Cannot Earthquakes Be Predicted

Robert J. Geller et al. are on shaky ground when they state, in the title of their Perspective (1), that earthquakes cannot be predicted. In spite of their advice, we should not stop studying the physics of preparation for catastrophic rupture, in the field, the laboratory, and theoretically; neither should we stop measuring crustal parameters that might furnish constraints for physical models; and we should continue researching statistical methods to evaluate prediction claims and to test hypotheses quantitatively. Some of the arguments Geller et al. put forward are incorrect. For example, the “slip on geological faults” is not always as “sudden” as they state. In 10% to 30% of large earthquakes, foreshocks occur days (2) to months (3) before the main shock. Seismologists agree that foreshocks are a symptom of some preparatory process to the main rupture. Thus, foreshocks are precursors. If that process could be detected and understood by measuring the several physical parameters of Earth’s crust that probably change during it, then prediction would be possible, even if foreshocks themselves can be identified with a low probability only (4). When Geller et al. state that “[t]here are no objective definitions of ‘anomalies’” and that “statistical evidence for a correlation is lacking,” they appear to be referring to specific papers that have been criticized (5). However, there are examples of clearly formulated, even tested, hypotheses. Evison and Rhoades (6) formulated a rigorous statistical test and applied it in real time to their well-defined hypothesis of precursory earthquake swarms. The algorithm M8 (7) has been tested in real time, and critically evaluated by others (8). The hypothesis of precursory quiescence also has been clearly stated (9), and specific predictions have been made to test it (10). On the basis of a mathematical model of failure of earth materials, the hypothesis of increasing moment release has been formulated (11) and tested by predictions (12). Geller et al. are also incorrect in stating that “no quantitative physical mechanism links the alleged precursors to earthquakes. ” Laboratory rock fracture experiments have shown that dilatancy occurs in rocks under high deviatoric stresses and that rock properties are drastically altered by this phenomenon (13). Dilatancy could explain many precursors, as proposed by Scholz et al. (14). An alternate mechanism to explain precursors is a reduction in ambient stress level that results from strain softening (15) during days to years before catastrophic failure in a major earthquake. This phenomenon is routinely observed in the laboratory in stiff rock presses, and it has been modeled quantitatively by modern friction laws (16), with the result that years before large subduction shocks occur, a reduction of stress is expected near the source volume (17). Thus, several types of measurements could furnish observable precursors. Finally, the pore pressure of underground fluids, which is known to play an important role in rupture initiation along many faults (18), can be altered in a number of ways, which also could lead to precursors. Some of these models are quantitative, others not yet because too few constraints exist at this time. Geller et al. are again incorrect when they say that “the leading seismological authorities of each era have generally concluded that earthquake prediction is not feasible. ” They should have added “with the current knowledge” to this sentence. I remember the frustration of Richter, when asked sensationalist questions about unfounded predictions, instead of the science of earthquakes. However, as cited by Geller et al., Richter did not advocate an opinion that earthquake prediction was inherently impossible. Geller et al. state that they believe that earthquakes occur at random. Randomness would require the assumptions that the tectonic stress is near failure everywhere and at all times, and that the stress drops are small, depleting the local elastic energy available for further ruptures only to an insignificant degree. But the accumulation and release of strain that has been measured leading up to and following earthquakes, respectively, suggests otherwise. For example, the M7.2, Kalapana, Hawaii, earthquake in 1975 was anticipated on the basis of the observed accumulated strain (19). Over decades, compressive strain of 4 (1024) accumulated, and during the earthquake the same amount of strain was released (20). This conclusion was also reached by Reid et al. (21) for the 1906 San Francisco earthquake. Precise leveling along the coast of Japan (22), and geological records of recent sedimentation along beaches (23) show that strain is released in great earthquakes only after it has been accumulated over centuries. The figure in the Perspective by Geller et al. (p. 1617) does not support the idea that earthquakes are unpredictable. It shows, instead, that the sizes of cracks available for failure in earthquakes are fractally distributed. In volumes where no strain energy has been built up by tectonic processes, however, large earthquakes cannot occur, regardless of the nature of the crack distribution. Along the plate margins, where the vast majority of all earthquakes occur, the stresses are likely to be low, and the stress release may be nearly complete (24). At the time of Columbus, most experts asserted that one could not reach India by sailing from Europe to the west and that funds should not be wasted on such a folly. Geller et al. make a similar mistake, but I doubt that human curiosity and ingenuity can be prevented in the long run from exploring fully the extent to which at least some earthquakes are predictable, although it is not easy. Such discoveries will be made in Japan, Europe, or China if the current lack of funding for earthquake prediction research continues in the United States. Max Wyss Geophysical Institute, University of Alaska, Fairbanks, AK 99775–0800, USA