Debate in 1999

Predicting earthquakes requires an understanding of the underlying physics, which calls for novel multidisciplinary approaches at a level never yet undertaken. Notwithstanding past efforts in several countries in the last decades, I fail to see that the scientific community has used the full potential of artificial/computational intelligence, statistical physics, super-computer modelling, large scale monitoring of a full spectrum of physical measurements, coupled together with more traditional seismological and geological approaches to make a dent in the earthquake problem. What we have learned is that past failures in earthquake prediction reflect the biased view that it was a simple problem.

This debate has highlighted both a degree of consensus and a degree of continuing controversy within the thorny subject of the predictability of earthquakes. In terms of the four levels of prediction of seismicity I introduced at the start of this debate, a consensus has emerged that at least some form of time-dependent seismic hazard can be justified on both physical and observational grounds. The phenomenon of earthquake triggering leads to a transient, local increase in probability of future earthquakes, for example as aftershocks, but also sometimes in the form of subsequently larger events. In fact warnings based on such clustering are already in use in California (Michael, week 2). On the other hand, all of the contributors to this debate who expressed an opinion agree that the deterministic prediction of an individual earthquake, within sufficiently narrow limits to allow a planned evacuation programme, is an unrealistic goal.

Geller and Jackson have both reproached me for not citing all of the Jackson and Kagan papers in my earlier statement. Space requirements did not allow for a fuller discussion it that time.

The question at the heart of this debate appears to be whether earthquake prediction should be recognised as a distinct and independent research field, or whether it is just one possible research topic in the general field of study of the earthquake source process. As there are no known grounds for optimism that reliable and accurate earthquake prediction (as defined in my first article) can be realized in the foreseeable future, the case for the latter position appears clear-cut. As a consequence there is no obvious need for specialised organisations for prediction research. Besides the benefits that always accrue from pruning deadwood, abolition of such organisations would force prediction proponents and critics to confront each other in common forums, thereby speeding the resolution of the controversy.

As we all agree that we know little about how earthquakes initiate and how to predict them, it follows that we will study the problem and eventually reach a relatively satisfactory solution. The question is, will we do it with the significant financial support and expedience this humanitarian effort deserves, or will we force individual scientists to do it in their non-existent spare time?

Wyss's letter in week 5 of this debate claims that I made a 'significant error' in my week 3 article. I explain here why this claim should not be accepted, placing my rebuttal in a more general context.

Testing hypothesises is an essential part of earthquake prediction. The 'game rule' associated with this test is especially important because it leads to the criteria for accepting or rejecting a statistical hypothesis related to earthquake prediction.

All discussions so far have referred (perhaps not surprisingly) to the properties of earthquakes, their times, locations, nucleation mechanisms, physics of the source, possible precursors, etc. I think this will lead nowhere. Earthquakes are extraordinarily varied and impossible to average. Perhaps the only feature of earthquakes that can be relied on is that they release a large amount of stress which, because rock is weak, has necessarily accumulated over a large volume of rock. If this build up of stress can be monitored then the time and magnitude of the earthquake when fracture criticality is reached can be subject to 'stress-forecast'. I suggest that we already know how to do this. The effects have been seen with hindsight for eight earthquakes worldwide, and the time and magnitude of an M=5 earthquake has been successfully stress-forecasted.

Scholz omitted crucial parts of the the recent history of the seismic gap forecast and test. He remarked that our test of the gap theory was 'flawed' because it used earthquakes 'smaller than system-sized'. This was also asserted in a published comment by Nishenko & Sykes¹ and answered by Jackson & Kagan². But 'system-sized' was never defined in the original seismic gap model³.

Earthquake prediction is strictly related to empirical precursors. Despite the results presented in recent decades in support of the existence of empirical precursors there is scepticism in the scientific community about whether they exist^1-3. The widespread argument is that precursor signals reported are unrelated to earthquake activity and that they could have occurred by chance. If this were true, earthquake prediction would not be possible.

Wyss asserted that without funding at "a scale comparable to the funding of astrophysical research ... serious progress toward learning how to predict earthquakes" was impossible. However, extensive prediction efforts in several countries in several eras have all failed. Further allocation of public funds appears unwarranted unless there are specific and compelling grounds for thinking that a proposed new prediction programme will be successful.

As anyone who has ever spent any time in California can attest, much public attention is being focused on the great earthquake-prediction debate. Unfortunately, this attention focuses on deterministic predictions on the day-to-week timescale. But as some of the participants in this debate have pointed out^1,2 current efforts to identify reliable short-term precursors to large earthquakes have been largely unsuccessful, suggesting that earthquakes are such a complicated process that reliable (and observable) precursors might not exist. That is not to say that earthquakes do not have some 'preparatory phase', but rather that this phase might be not be consistently observable by geophysicists on the surface. But does this mean that all efforts to determine the size, timing and locations of future earthquakes are fruitless? Or are we being misled by human scales of time and distance?

It is certainly possible to define the reliable prediction of individual earthquakes so narrowly that success is impossible. For instance, in Main's level 4 he refers to predictions with such precision and accuracy that a planned evacuation can take place. None of the contributors have yet to suggest that this is a possibility and I agree with Wyss that using this straw man as the standard will not lead to a useful debate. However, Main's levels 2 and 3 may lead to socially useful tools regardless of whether we call them predictions or probabilistic forecasts.

For a prediction to be successful, the probability of occurrence in a time interval and a space domain must be specified in advance, as must the lower magnitude. There are two important additional constraints: a utilitarian constraint demands that the lower magnitude bound be appropriate to societal needs; in other words, we are especially interested in strong destructive earthquakes.

The contributions to the debate about earthquake prediction research in Nature so far, clearly show that we have hardly scratched the surface of the problem of how earthquake ruptures initiate and how to predict them. This arises from the difficulty of the problem and the lack of a vigorous program to study these questions. As Andrew Michael has said, funding for earthquake prediction research is a small fraction of the seismology program, in the U.S., and seismology is poorly funded compared to disciplines like astronomy.

One of this week's email contributors considers the consequences for earthquake predictions if they are indeed self-organized critical phenomena.

The topic posed by the editors for this debate, is whether the reliable prediction of individual earthquakes is a realistic scientific goal. Translated into everyday language, this becomes: given the present state of the art, does earthquake prediction research merit a significant investment of public funds? My initial contribution to this debate stated the negative case. None of the other debaters appears to have made strong arguments for the affirmative.