ACTIVE FAULTING AND GROWTH FOLDING IN THE EASTERN SANTA-BARBARA CHANNEL, CALIFORNIA

We develop new methods to identify blind-thrust fault systems, determi ne fault slip rates, and estimate potential earthquake magnitudes and recurrence intervals in active fold-and-thrust belts. These methods ar e applied to compressive folds along the Offshore Oak Ridge and Blue B ottle trends, which overlie active blind-thrust faults in the eastern Santa Barbara Channel. These folds and their causative faults are inte rpreted using fault-bend fold theory and are represented in balanced m odels and cross sections that integrate surface and subsurface data. T he structures are mapped using a new technique of axial-surface mappin g in seismic reflection grids, which defines three-dimensional structu ral geometries and shows changes in slip and subsurface fault geometry along strike. Analysis of syntectonic (growth) sediments yields Plioc ene and Quaternary fault slip rates of 1.3 mm/yr on a deep thrust (gre ater-than-or-equal-to 16 km) and 1.3 mm/yr on shallower faults (2-5 km ). The combined 2.6 mm/yr slip rate represents only part of the 6 mm/y r of shortening measured by geodesy across the channel and estimated f rom relative Pacific-North American plate motions across the Transvers e Ranges. Additional shortening is probably accommodated on other acti ve thrusts in the western Transverse Ranges and in the northern channe l along the Santa Barbara coast. Deformed seafloor sediments and a swa rm of axial surface seismicity along the fold trends indicate that the underlying thrusts are active and may pose significant earthquake haz ards to coastal southern California. Unsegmented fault surfaces are us ed through empirical relationships between fault surface area and rupt ure magnitude to estimate the sizes of potential earthquakes. This ana lysis suggests that a ramp in the Channel Islands fault beneath the Of fshore Oak Ridge trend is capable of rupturing in a M(s) greater-than- or-equal-to 7.2 earthquake. Earthquakes of this magnitude may release approximately 2 m of slip, which, when combined with the estimated sli p rate (1.3 mm/yr), yields an earthquake recurrence interval of approx imately 1500 yr for this Channel Islands fault ramp.