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Stress changes produced by the 1906 San Francisco earthquake had a profound effect on the seismicity of the San Francisco Bay region (SFBR), dramatically reducing it in the twentieth century. Whether the SFBR is still within or has emerged from this seismic quiescence is an issue of debate with implications for earthquake mechanics and seismic hazards. Historically, the SFBR has not experienced one complete earthquake cycle (i.e., the accumulation of stress, its release primarily as coseismic slip during surface‐faulting earthquakes, its re‐accumulation in the interval following, and its subsequent rerelease). The historical record of earthquake occurrence in the SFBR appears to be complete at about M 5.5 back to 1850 (Bakun, 1999). For large events, the record may be complete back to 1776, which represents about half a cycle. Paleoseismic data provide a more complete view of the most recent pre‐1906 SFBR earthquake cycle, extending it back to about 1600. Using these, we have developed estimates of magnitude and seismic moment for alternative sequences of surface‐faulting paleoearthquakes occurring between 1600 and 1776 on the region’s major faults. From these we calculate seismic moment and moment release rates for different time intervals between 1600 and 2012. These show the variability in moment release and suggest that, in the SFBR regional plate boundary, stress can be released on a single fault in great earthquakes such as that in 1906 and in multiple ruptures distributed on the regional plate boundary fault system on a decadal time scale.

Evidence of rapid relative sea level changes preserved in sediment of coastal estuaries along the north coast segment of the San Andreas fault provides information on the dates of past earthquakes. This approach, although successfully used to document large subduction zone earthquakes, has not been used previously to constrain the dates of San Andreas fault or other strike-slip fault earthquakes. Data on prehistoric San Andreas fault earthquakes is needed for development of robust probabilistic hazard assessments in northern California and the San Francisco Bay area. Evidence of earthquake-induced subsidence is preserved in marshes at the northern margin of Bolinas Lagoon and the southeastern margin of Bodega Harbor. These sites occupy structural basins or troughs along the northern San Andreas fault. Radiocarbon dating and identification of the first occurrence of nonnative pollen near abrupt sedimentological changes in cores are used to constrain the dates of San Andreas fault earthquakes over about the last 800 years. Estuarine sediment from northern Bolinas Lagoon preserves evidence of an earthquake that occurred about 750 years ago. Evidence for this earthquake includes a marsh soil that was abruptly buried by tidal flat mud or coarse, poorly sorted deltaic deposits containing a mixture of terrestrial sediment and marine mud. Evaluation of diatom assemblages from above and below the buried soil horizon provides evidence of at least several decimeters of sustained relative sea level rise. Three buried soils in a marsh at the southern end of Bodega Harbor are likely a result of coseismic subsidence. All three buried marsh soils have abrupt upper contacts, although diatom evidence for decimeters of sustained subsidence is not as strong as it is for the buried soil in Bolinas Lagoon. Existing paleoseismic data on the northern San Andreas fault is compared with results of this study in order to evaluate the dates of prehistoric large earthquakes and length of past ruptures. These data indicate that an earthquake occurred about 400 years ago and another earthquake occurred about 700 years ago. The data allow for 1906-type ruptures of the fault from the Santa Cruz Mountains to at least Point Arena. However, a sequence of closely timed, smaller earthquakes would produce a similar set of paleoseismic data.

Paleoseismic investigation of the Northridge Hills fault in the northern San Fernando Valley, California, helps assess the timing and style of near-surface late Quaternary deformation in the epicentral area of the 1994 Northridge earthquake. The Northridge Hills fault, a 15-km-long, north-dipping reverse fault, exhibits geomorphic evidence of late Quaternary surface deformation, including topographic scarps across late Quaternary fluvial terraces and aligned alluvial-fan apices on the footwall block. We excavated one 40-m-long trench and six test pits, and drilled nine boreholes across a 2-m-high scarp developed on a probable Holocene fluvial terrace adjacent to Aliso Canyon Wash. A continuous clayey gravel identified in the trench, test pits, and boreholes defines a south-facing monocline with 6 ± 1 m of vertical separation across the Northridge Hills fault. Based on pedochronology, the clayey gravel ranges in age from 6 to 30 ka. The borehole data also suggest that an unconformity developed on the Plio-Pleistocene Saugus Formation is warped into a monocline that has 13 ± 2 m of vertical separation across the fault. These preliminary data yield a dip-slip rate of 1.0 ± 0.7 mm/yr for the Northridge Hills fault. The absence of distinct scarp-derived colluvium in trench exposures at the base of the scarp and secondary brittle fracturing or faulting suggests that the monocline is related to folding during small, incremental uplifts rather than large uplifts that generate distinct scarp relief. We postulate that such uplift could be produced via moderate-magnitude earthquakes ( M W 6¼) on the Northridge Hills fault, or secondary deformation induced by earthquakes on other faults. For instance, evidence of surface uplift near the trench site during or following the 1994 earthquake suggests that all or part of the observed deformation is a result of secondary slip on the Northridge Hills fault produced by movement on the underlying Northridge blind reverse fault or other nearby large structures. Based on our geologic investigations, the distribution of aftershocks following the 1994 earthquake, and pre- and post-1994 leveling and geodetic surveys, we interpret that the Northridge Hills fault underwent triggered slip during the 1994 earthquake.

