colloquium entitled " Earthquake Prediction : The Scientific Challenge , " organized

Earthquake prediction research has searched for both informational phenomena, those that provide information about earthquake hazards useful to the public, and causal phenomena, causally related to the physical processes governing failure on a fault, to improve our understanding of those processes. Neither informational nor causal phenomena are a subset of the other. I propose a classification of potential earthquake predictors of informational, causal, and predictive phenomena, where predictors are causal phenomena that provide more accurate assessments of the earthquake hazard than can be gotten from assuming a random distribution. Achieving higher, more accurate probabilities than a random distribution requires much more information about the precursor than just that it is causally related to the earthquake. Research in earthquake prediction has two potentially compatible but distinctly different objectives: (i) phenomena that provide us information about the future earthquake hazard useful to those who live in earthquake-prone regions and (ii) phenomena causally related to the physical processes governing failure on a fault that will improve our understanding of those processes. Both are important and laudable goals of any research project, but they are distinct. We can call these informational phenomena and causal precursors. It is obvious that not all causal precursors are informational. We investigate phenomena related to the earthquake process long before we have enough information to tell the public about a significant change in the earthquake hazard. In addition, however, not all informational phenomena are causal. We use the geologically and seismologically determined rates of past earthquake occurrence to assess the earthquake hazard in a socially useful way without knowing anything about the failure processes active on a fault. What, if anything, lies at the intersection of these two groups is perhaps the greatest goal of earthquake prediction (Fig. 1). Much of the early research in earthquake prediction in the 1970s was directed toward causal earthquake precursors. The laboratory research showing dilatancy and strain softening preceding rock failure encouraged many to believe that precursors arising from these processes could be measured in the field (e.g., refs. 1 and 2). When field measurements did not live up to this expectation, research in the 1980s began to shift toward nonprecursory processes, earthquake rates, and earthquake forecasting (e.g., ref. 7). In the last few years, as the informational capacity of rate analyses has been more fully explored, interest is beginning to return to possibly causal precursors. However, our approach to the research and our communication with the public now rests on a different The publication costs of this article were defrayed in part by page charge payment. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. §1734 solely to indicate this fact. 3721 foundation. The public has become accustomed to earthquake probabilities and processes further information within this