Elementary school children now hear this fairly straightforward description of why earthquakes are inevitable, and yet 30 years ago geophysicists were just beginning to piece it together, marking a new era in seismology, a field born at the end of the nineteenth century with the development of recording instruments designed to measure the shaking of the earth (seismometers or seismographs), and the "pictures" they produce (see Box on p. 162). But even the seismograms don't reveal the picture of plate tectonics described above. They simply don't contain that information. "Very clever people," says Ellsworth, "working with very sparse information, cracked the plate tectonics puzzle in the early 1960s, and the dramatic advances in seismology since then rest on better data gathering through advances in tools and technology, the enhanced power to compute complex numerical solutions, and advances in the theory.''

What Agnew and Ellsworth call the first "modern" theory of earthquake prediction, by G. K. Gilbert in 1883, was conceived without benefit of the plate tectonics model. Gilbert's notion, as idealized in the block-and-spring model, was developed by another American, Harry Fielding Reid, into what has come to be known as the elastic rebound theory. Reid's study of the fault region that broke during San Francisco's notorious 1906 earthquake provided the first solid evidence for the elastic rebound of the crust after an earthquake. After Reid, scientists began to look at individual earthquakes as passing through a seismic cycle that they could examine and analyze quantitatively, searching for patterns that could be used to develop a prediction of the next rupture. As a generalization, stress accumulates slowly over time on a given fault, it reaches a critical point of failure, the fault slips, and energy is released as an earthquake. Go into the lab—or, more often these days, boot up your computer for a simulation—attach a spring to a block, and exert a pull, and you can watch the basic idea on which Gilbert and Reid built the earthquake cycle hypothesis in action. You will find that the amount of force required before the spring slips and the distance the block moves are predictable.

But the crucial question must be probed: Predictable in what sense? Ellsworth says that in the late 1970s, as elaborate experiments with block and spring models were being conducted in laboratories around the world (this construction of analog earthquake models is still going strong in such places as the Institute for Theoretical Physics in Santa Barbara), an important conceptualization came from Japanese seismologists Shimizaki and Nakata. They realized that, assuming the spring is being pulled at a constant rate, if the block always moves when a given force is



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