do not lock (i.e. experience a series of small motions and thus tiny earthquakes, are said to creep Locked sections = seismic gaps o The locked sections can go for some period of time without any significant movement (thus no significant earthquakes) are known as seismic gaps. Case Study 1: San Andreas Fault Be sure you understand how/why San Andreas developed, what the relative motion is, how much annual movement on creep sections o In Chapter 5 I showed how subduction of the Farallon Plate beneath the North American Plate has left only a few small fragments, the most interesting to us being the Juan de Fuca Plate. The San Andreas Fault developed out of this same activity. o As more and more of the Farallon went under, the transform fault lengthened to the south and to the north; that transform fault is the San Andreas. o The San Andreas Fault of California (Fig. 3) is exactly such an example ( and we’ll look at it in a case study in this chapter ). Because of the great thickness of continental crust crossed by these faults, sections get “locked” by friction while great tension builds; eventually the strength of the rock is overcome, and the release of energy produces devastating earthquakes. o
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Chapter 7 - Chapter 7 General Review mechanism 5 Ocean...

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