It was found that the strength of ductile rocks decreases rapidly with increasing temperature and that their rheology approaches that of a viscous fluid. The brittle-ductile transition thus explained the plate-like behavior of the oceanic lithosphere and the fluid-like behavior of its subjacent, convecting mantle. Rock mechanics experiments further revealed that ductility sets in at lower temperatures in quartz-rich rocks than in olivine-rich rocks, typically at midcrustal depths in the continents. The ductile behavior of the lower continental crust inferred from laboratory data, which was consistent with the lack of earthquakes at these depths, thus explained the less plate-like behavior of the continents (80).

2.5 EARTHQUAKE MECHANICS

Gilbert and Reid recognized the distinction between fracture strength and frictional strength (81), and they portrayed earthquakes as frictional instabilities on two-dimensional faults in a three-dimensional elastic crust, driven to failure by slowly accumulating tectonic stresses—a view entirely consistent with plate tectonics. Although earthquakes surely involve some nonelastic, volumetric effects such as fluid flow, cracking of new rock, and the expansion of gouge zones, Gilbert and Reid’s idealization still forms the conceptual framework for much of earthquake science, both basic and applied. Nevertheless, because the friction mechanism was not obviously compatible with deep earthquakes, as described below, their view that earthquakes are frictional instabilities on faults had, by the time Wilson wrote his 1965 paper on plate tectonics, been considered and rejected by some scientists.

The Instability Problem

Deep-focus earthquakes presented a major puzzle. Seismologists had found that the deepest events, 600 to 700 kilometers below the surface, are shear failures just like shallow-focus earthquakes and that the decrease in apparent shear stress during these events is on the order of 10 megapascals, about the same size as the stress drops estimated for shallow shocks. According to a Coulomb criterion (Equation 2.1), the shear stress needed to induce frictional failure on a fault should be comparable to the lithostatic pressure, which reaches 2500 megapascals in zones of deep seismicity. Shear stresses of this magnitude are impossibly high, and if the stress drop approximates the absolute stress, as most seismologists believe, they would conflict with the observations (82).

Furthermore, if earthquakes result from a frictional instability, the motion across a fault must at some point be accelerated by a drop in the frictional resistance. A spontaneous rupture like an earthquake thus re-



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