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Faults are surfaces along which rocks have fractured and been displaced. There are three major types of faults: strike-slip, normal, and reverse. The tectonic stresses caused by plate motions (see previous section) build up over time and eventually cause breaks in the crust of the Earth along which the rocks sporadically grind past one another. When this happens, earthquakes occur.
Each different type of plate boundary is characterized by one of the three main types of faulting. Normal faulting is associated with crustal extension, and so can be found at divergent boundaries. Reverse faulting is associated with crustal shortening, and so can be found at convergent boundaries. Strike-slip faulting is associated with lateral motion of the crust, and so can be found at transform boundaries. The following images show pictures of actual faults in Southern California. The first is a photograph of a trench that was excavated across the Raymond Fault in the city of San Marino by James Dolan and his graduate student Kristin Weaver (University of Southern California). They found several displaced layers caused by earthquakes on the fault, including the dark, organic-rich layer we see displaced in the photograph. Using material within this layer in order to find a radiocarbon date, they can then determine when the event that displaced this layer occurred. This can help to constrain the rate at which the Raymond fault is moving.
The next few photographs are of the Sierra Madre fault, a north-dipping thrust fault that runs along the front of the San Gabriel mountains.
Notice that the two layers are separated by a gray layer that looks very ground up. This layer separating the two blocks of the fault is known as fault gouge, and consists of broken up and smashed pieces of rock (some pieces are so small that they form clay) which forms as the result of the two sides of the fault moving against one another.
Structure
of the Earth
Last modified on 10/15/98 by Maggi Glasscoe (scignedu@jpl.nasa.gov)
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