reached, one can predict when this will occur—what they called time-predictable behavior. Conversely, if the block always springs back to the same point, the result is slip predictable, though it can't be said when (over the course of exerting the force by pulling the spring) this will happen. In terms of prediction, says Ellsworth dryly, the slip-predictable model is very pessimistic. Conversely, as geophysicists continue to improve their ability to actually measure the tectonic stresses that build up in fault zones, the time-predictable model could one day lead to a more precise means of forecasting.
What is an earthquake prediction? "Any serious prediction," say Agnew and Ellsworth, "must include not only some statement about time of occurrence but also delineate the expected magnitude range [see Box on p. 158] and location." Without these elements, a prediction could not with certainty distinguish between what is being foreseen and what might actually occur, given how much seismicity occurs near plate boundaries, so seismologist and writer Bruce Bolt of the University of California-Berkeley adds a fourth element: ''A statement of the odds that an earthquake of the predicted kind would occur by chance alone and without reference to any special evidence" (Bolt, 1988, p. 160). Seismicity means the distribution of earthquakes in space and over time. "We currently record an average of 40 earthquakes per day in Southern California," said Tom Heaton (USGS-Pasadena), "and there is every reason to believe that there are many more that are too small for us to record."