have made six-month advance predictions for magnitude thresholds 7.5 and 8.0 accessible on their web page (142), and fully prospective statistical tests will be possible in the near future.

The “Holy Grail” of earthquake science has always been short-term prediction—anticipating the time, place, and size of a large earthquake in a window narrow and reliable enough to prepare for its effects (143). However, interest in the possibility of detecting earthquake precursors grew as new technologies were developed to monitor the crustal environment with increasing sensitivity. In the year following the destructive 1964 Alaskan earthquake, a select committee of the White House Office of Science and Technology issued a report called Earthquake Prediction: A Proposal for a Ten Year Program of Research, which called for a national program of research focused on this goal (144).

Optimism about the feasibility of short-term prediction was height-ened in the mid-1970s by the apparent successes of empirical prediction schemes and the plausibility of physical process models, such as dilatancy diffusion. Laboratory studies had measured dilatant behavior in rocks prior to failure, caused by pervasive microcracking. Dilatancy creates measurable strain, changes the material properties, and increases the permeability of the samples (145). Field evidence for such effects came from the Garm region of the former U.S.S.R., where Soviet seismologists had identified changes in the ratio of shear and compressional velocities, V_S/V_P, as precursors to some moderate earthquakes (146). Positive results on V_S/V_P precursors were also reported in the United States (147). These observations prompted refinements of the dilatancy diffusion model and a wider search for related precursors.

A reported prediction of an M 7.3 earthquake in Haicheng, China, is widely regarded as the single most successful earthquake prediction. An international team that visited China shortly after the quake (148) reported that the region had already been subject to an intermediate-term earthquake forecast based on seismicity patterns, magnetic anomalies, and other geophysical data. Accelerating seismic activity (Figure 2.14) and rapid changes in the flow from local water wells prompted Chinese officials to issue a short-term prediction and to evacuate thousands of unsafe buildings. At 7:36 p.m. (local time) on February 4, 1975, less than 24 hours after the evacuation began, the main shock destroyed 90 percent of the city. Chinese officials stated that because of the evacuation the number of casualties was extremely low for such an earthquake. This reported success stimulated great optimism in the earthquake prediction community, but it did not signal a widespread breakthrough in predic-