Abstract

We develop new methods to identify blind-thrust fault systems, determine fault slip rates, and estimate potential earthquake magnitudes and recurrence intervals in active fold-and-thrust belts. These methods are applied to compressive folds along the Offshore Oak Ridge and Blue Bottle trends, which overlie active blind-thrust faults in the eastern Santa Barbara Channel. These folds and their causative faults are interpreted using fault-bend fold theory and are represented in balanced models and cross sections that integrate surface and subsurface data. The structures are mapped using a new technique of axial-surface mapping in seismic reflection grids, which defines three-dimensional structural geometries and shows changes in slip and subsurface fault geometry along strike. Analysis of syntectonic (growth) sediments yields Pliocene and Quaternary fault slip rates of 1.3 mm/yr on a deep thrust (≥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/yr of shortening measured by geodesy across the channel and estimated from relative Pacific-North American plate motions across the Transverse Ranges. Additional shortening is probably accommodated on other active thrusts in the western Transverse Ranges and in the northern channel along the Santa Barbara coast.

Deformed seafloor sediments and a swarm of axial surface seismicity along the fold trends indicate that the underlying thrusts are active and may pose significant earthquake hazards to coastal southern California. Unsegmented fault surfaces are used through empirical relationships between fault surface area and rupture magnitude to estimate the sizes of potential earthquakes. This analysis suggests that a ramp in the Channel Islands fault beneath the Offshore Oak Ridge trend is capable of rupturing in a Ms ≥7.2 earthquake. Earthquakes of this magnitude may release ∼2 m of slip, which, when combined with the estimated slip rate (1.3 mm/yr), yields an earthquake recurrence interval of ∼1500 yr for this Channel Islands fault ramp.

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