The current(Oct)issue of the Bulletin of the Seismological Society of America has several interesting articles. Here is a passage from the conclusion of one of them, "Repeat Times of Large Earthquakes: Implications for Earthquake Mechanics and Long-Term Prediction," by Lynn R. Sykes and William Menke: (the abstract of the article is below this passage)

"The Hayward fault and possibly at two other fault segments in California appear to be advanced in their cycles of stress buildup to their next large earthquakes. We conclude that each has been assigned too low a probability of rupture in the next few decades by either WG (1995) or WG (2003). They used a much larger value of CV than we think is appropriate. The Peninsular segment of the San Andreas fault has a moderate to high, but poorly constrained, probability of rupture in an event of about M 7 during the next 30 years. The segment of the San Andreas fault in San Gorgonio Pass does not appear to have ruptured in a great earthquake since about 1680. That segment, however, is structurally complex and its long-term rate of deformation is uncertain. Especially in this year of the centenary of the 1906 San Francisco earthquake, more attention in California needs to be devoted to those three segments. Likewise, the rupture zone of the 1938 earthquake of Mw 8.3 off the Alaska Peninsula (Davies et al., 1981), which broke in yet larger events in 1788 and 1847, deserves greater study than the little presently being devoted to it."

Here is the abstract:

"Information on the time intervals between large earthquakes is now available for several fault segments along plate boundaries in Japan, Alaska, California, Cascadia, and Turkey. When dates in a sequence are known historically, as along much of the Nankai trough, they provide information on the natural (intrinsic) variability of the rupture process. Most sets of repeat times, however, are dominated by paleoseismic determinations of dates of older large earthquakes, which contain measurement uncertainties in addition to intrinsic variability. A Bayesian technique along with prior information on measurement uncertainties is used to make maximum-likelihood estimates of intrinsic repeat time and its normalized standard deviation, the coefficient of variation (CV). It is these intrinsic parameters and their uncertainties that are most useful for understanding the mechanics of earthquakes and for prediction for timescales of a few decades. Our estimates of intrinsic CV are small, 0 to 0.25, for several very active fault segments where deformation is relatively simple, large events do not appear to be missing in historic and paleoseismic records, and data are available at or near major asperities and away from the ends of rupture zones. CV is larger for regions of multibranched faulting, overlapping slip near the ends of rupture zones and for data from uplifted terraces at subduction zones. A Poisson process is an inferior characterization of all of the 11 segments we examined. Scenarios used by recent working groups that assume either Poissonian behavior or renewal processes with CV of 0.5 ± 0.2 for the most active fault segments in the San Francisco Bay area likely lead to incorrect 30-year probability estimates. The Hayward fault and perhaps the Peninsular segment of the San Andreas fault in the San Francisco Bay area appear to be advanced in their buildup of stress that will be released in future large earthquakes. Multibranched faulting may account for why the predicted Tokai earthquake in Japan has not occurred as of 2006. Parkfield earthquakes from 1857 to 2004 were characterized by the largest uncertainty of the sequences we studied, CV = 0.37, which may account for the failure of past predictions. The large CV for Parkfield fits our hypothesis that relatively weak fault segments are characterized by more irregular earthquake recurrence. Paleoseismic data from coastal sites along the Cascadia subduction zone are characterized by CVs of about 0.3."

Michael F. Williams
Arroyo Grande, CA USA