ALBERT

Beschreibung: 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+ or -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.

Beschreibung: A catalog of 383 earthquakes in southeastern New York, southwestern Connecticut, northern New Jersey, and eastern Pennsylvania, including metropolitan New York City and Philadelphia, is compiled from historical and instrumental data from 1677 through 2006. A magnitude-felt area relationship is used to calculate the equivalent magnitude m (sub b) Lg prior to the advent of abundant instrumental data in 1974. Revised locations are computed for a number of historic earthquakes. Most hypocenters are concentrated in older terranes bordering the Mesozoic Newark basin in the Reading, Manhattan, and Trenton prongs and in similar rocks found at a shallow depth beneath the coastal plain from south of New York City across central New Jersey. Historic shocks of m (sub b) Lg 3 and larger were most numerous in the latter zone. The largest known event, m (sub b) Lg 5.25, occurred just offshore of New York City in 1884. Many earthquakes have occurred beneath the 12-km wide Ramapo seismic zone (RSZ) in the eastern part of the Reading prong, where station coverage was the most extensive since 1974. The southeastern boundary of the RSZ, which is nearly vertical, extends from near the surface trace of the Mesozoic Ramapo fault to depths of 12-15 km. Because the Mesozoic border fault dips about 50 degrees -60 degrees southeast, earthquakes of the RSZ are occurring within middle Proterozoic through early Paleozoic rocks. Which faults within the RSZ are active is unclear. Well-located activity in the Manhattan prong since 1974 extends to similar depths but cuts off abruptly at all depths along a northwest-striking boundary extending from Peekskill, New York, to Stamford, Connecticut. That boundary, which is subparallel to brittle faults farther south, is inferred to be a similar fault or fault zone. Those brittle features may have formed between the Newark, Hartford, and New York bight basins to accommodate Mesozoic extension. The Great Valley in the northwestern part of the study region is nearly devoid of known earthquakes. While few focal mechanism solutions and in situ stress measurements of high quality are available, the maximum compressive stress is nearly horizontal and is oriented about N64 degrees E, similar to that in adjacent areas. The catalog is likely complete for events of m (sub b) Lg〉5 since 1737, 〉 or =3.5 since 1840, and 〉 or =3.0 since 1928. Extrapolation of the frequency-magnitude relationship indicates that an event of m (sub b) Lg〉 or =6.0 is expected about once per 670 yr.