SUMMARY

To recapitulate, the simplest interpretation of earthquakes in terms of the frictional fault model and laboratory measurements of rock friction leads to fault stresses many times larger than the limits suggested from heatflow and fault energetics. This interpretation depends on three assumptions: (1) that the average coefficient of friction on real faults is comparable to typical laboratory values (mu ~ 0.6-0.9), (2) that the pore-fluid pressure throughout the depth of faulting is comparable to the weight of the overlying column of water, and (3) that the intrinsic resistance of the Earth to sliding is isotropic - that is, no weak directions exist. To reduce the high estimates of friction obtained from rock mechanics to the low ones obtained from heat flow, we must assume either smaller values of the coefficient of friction mu or larger values of fluid pressure P (see eq. 15). Of particular interest in this connection is reported evidence that the trend of the San Andreas fault in California might occupy an anomalous weak direction. According to Mount and Suppe (1987) and Zoback and others (1987), the fault plane is nearly perpendicular to the direction of maximum compression (theta ~ pi/2, fig. 10.10), a direction in which the resolved shear stress is very small. Such a condition could be consistent with the low friction required by heat flow, while permitting high stresses to accommodate the subsidiary faulting observed on more favorably situated planes. This model, however, raises some basic questions regarding the mechanics of faulting; for the fault to slip, it must have a low shear strength, as well as the low shear stress suggested by its orientation. If conventional friction theory applies, anomalously high fluid pressure along the fault cannot readily account for the required low friction because, unless mu is unusually low, the fault becomes exceedingly resistant to shear failure as theta begins to approach pi/2 (fig. 10.10).