warnings. While they may have had some value in terms of public preparedness, they were, in fact, false alarms.
If it had been realized that the Lake Elsman events were on a nearby but different fault, an intermediate-term warning would have been more appropriate. They probably indicated the return of stresses in that area to pre-1906 levels rather than the initiation of accelerated precursory slip on the San Andreas fault itself. Another example of an intermediate-term seismic precursor is the northward growth in aftershock activity in the Joshua Tree earthquake sequence in southern California between its mainshock on April 23 and the Landers shock of June 28, 1992 (41). While these precursors are subtle in character, they, and other examples, indicate that precursory phenomena likely exist on time scales of months to a decade.
How earthquake prediction is and has been viewed in the United States has a number of parallels to skepticism about continental drift and paleomagnetism prior to the late 1960s. Like them, prediction invokes strong views about what problems are “worth working on.” Earthquake prediction has suffered in this regard; only 10–20 scientists in the U.S. are currently working on intermediate-term prediction. Work in prediction also has suffered from a general belief that only short-term predictions would have social value. While not possible now, a well-founded 5-year prediction could be of greater value since serious mitigation measures could be undertaken.
Several observations of precursors have turned out upon reexamination to be artifacts of either environmental changes affecting instruments or changes in earthquake catalogs that are of human, not natural, origin. A superficial application of the ideas of chaos has led some to conclude that earthquakes are not predictable. Several workers active in studying earthquakes as an example of deterministic chaos, however, are moderately optimistic about prediction. Long- and intermediate-term prediction are areas where I think progress is possible in the next 20 years. Much remains to be done in understanding the physics of earthquakes and the role of fluid pressures at depth in fault zone and in deploying dense networks of a variety of observing instruments.
I thank J.Deng, S.Jaumé, C.Scholz, and B.Shaw for critical comments and discussions. This work was supported by grants from the U.S. Geological Survey, the National Science Foundation, and the Southern California Earthquake Center (SCEC). This is Lamont-Doherty Earth Observatory Contribution 5486 and SCEC contribution 319.
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