INTRODUCTION

As the Pacific plate slides northward past the North American plate along the San Andreas fault, the frictional stress that resists plate motion there is overcome to cause earthquakes. However, the frictional heating predicted for the process has never been detected. Thus, in spite of its importance to an understanding of both plate motion and earthquakes, the size of this frictional stress is still uncertain, even in order of magnitude.

As one of the best exposed tectonic-plate boundaries in the world, the San Andreas fault provides an excellent opportunity to study the forces causing interplate motion and the associated great earthquakes. Thus, there is considerable motivation, scientific, social, and economic, to understand the thermomechanics of the San Andreas fault system, which has been the subject of intensive studies for the past several decades.

Although substantial progress has been made in unraveling the complex kinematics of the San Andreas fault system (Atwater, 1970; Minster and Jordan, 1984; Weldon and Humphries, 1986), efforts to determine the stresses that give rise to San Andreas fault slip, to date, have not led to anything resembling scientific consensus. The uncertainty results from widespread disagreement over the implications of different methods of assessing the stresses.

The question of how much shear stress acts on the San Andreas fault to cause dextral slip began to acquire definition in 1968, when the first heat-flow data adjacent to the fault zone (fig. 10.1) were gathered and analyzed by Henyey (1968). Because these data did not reveal any anomalous heat flow near the major active faults of the San Andreas system, upper bounds of about 10 to 20 MPa on the average frictional stress resisting fault motion could be calculated (Brune and others, 1969; Henyey and Wasserburg, 1971). These upper bounds were taken as evidence confirming speculation on the low strength of the crust based on earthquake stress drops, almost invariably in the range 0.1-10 MPa (for example, Chinnery, 1964; Brune and Allen, 1967). At the same time, however, laboratory experiments indicated typical frictional strengths for precut rock samples of about 100 MPa under pressure and temperature conditions thought to obtain in the upper crust (Byerlee and Brace, 1968, 1969; Byerlee, 1970).

Over the next several years, new heat-flow measurements supported the absence of any local heat-flow anomaly associated with the San Andreas fault (Lachenbruch and Sass, 1973) and thus augmented the position for low frictional fault strength. The recognition of a broad heat-flow high coincident with the Coast Ranges of California led Lachenbruch and Sass (1973) to suggest that partial decoupling at the base of the seismogenic part of the crust might account for both the weak fault (minimum in shear stress at the fault trace) and the broad thermal anomaly.

Additional laboratory experiments on different rock types, and in conditions of higher temperature and confining pressure than had been obtained previously, continued to support high frictional strength in the top 15 to 20 km of the fault zone (Stesky and Brace, 1973). The experimental results are most simply characterized in terms of a coefficient of friction that varies little with rock type (Byerlee, 1978), slip rate, or slip history (Dieterich, 1979; Ruina, 1983). As emphasized by Brace and Kohlstedt (1980) and Kirby (1980), these results still indicate a high-strength upper crust.

Beginning in the late 1970's, inplace stress measurements have provided another way to assess the stress acting on the San Andreas fault (Zoback and others, 1977), especially with the advent of stress measurements at depths approaching 1 km only a few kilometers distant from the fault (Zoback and others, 1980). If the observed depth gradient for the component of shear stress thought to act on the San Andreas fault could be extrapolated to the base of the seismogenic zone, as argued by McGarr and others (1982), then the corresponding frictional stress resisting fault motion is a factor of 3 greater than the upper bound from the heat-flow analyses, as presented most recently by Lachenbruch and Sass (1980).

The most recent developments, if accepted at face value, could be construed as additional evidence favoring a low-strength San Andreas fault. Specifically, stress-direction indicators on either side of the fault have been interpreted to mean that there is almost no shear stress resolved on the fault plane, thus implying a very weak fault zone (Mount and Suppe, 1987; Soback and others, 1987). If so, then the question regarding the strength of the fault would be answered, and the outstanding problem would be the equally vexing one of understanding the nature of a remarkably weak fault zone.

This chapter is largely a review and commentary on the different approaches taken to estimate the tractions acting on the San Andreas fault. We restrict our attention to three main methods: (1) inferring stress from the fault's energy budget (thermal and kinetic), (2) inferring fault strength from laboratory measurements of the stresses needed to slide rocks past one another under pressure, and (3) inferring stress on the fault from observations of the crustal state of stress.