Paleoseismic trenching was performed to assess the slip rate and earthquake history of the northern segment of the Calaveras fault at a site along Welch Creek in the eastern San Francisco Bay area, California. At Welch Creek, the northern Calaveras fault crosses a series of fluvial terraces and displaces the intervening terrace risers. We derive a late Holocene slip rate using two independent methods: (1) by measuring the offset of the back-edge (i.e., terrace angle) of one of the offset terraces and (2) by using isopach contours to measure the offset of a debris flow unit within the terrace sediments. The terrace back-edge is offset 39 ± 1 m and is between 5 and 13 ka old. The debris flow deposit is offset 27 ± 1 m; the deposit age is estimated to be between 4840 and 5325 cal yr B.P. These findings suggest a late Holocene slip rate of 6 ± 1 mm/yr for the northern Calaveras fault. We recognize as many as seven surface-rupturing earthquakes at Welch Creek, although the amount of terrace back-edge displacement suggests that several more events must have occurred that are not discernable in the stratigraphic or structural record. Based on the maximum amount of time between age-constrained paleoearthquakes that are preserved in the record at Welch Creek, we derive an estimate of the maximum recurrence interval of between 1375 and 3425 yr. Using the assumption that additional events are required to account for the 39 ± 1 m of back-edge offset, we derive a recurrence estimate of between 125 and 685 yr.

Active thrust faults pose a significant seismic hazard worldwide. Many of these faults include “blind” thrusts, where the propagating fault tip does not reach the Earth's surface, and “buried” faults, where the geomorphic expression of the fault is obscured by subsequent sedimentation and/or erosion. This raises the issue of whether conventional geologic, geomorphic, and paleoseismic methods can be used to identify and characterize thrust faults for the assessment of seismic hazards or whether these faults sometimes are truly “blind.” We compiled a data base of 148 worldwide moderate- to large-magnitude thrust/reverse earthquakes to evaluate whether or not the event occurred on a fault that could have been identified prior to the earthquake on the basis of recognizable Quaternary surface deformation (i.e., a pre-existing fault or fold). Analysis of the data shows that interplate reverse earthquakes almost always are associated with pre-existing Quaternary deformation that was or could have been recognized prior to the earthquake. In particular, most interplate reverse earthquakes are associated with an active reverse fault at the surface and/or an active anticline. In contrast, intraplate reverse earthquakes seldom occur on faults associated with pre-existing recognizable surface deformation. We conclude that thrust faults can be detected in interplate regions with careful Quaternary geologic and geomorphic mapping; furthermore, the absence of Quaternary surface deformation can be used to infer the absence of an underlying active blind thrust fault in interplate tectonic settings. However, the data show that Quaternary geologic mapping techniques alone likely are insufficient to characterize blind thrusts in intraplate regions. In these areas, inclusion of a floating or random earthquake may be necessary to assess earthquake hazards.

This reply addresses the three main comments by Shlemon et al. (2001), namely that our article (1) provided inadequate reference to a large body of earlier work, (2) yielded a conclusion that had already been made, and (3) estimated a range in slip-rate uncertainty that is too large. We address these three issues individually. First, we acknowledge that the Northridge Hills fault was identified many years ago and that it was previously interpreted as a probable active fault. However, Shlemon et al. (2001) implied that Baldwin et al. (2000) failed to acknowledge the earlier work performed by others (Barnhart and Slosson, 1973; Saul, 1975; and Johnson et al., 1996). They also suggest that earlier geomorphic and geologic observations by Barnhart, Slosson, and Saul are sufficient to define the seismogenic potential of the Northridge Hills fault. We disagree. As stated in our article, the Northridge Park trench site “... was selected on the basis of detailed geologic and geomorphic mapping of surficial deposits in the northern San Fernando Valley (Hitchcock and Kelson, 1996; Wills and Hitchcock, 1999),” (p. 631) in addition to the earlier work performed by others (e.g., Barnhart and Slosson, 1973; Smith, 1977; Saul, 1979; Dibblee, 1992). We also referenced Johnson et al. (1996) throughout the manuscript. The detailed 1:24,000-scale mapping of surficial deposits summarized by Hitchcock and Kelson (1996) and Wills and Hitchcock (1999) has recently been published by the California Division of Mines and Geology as Map Sheet 50 (1:48,000-scale) (Hitchcock and Wills, 2000). Second, we acknowledge that the Northridge Hills fault was interpreted as a possible active fault (Barnhart and Slosson, 1973), but also note Barnhart and Slosson (1973) state “Proper credence to the …