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With the increasing public concern about the potential for destructive earthquakes in California since the great Alaska earthquake of 1964, and the general acceptance of the concept of plate tectonics and sea_floor spreading by earth scientists in the late 1960's, the San Andreas fault has gained wide attention. The fault has long been recognized as the source of the destructive San Francisco earthquake of 1906 and of the similarly large Fort Tejon earthquake of 1857, as well as the smaller (M=7.1), but also destructive, Loma Prieta earthquake of 1989. Many textbooks in recent decades have included photographs, maps, and descriptions of the San Andreas fault, and so among earth scientists around the world, few geologic features have greater identity than the San Andreas fault. The fault__better designated a "fault system" because of its complexities__represents an exceptional example of a plate margin that can be seen and studied on land; many other plate margins are covered by the oceans.

This volume is addressed to a varied audience, but especially to earth scientists who wish to gain a brief overview of the San Andreas fault system. We hope that the nontechnical reader also will find the volume interesting and useful. Each chapter has its own references that direct the investigator more to specific literature; suggestions for additional reading and sources of information are included at the end of the volume.

The public_safety issues of earthquake_hazard mitigation and earthquake prediction are not covered in this volume except by implication. Fundamental concepts and understanding of how the solid Earth works are essential to the development of realistic and effective procedures for hazard mitigation. Without such understanding, mitigation measures may be ineffective and wasteful of resources. Earthquake prediction requires geologic and geophysical models and data that constitute valid bases for extrapolation into the future. We believe that these concerns are well served by the reviews in this volume.

The 10 chapters that follow review geologic, geomorphic, geophysical, geodetic, and seismologic information about the San Andreas fault system. Although the material is intended to represent our current state of knowledge and understanding, many investigators will find their own specialities inadequately treated. A full discussion of the more rapidly changing and controversial ideas currently in play is impossible, given the constraints of this volume.

The need for such a volume has been recognized since the 1960's, but until recent years, data about the San Andreas fault system was so incomplete that a significant review seemed premature. Now, a general overview of many significant elements seems possible, but even so, the questions far outnumber the answers, and if history is any guide, many of the concepts put forth here will have changed markedly within the next few years. Indeed, one of the purposes of this volume is to assist and hasten the evolution of our understanding through the consolidation, under a single cover, of some of the current ideas and models.

Reference to a few of the more outstanding problems concerning the San Andreas fault system may serve to suggest how little is yet known about the fault system and how much remains to be learned. In studying the San Andreas fault system, opportunities abound for learning how the Earth works in a general sense.

A group of particularly significant problems can be collectively expressed under the question: "How does a fault system that has primarily strike slip terminate at its northwest and southeast ends, and how does it bottom out at depth?"

At the northwest end of the fault system, the Mendocino triple junction represents an intriguing structural knot where the North American, Pacific, and Gorda plates join. A fourth block at depth, made up of material below the North American plate but east of the San Andreas fault and south of the Gorda plate, also is juxtaposed with these three named plates. How are the diverse motions of these four plates or blocks accommodated where they join? Clearly, severe space problems must occur at detailed scales, even though the gross theoretical geometry of triple junctions has been fairly well described. How do these four structural blocks interact to influence the energetics of the fault system (see chap. 10)? How did the triple junction migrate over time, and how are the consequences of that migration recorded geologically?

Crustal convergence also strongly influences the fault system. Where major bends occur, as in the Transverse Ranges region, structural complexities arise. There, major left_lateral faults splay from the main San Andreas fault, and dense clusters of earthquakes extend to depths of 20 km. Elsewhere, as in the Santa Cruz Mountains, some segments of the fault dip at steep, but not vertical, angles.

Adjacent to the San Andreas fault on its east side lies the North American plate, at least in the upper few tens of kilometers; but below this plate, as indicated above, is a block of almost unknown characteristics left in the wake of the eastward_moving and subducting plate now represented by its remnants, the Gorda and Juan de Fuca plates. After pulling away from the Pacific plate in its eastward passage, did this plate leave remnants here and there under the North American plate? Did the spreading center spread continuously, or did it move eastward in one or more leaps? What sort of mantle material rose to fill in behind the stern of the eastward_moving plate? Was this newly emplaced material similar to that being generated at modern spreading centers, and to what extent did the overlying blanket of continental material alter both the geometry and thermal histories of the emplaced rocks?

