andesitic stratovolcanoes characteristic of these arcs (60). Benioff’s model was based on several misconceptions, however, including the assumption that intermediate- and deep-focus seismicity could be explained by extrapolating trench-type reverse faulting into the mid-mantle transition zone. In fact, the focal mechanism of most earthquakes with hypocenters deeper than 70 kilometers does not agree with Benioff’s model of reverse faulting (61).

The definitive evidence for “thrust tectonics” finally arrived in the form of the great 1964 Alaska earthquake (Box 2.3). The enormous energy released in this event (~3 × 1018 joules) set the Earth to ringing like a bell and allowed precise studies of the terrestrial free oscillations, whose period might be as long as 54 minutes (62). A permanent strain of 10–8 was recorded by the Benioff strainmeter on Oahu, more than 4000 kilometers away, consistent with a fault-dislocation model of the earthquake (63). However, the high-amplitude waves drove most of the pendulum seismometers offscale (64). Moreover, field geologists could not find the fault; all ground breaks were ascribable to secondary effects. What they did observe was a systematic pattern of large vertical motions—uplifts as high as 12 meters and depressions as deep as 2.3 meters, which could easily be mapped along the rugged coastlines by observing the displacement of beaches and the stranded colonies of sessile marine organisms such as barnacles (just as Darwin had done for the 1835 Chile earthquake). By combining this pattern with the seismological and geodetic data, they inferred that the rupture represented the slippage of the Pacific Ocean crust beneath the continental margin of southern Alaska along a huge thrust fault. Geologist George Plafker concluded that “arc structures are sites of down-welling mantle convection currents and that planar seismic zones dipping beneath them mark the zone of shearing produced by downward-moving material thrust against a less mobile block of the crust and upper mantle” (65). By connecting the Alaska megathrust with the more steeply inclined plane of deeper seismicity under the Aleutian Arc, Plafker articulated one of the central tenets of plate tectonics.

Plafker’s conclusions were bolstered by more accurate sets of focal mechanisms that William Stauder and his colleagues at St. Louis University derived (66). Dan McKenzie and Robert Parker took the next major step toward completion of the plate theory in 1967, when they showed that slip vectors from Stauder’s mechanisms of Alaskan earthquakes could be combined with the azimuth of the San Andreas fault to compute a consistent pole of instantaneous rotation for the Pacific and North American plates (67). At the same time, Jason Morgan’s analysis of seafloor spreading rates and transform-fault azimuths demonstrated the global consistency of plate kinematics (68).

Clarity came with the realization that the plate is a cold mechanical



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