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Proceedings of the National Academy of Sciences of the United States of America
and Northridge (1994) earthquakes. Progress in this field has been due to the development of a dense network of seismographs, a program that took a number of years to install. Similarly, better tools are available for the study of precursory creep and changes in the earth’s magnetic and electric fields. With the new generation of instruments, and with increased resolution in devising solutions to the inverse problem, some of the older presumptions about precursors have disappeared from the repertoire as noted above, others survive with increased intensity of attack, and a few new methods appear.
We remain in the case-history stage of the study of precursory clustering of earthquakes prior to strong earthquakes; these accounts of clustering remain without statistical substantiation because of the paucity of strong earthquakes for study: These comments should not be interpreted to be a plea that more strong earthquakes should take place.
Great progress has been made on the modeling front, partly through the extraordinary development of large-scale computing resources. There has been a remarkable increase in our understanding of the behavior of the deformation of rocks through laboratory measurements, and especially in the behavior of prefractured rocks, in the times before large-scale rupture. There has been unusual activity in the modeling, usually numerical, of processes of self-organization of the stress field due to the occurrence of extended fractures in faulted systems. In particular, we have acquired insights into the physics of fracture, on preexisting, nonuniform faults.
Despite the optimistic tone of the above remarks, an ability to predict earthquakes either on an individual basis or on a statistical basis remains remote. It is clear that the scientific issues must be understood before routine predictions can be announced, which in a generalized sense is an engineering problem.
There are other issues connected with earthquake prediction that were not discussed at the colloquium presented here: neither the organization of national programs in earthquake prediction, nor the engineering problems, nor the problems of societal response to possible future predictions in the three different time scales. With regard to the scientific issues, the colloquium committee developed a program that focused in roughly equal amounts on the laboratory and modeling research that is currently being performed on the one hand and on the observations relevant to the three time scales on the other. The papers that follow are an excellent representation of the thoughts that were aired and cover the full range from the pessimistic to the optimistic.
It is a certainty that the problems of societal response and engineering response to earthquake predictions are not going to be solved until the scientific problems can be brought under control. These are no more difficult than they were several decades ago; they are only more clearly defined today. We recognize today that the scientific problems are not simple.
A significant number of graduate students and young postdoctoral scholars were able to attend the Colloquium through generous support by the National Science Foundation and the Southern California Earthquake Center.