Some of the models mentioned above imply decoupling between the subducting plate and the North American plate, as well as a rather significant discontinuity between the North American plate and the underlying backfill behind the stern of the eastward_moving plate. Some of the problems of this "window" behind the subducting plate are discussed in chapter 3. The San Andreas fault, indeed, may bottom in a zone of decoupling, either within the crust or below, possibly a low_angle thrust fault, as described in chapter 1, or perhaps involving gravitationally driven detachments. The characteristics of this decoupling are almost unknown. Much needs to be learned before the style of stress and strain propagation across such discontinuities can be addressed adequately.

Present_day strain is demonstrable by geodetic techniques (see chap. 7), and longer_term strain is represented geologically by the pattern of folds, faults, and magmatic intrusion into the upper crust (see chaps. 3, 4). Release of elastic strain, its timing and spatial distribution, is nicely displayed by seismicity, especially microseismicity (see chap. 5). Heat_flow measurements provide an important insight into the energetics of the San Andreas fault system (see chap. 10). Integration and comparison of these data sets, however, reveal numerous unresolved problems and apparent paradoxes. As reported in chapter 10, no sharp increase in heat flow is found directly over the San Andreas fault, even though heat would be expected to be generated in the narrow fault zone by the annual slip of several centimeters on the fault. Instead, the heat flow is distributed across a broad zone, further suggesting distributed slip on a subhorizontal plane that decouples the upper_crustal materials from those below.

The detailed characteristics of the fault zone itself are far from fully understood. Low values of stress drop that occur during seismic events have long been known (see chaps. 5, 10). The dominance of right_lateral slip along the fault, despite evidence for fault_normal compression, together with the absence of a pronounced heat_flow anomaly, attests to general weakness of the fault. How the fault zone has grown to its present 0.5_ to 1_km width, given this weakness, is also a puzzle. Furthermore, at many places along the fault, as in central California, the width of the fault zone is appropriately considered to be 10 km or more wide; that is the width of highly sheared and deformed rocks which lie between relatively undeformed terrane to the northeast and southwest. What is the nature of asperities, or strong points, on the fault, and how do its stronger and weaker parts interact? To what extent does plastic_behaving fault gouge move within the fault zone to change the overall geometry?

The role of water in the kinematics and dynamics of the fault system can scarcely be overemphasized, and yet almost nothing is known about the actual hydrotectonic relations. To what extent does elevated or reduced pore pressure modify the properties of the lithologic packages of rocks at different places along and across the fault? What geochemical changes in the fault zone are enhanced by the movement of fluids along the fault and through rocks adjacent to the fault? What mineralogic changes take place as a result of thermal changes related to friction, possibly accompanied by exsolution of water from minerals? Can localized, tectonically elevated pore pressure initiate slip, and once slip acceleration occurs, what role does friction play in the dynamics of slip? Why are so few volcanic rocks associated with the fault except for those related to the passing triple junction?

What are the rates of fault slip, folding, and the overall budget of deformation among various forms of strain? How do erosional rates compare with tectonic rates in changing the landscape, and to what extent do these processes deviate from linearity? How do complex geomorphic processes interact among themselves, as well as with the tectonic processes? How can the ubiquitous landforms be interpreted to illuminate the younger history of the San Andreas fault?

Strands and branches of the San Andreas fault system bound numerous exotic terranes, those aggregates of rocks which are so dissimilar that they could not have been born in their present relation to one another. The patterns of movement and distances traveled by these terranes may constitute the most significant characteristic and role of the San Andreas fault in the overall scheme of global tectonics. By whatever mechanisms these exotic blocks or terranes were transported, the western part of North America has been enlarged by the accretion of these "strangers," while at the same time other pieces of older continental material have been plucked away, eventually to join land somewhere to the northwest.

This volume represents but a small punctuation mark in the early stage of our understanding of the San Andreas fault system and the tectonics that it highlights. Most of the story has yet to be learned.