A Possible Tsunamigenic Near‐Trench Strike‐Slip Fault, Offshore North Ecuador–South Colombia

Abstract Seismic reflection and bathymetry collected along the Ecuador–Colombia obliquely convergent margin allow the first characterization of the NNE‐trending, near‐trench strike‐slip Ancon Fault in the possible source region of the 1906‐Mw8.6–8.8 and 1979‐Mw8.2 earthquakes, which produced devastating tsunamis. This study aims at highlighting the possible tsunami contribution of the fault during subduction earthquakes. The fault, which correlates with a zone of strong interseismic, inter‐plate coupling, is 200‐km‐long, segmented and bordered by remarkable slump scars. It bounds a tectonic sliver characterized by structural and rheological variations. The south‐fault segment bounds a pop‐up structure that comprises an up‐to‐25‐km‐wide accretionary wedge, and a mid‐slope oceanic basement block uplifted by dextral transpression. The Ancon Fault becomes dominantly reverse in a seamount collision zone, where the East‐directed Galera fault takes over toward the central‐fault segment. This segment shows extension reflecting a releasing fault bend. The northern‐fault segment is transpressive and fans out northward. It separates the fore‐arc basin from a near‐trench, ∼20‐km‐wide, pop‐up oceanic basement block. Morphology, geological structures and sediment dating support a late‐Pleistocene/Holocene activity of the Ancon Fault. The fault could have ruptured concurrently with the 1906 and possibly 1979 earthquakes, and contributed to the tsunamis by producing lateral displacement and differential uplift of the tectonic sliver in a similar way as a normal fault rupture contributed to the 2011 Tohoku‐Oki tsunami. Transpressional uplift and landslides associated with the rupture of strike‐slip faults are plausible contributing factors to tsunamis offshore North Ecuador‐South Colombia and should be considered in seismic hazard models.