to approximate the spectral observations rather well, especially for small earthquakes.

The orientation of an elementary dislocation depends on two directions, the normal direction to the fault plane and the slip direction within this plane, so that the double-couple for a dislocation source is described by a three-dimensional, second-order moment tensorM proportional to M0 (106). By 1970, it was recognized that the seismic moment tensor can be generalized to include an ideal (spherically symmetrical) explosion and another type of seismic source called a compensated linear vector dipole (CLVD). A CLVD mechanism was invoked as a plausible model for seismic sources with cylindrical symmetry, such as magma-injection events, ring-dike faulting, and some intermediate- and deep-focus events (Figure 2.8) (107).

The Stress Paradox

Plate tectonics accounted for the orientation of the stress field on simple plate boundaries, which could be classified according to Anderson’s three principal types of faulting: divergent boundaries (normal faults), transform boundaries (strike-slip faults), and convergent boundaries (reverse faults). The stress orientations mapped on plate interiors using a variety of indicators—wellbore breakouts, volcanic alignments, and earthquake focal mechanisms—were generally found to be coherent over distances of 400 to 4000 kilometers and to match the predictions of intraplate stress from dynamic models of plate motions (108). This behavior implies that the spatial localization of intraplate seismicity primarily reflects the concentration of strain in zones of crustal weakness (109). Explaining the orientation of crustal stresses was a major success for the new field of geodynamics.

About 1970, a major debate erupted over the magnitude of the stress responsible for crustal earthquakes. Byerlee’s law implies that the shear stress required to initiate frictional slip should be at least 100 megapascals, an order of magnitude greater than most seismic stress drops (110). The stresses measured during deep drilling generally agree with these predictions. If the average stresses were this large, however, the heat generated by earthquakes along major plate boundaries would greatly exceed the radiated seismic energy and the heat flowing out of the crust along active fault zones should be very high. Attempts to measure a heat flow anomaly on the San Andreas fault found no evidence of a peak (111). The puzzle of fault stress levels was further complicated as data became available in the middle to late 1980s on principal stress orientations in the crust near the San Andreas (112); the maximum stress direction was found to be steeply inclined to the fault trace and to re-



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