A likelihood analysis of causative faulting of the 7 ka coastal uplift in Ilocos Region, Philippines

Abstract

Tectonic deformation can result in hazards like tsunamis and earthquakes, which have serious consequences on human livelihoods. A better understanding of tectonic structures can improve tectonic hazard identification in a region, reducing the impact of such hazards. There are gaps in the understanding of tectonic structures in the Ilocos region of the Philippines. The region is characterized by complex fault systems including the Manila Trench subduction zone, giving rise to deformation along the coast. Attempts have been made to understand the tectonics in the region by reconstructing the relative sea-level (RSL), but the tectonics in the region are still poorly understood. The reconstruction recorded a RSL fall of ~0.85 m at two sites in the Ilocos region at ~7480 cal yr BP, indicating coseismic uplift. An elastic dislocation model was used to recreate the uplift and identify the faults that caused the event. The model processed hypothetical fault scenarios, producing the predicted surface deformation. Each fault scenario was evaluated by a likelihood function to deduce its likelihood of producing the observed uplift. The function identified the fault scenarios most likely to cause the observed uplift at the two sites. The results suggest that the uplift was caused by a shallow dipping offshore thrust fault located along the accretionary wedge of the Manila Trench. These findings indicate that tectonic deformation in the Ilocos region can be caused by unmapped offshore faults. Thus, future efforts must be made to map the offshore faults to understand the tectonics of the region.