Abstract

In southern California, high rates of measured geodetic shortening occur where active basin-bounding faults thrust early-Cenozoic rocks over young unconsolidated sediments. This implies that compaction, subsidence, and other nonelastic processes of footwall deformation may play an important role in contributing to the high rates of observed crustal strain. Even in the absence of active tectonic shortening, sediment compaction alone can produce surficial motions that mimic deep fault slip or elastic strain accumulation. Differential compaction and subsidence of footwall sediments relative to hanging-wall rocks can lead to increased vertical separation, basinward collapse, and fault rotation about horizontal axes. Such effects contribute to net horizontal and vertical motions in both geologic and geodetic data, and—if not properly accounted for—result in incorrect estimates of the inferred seismic hazard.

Subsidence and compaction also increase the potential for gravity sliding toward the basin and the development of significant nonplanar 3D fault geometry. A prime example occurs along the San Cayetano fault that bounds the eastern Ventura basin. At shallow levels, a large thrust sheet (the Modelo Lobe) with low dip extends out in front of the more steeply dipping, planar fault segment by over 4 km, is nearly 2 km thick, and occupies over 60 cubic km. This geometry is strongly indicative of gravity-driven failure resulting from hanging-wall uplift, basinward tilt, and collapse, enhanced by footwall subsidence and compaction. Failure of this mega-slide off the hanging-wall block most likely occurred within the Rincon Formation, a thick ductile shale sequence that often accommodates detachment slip. This 3D geometry has significant implications for how the fault may behave during dynamic rupture and implies that additional care should be taken in extrapolating near-surface measurements or estimates of fault slip and dip to seismogenic depths.

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