This 100-mile left step, or so-called "Big Bend", acts to impede motion along the San Andreas' natural course (U.S. Geological Survey 389-396). Thus, the Los Angeles basin and the San Fernando Valley are continuously subject to compressive tectonic forces. In fact, the relative plate motions are currently squeezing the entire area together at a rate of approximately a quarter of an inch per year (Blakeslee A20).
One manifestation of these compressive forces is mountain-building activity. Such ranges as the Santa Monica Mountains and the San Gabriel Mountains are presently being shoved skyward. Moreover, a second manifestation of the compression is the development of a broad system north-dipping and south-dipping subparallel thrust faults. These faults act to absorb any energy created by the Earth's "accordion-like" movement (Blakeslee A20). In fact, the east-striking thrust faults and folds of the Transverse Ranges and Los Angeles basin are so extensive that the United States Geological Survey has aptly named the area the "B
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