The thickness of subduction plate boundary faults from the seafloor into the seismogenic zone

Abstract

The thickness of an active plate boundary fault is an important parameter for understanding
\nthe strength and spatial heterogeneity of fault behavior. We have compiled direct
\nmeasurements of the thickness of subduction thrust faults from active and ancient examples
\nobserved by ocean drilling and fi eld studies in accretionary wedges. We describe a general
\ngeometric model for subduction thrust décollements, which includes multiple simultaneously
\nactive, anastomosing fault strands tens of meters thick. The total thickness encompassing all
\nsimultaneously active strands increases to ~100–350 m at ~1–2 km below seafl oor, and this
\nthickness is maintained down to a depth of ~15 km. Thin sharp faults representing earthquake
\nslip surfaces or other discrete slip events are found within and along the edges of the tens-ofmeters-
\nthick fault strands. Although fl attening, primary inherited chaotic fabrics, and fault
\nmigration through subducting sediments or the frontal prism may build mélange sections that
\nare much thicker (to several kilometers), this thickness does not describe the active fault at
\nany depth. These observations suggest that models should treat the subduction thrust plate
\nboundary fault as <1–20 cm thick during earthquakes, with a concentration of postseismic
\nand interseismic creep in single to several strands 5–35 m thick, with lesser distributed interseismic
\ndeformation in stratally disrupted rocks surrounding the fault strands.