only other place to look is the past, to recorded history and, with the intriguing new science of paleoseismology, into prehistory as well. Second, as people living in fault zones—and the politicians who represent them—go about their business under the shadow of "The Big One" and other less mythic earthquakes, they want to know what to do. To scientists and the USGS falls the role of providing the best information possible, both to forecast quakes and to try to mitigate their effects. Third, geophysicists are developing other analytical and theoretical ideas (besides the block and spring and characteristic models), laboratory techniques, and monitoring strategies to try to better understand the genesis and mechanics of earthquakes. Finally, though the Parkfield quake didn't erupt in October 1992, another furor of sorts did, shining a spotlight on both the science and sociology of earthquake prediction.

Hitting the Trail into Past Time

As any scientist will readily concede, your conclusions are only as good as your data. And for models as heavily reliant on the particularities of data as are the seismic cycle, elastic rebound, and characteristic earthquake theories, interevent times for a particular fault shine like diamonds in a coal mine. The more precisely known the year of past events, the closer can be the statistical analysis of the spectrum of data. The greater the number of events or data points, the more likely that subpatterns will be expressed and the greater confidence scientists can take in their conclusions. In recurrence models, scientists reflect their subjective judgments about the quality of the data used to produce a particular prediction with a reliability measure (such as A through E for strongly to weakly confident).

A major hurdle faced by those trying to unravel the mystery of earthquakes is the recency of recording devices. Since John Milne began to install his worldwide network of seismographs in 1896, there is less than a century of seismograms to pore over for evidence of repeating earthquakes. The elastic rebound theory predicts that the larger the quake, the longer the time necessary for the fault to reaccumulate that amount of stress; thus, the interevent times for quakes above M 6 are measured in centuries, and not even one full cycle has run since seismology ushered in the era of instrumentation. Ellsworth, Agnew, and many others have extended their research farther back into the preinstrumental historical record to see what settlers had to say about nineteenth-century earthquakes.

By adding history to seismology, the potential time series for the



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