INTRODUCTION [c1, p3]

"The San Andreas fault system, a complex of faults that display predominantly large-scale strike slip, is part of an even more complex system of faults, isolated segments of the East Pacific Rise, and scraps of plates lying east of the East Pacific Rise that collectively separate the North American plate from the Pacific plate."

This chapter briefly describes the San Andreas fault system, its setting along the Pacific Ocean margin of North America, its extent, and the patterns of faulting. Only selected characteristics are described, and many features are left for depictions on maps and Figures. The other chapters in this volume elaborate on the history and evolution of the fault system, and the behavior of the Earth's crust and upper mantle within the fault system.

Because of the extent and complexity of the San Andreas fault system, it is helpful to distinguish between the broad, complex feature as seen on a map of the Western United States and the individual faults on which displacements occur to produce single earthquakes. From larger to smaller features, the terms "fault system," "fault zone," "fault," and "fault branches, splays, strands, and segments" are useful.

The term "San Andreas fault system" refers to the network of faults with predominantly right-lateral strike slip that collectively accommodate most of the relative motion between the North American and Pacific plates. The boundaries of this fault system are poorly defined, but to separate the San Andreas fault system from other tectonic provinces and systems, it is useful to limit the term to the set of faults along the Pacific rim of North America, both on land and off shore. Accordingly, at the latitude of San Francisco, the system is approximately 80 km wide, and at the latitude of San Diego approximately 150 km wide (see Figure 1 and maps at front of book).

The term "fault zone" refers to the complex zone of sheared rock that may be from 0.5 to more than 1 km wide and hundreds of kilometers long. The fault zone has developed over a period of millions of years while growing in width and complexity. The terms "fault," "fault branches," and "fault strands" refer to smaller elements and can be applied as needed. For example, surface rupture accompanying an earthquake commonly produces a complex pattern of fractures, and detailed elements can be discussed more effectively by using such terms as "fault branch, splay, or strand." The term "fault segment" recognizes that the fault is not completely continuous but is in sections or parts with poorly defined boundaries, as discussed below in the subsection entitled "Segmentation."