Complex Kinematics of Upper‐Plate Faulting Along the Central Cascadia Forearc Inferred From the Lateral Displacement of Marine Terraces

Abstract Recent earthquakes along the shallow portion of the Hikurangi subduction zone (the 2016 Kaikoura earthquake sequence, New Zealand) indicate that faults within the overriding plate of subduction zones may exhibit uncharacteristically large surface displacements relative to their fault length. Whether this style of upper‐plate faulting is common elsewhere along subduction margins is unclear. We evaluate the displacement history along two upper‐plate faults in the Cascadia forearc that displace a sequence of marine terraces dated between 85 ka and 125 ka. Systematic differences in the width of terrace platforms across these faults suggest sustained lateral slip rates that average 1–3 mm/yr. Differential widths of terrace platforms suggest, however, that these faults experienced periods of significantly higher displacement rates that were limited in duration. Our results indicate that known high‐angle faults in the upper plate of the Cascadia subduction zone have more complicated slip histories than previously recognized and may represent a significant and underappreciated hazard.