precise kinematic framework for estimating the rates of geological deformation across plate boundaries, specifying a deformation budget that could be balanced against historic seismic activity. For example, Sykes divided the amount of co-seismic slip during the 1957, 1964, and 1965 Aleutian Trench earthquakes by the rate of relative motion between the North American and Pacific plates, obtaining recurrence intervals of a century or so for each of the three segments (120). Self-consistent models of the relative plate motions were derived from global data sets that included seafloor magnetic anomalies tied to the precise magnetic reversal time scale (121), allowing Sykes’s calculation to be repeated for many of the major plate boundaries. Sykes and his colleagues produced maps in 1973 and 1979 showing plate boundary segments with high, medium, and low seismic potential based on the recent occurrence of large earthquakes (122) and published a more refined forecast in 1991 (123) (Figure 2.12).
While some form of the Gutenberg-Richter distribution is observed for almost all regions, Schwartz and Coppersmith (124) proposed that many individual faults, or segments of faults, behave quite differently. They proposed that most of the slip on a fault segment is released in large “characteristic” earthquakes having, for a given segment, similar magnitude, rupture area, and average displacement. It follows that characteristic earthquakes must be much more frequent, relative to smaller and larger earthquakes, than the Gutenberg-Richter relationship would predict. Wesnousky and colleagues (125) argue that earthquakes in a region obey the Gutenberg-Richter relationship because the fault segments there have a power-law distribution.
Characteristic earthquakes have profound implications for earthquake physics and hazards. For example, characteristic earthquakes can be counted confidently, and their average recurrence time would be an important measure of seismic hazard. The time of the last one would start a seismic clock, by which the probability of another such earthquake could be estimated. For Gutenberg-Richter earthquakes, the simple clock concept does not apply: for any magnitude of quake, there are many more earthquakes just slightly smaller but no different in character. The characteristic earthquake model has strong intuitive appeal, but the size of the characteristic earthquake and the excess frequencies of such events have been difficult to demonstrate experimentally (126).
The seismic-gap method met limited success as a basis for earthquake forecasting (127). Attempts to use it as a general tool were frustrated by the difficulty of specifying characteristic magnitudes and the lack of historical records needed to estimate the recurrence interval T. Moreover, the practical utility of the seismic-gap hypothesis was compromised by the intrinsic irregularity of the earthquake process and the tendency of earthquakes to cluster in space and time. The Gilbert-Reid idea that a