Late Pleistocene Slip Rate and 3D Geometry of the Mid-Channel Blind Thrust, Santa Barbara Channel, California, and Their Implications for Earthquake Hazards

Abstract

ABSTRACT The Mid-Channel fault is an active blind thrust overlain by a south-vergent anticline in the Santa Barbara channel of southern California. We use high-quality industry seismic reflection data to characterize the 3D geometry of the Mid-Channel fault, and define its lateral extent and interactions with other structures in the region. We use a detailed chronostratigraphic sequence of nine Pleistocene horizons ranging in age from 120 ka to 1.05 Ma to interpret the geometry and deformational history of the Mid-Channel anticline and blind thrust. These precisely dated horizons provide unique insights on the Pleistocene activity of the Mid-Channel fault because of the nearly complete preservation of sediment on the backlimb of the fold. Using a new method of relating structural relief of these horizons to slip on the underlying Mid-Channel fault, we calculate the faults late Pleistocene slip rate. Results indicate that the structure became active between 790 and 710 ka in the eastern part of the Channel and subsequently propagated west along strike. In the east, where the overlying anticline preserves a complete section of syntectonic growth strata, the Mid-Channel fault has a dip-slip rate of 1.8 mm/yr with a range of 1.6–2.1 mm/yr. Based on a map of total displacement, we infer that the maximum dip-slip rate on the Mid-Channel fault along strike is 2.1 ± 0.2 mm/yr, suggesting that it may accommodate about one-third of the observed geodetic contraction across the basin. We explore alternative ways that the fault may extend to depth and interact with surrounding structures, and consider the seismic hazard implications of these scenarios, including estimates of potential earthquake rupture areas, magnitudes, and average recurrence intervals.