FIGURE 2.4 The San Andreas Fault on east edge of the Carrizo Plain. Rugged, dissected terrain west (right) of the fault trace is the Elkhorn scarp. Photograph courtesy of R.E. Wallace, U.S. Geological Survey.

or continental rifts, or is converging, as in subduction zones and continental collisions. At these boundaries the faults range in angle from horizontal to vertical. Understanding the diverse types of fault motion in each of these three environments—strike-slip, spreading, or converging—was critical to reconciling the global distribution of earthquakes to large-scale plate tectonic processes.

North of the San Andreas Fault, in the Pacific Northwest, the major plate boundary is convergent, and the ocean crust plunges under the continent. Associated with this subduction zone is a linear chain of active volcanoes extending from British Columbia to Mount Lassen in northern California. The largest earthquakes that have occurred around the world in this century have been located in subduction zones, including the 1964 Alaskan event that devastated Anchorage. Events of comparable size may well occur in the Pacific Northwest.

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