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In recent years several welcome new trends have appeared in the study of earthquake prediction, especially in the U.S. First, we see more objective testing of prediction methodology by researchers other than the advocators as well as efforts on the part of predictors to make prediction testable by others. Secondly, probabilities have been increasingly used for transmitting earth science information on future earthquake hazards to officials in charge of emergency preparedness, earthquake engineers and the public. This trend is welcome because the probability conveys the information in a form which can be dealt with in response planning and cost-benefit analysis of mitigation. Thirdly, physicists outside the community of earth science have started to get involved in the study of earthquake prediction. Their involvement introduced new concepts such as self-organized criticality into earthquake study and led to a fundamental debate on whether the observed spatio-temporal complexity in seismicity is due to intrinsic heterogeneity of a fault zone or to non-linear dynamics of fault rupture. Central to the debate are the two scale lengths introduced by Rice ; one is the scale of intrinsic discreteness of a fault, and the other is the size of the nucleation zone which scales with the critical weakening slip of the state- and rate-dependent friction law. Numerous seismological observations on the scale dependence in earthquake phenomena may be understood in terms of these parameters, offering a promising basis for developing a truly physical theory of earthquake prediction.
Acknowledgments. This work was supported partly by the Southern California Earthquake Center under National Science Foundation Cooperative Agreement EAR-8920136 and U.S. Geological Survey Cooperative Agreement 14-08-0001-A0899. This is SCEC contribution #122.