Late Quaternary kinematics, slip-rate and segmentation of a major Cordillera-parallel transcurrent fault: The Cayambe-Afiladores-Sibundoy system, NW South America

We describe the recent activity of the Cayambe-Afiladores-Sibundoy Fault (CASF) and recognise it as one of the major potential active structures of northwestern South America, based on field observations, stereoscopic aerial photos of offset late Pleistocene-Holocene deposits and landforms, and crustal seismic activity. The CASF runs for at least 270 km along the sub-Andean zone of northern Ecuador and southern Colombia. We measured systematic latest Pleistocene-Holocene right-lateral strike-slip motion and right-lateral reverse motion consistent with earthquake focal mechanism solutions, and estimated a 7.7 ± 0.4 to 11.9 ± 0.7 mm/yr slip-rate. Magnitudes of the earthquakes that could be generated by possible fault-segment reactivation range up to M 7.0 ± 0.1. The CASF should be considered as a major source of possible future large magnitude earthquakes, presenting a seismic hazard for the densely populated regions to the west. The CASF is part of the tectonic boundary of the North Andean block escaping NNE-wards with respect to the stable South American plate.