Earthquake prediction: the scientific challenge.

As recently as 20 years ago the problems of earthquake prediction research were approached through a compilation of a succession of isolated case histories of presumed precursors to subsequent large and small earthquakes. The hope was that these precursory phenomena would appear before many, if not all, subsequent events. Alas, some of these hopes have either evaporated or have proved extremely difficult to document. Topics such-as anomalies in the ratio of Pto S-wave velocities, magnetic fields, resistivity, tilt, emission of noble gases, and so on are no longer at the leading edge of contemporary interest. Although interest in these areas is occasionally rekindled, the spark is difficult to fan into flame, and investment of support and effort in these areas has not been heavy in recent times. Today our approach is much the same as before: we continue to study a succession of case histories of events leading to strong earthquakes. Even today, there are occasional reports of new precursory anomalies, such as a change in the magnetic field before the Loma Prieta earthquake and, as will be discussed later in this collection of papers, observation of an increase of the concentration of chlorine and other ions in well waters before the Kobe earthquake. Whether these new areas will prove to be universals or disappear as others have remains for the future. But some avenues of phenomenology have continued to be pursued: clustering and anticlustering of earthquakes, creep measurements, changes in the attenuation factor, paleoseismicity methods, etc. A second thread of earlier prediction research was the presumption that small earthquakes were scaled-down versions of large ones, and hence the supposition was made that the study of small earthquakes would reveal important truths about large ones, a model if not driven by the scaling implicit in the Gutenberg-Richter distribution, then at least with the notion of scaling lurking in the background. These ideas suggest that simple isolated fractures in an elastic solid have a distribution of stresses and slips that are scaled only by the sizes of the cracks and hence whatever precursors, and postcursors for that matter, that might be observed will also be similarly scaled. This direction of research has also undergone modification over the years: on present-day models, earthquake fractures take place in a prestressed solid in which fluctuations are significant perturbations of the uniformity of the stress field. But small fractures take place in the shadows cast by the stress field of the larger and the largest fractures. The chain of self-similarity is broken for the largest earthquakes, since the largest fractures do not have stress fluctuations with even larger scales to contend with. Furthermore, the details of the fracture and of the properties of nearby rocks are not resolved observationally in smaller earthquakes, certainly not as clearly as in the case of large earthquakes. Today, the paradigms have shifted to the study of strong earthquakes and away from the