Interseismic Coupling and Quaternary Coastal Tectonics along the Andean Megathrust

Abstract

International audienceThe Andean margin is one of the most active subduction zone worldwide (e.g., the1960 Mw 9.5 Chile or the Mw 8.8 1906 Ecuador earthquakes). We here proposethat the interseismic coupling along the Andean megathrust relates to spatialvariations along the interface as well as the fore-arc geology and inherited faultzones. For such matter, we compiled information on the extent of earthquakeruptures for the last 500 years, geodetic data and uplift rates derived from marineterraces from Ecuador to Chile. First, we show that 1) the coastline geometry, characterized by the distance between the coast and the trench, 2) the latitudinalvariations of long-term uplift rates, 3) and the spatial pattern of interseismic couplingare correlated. In addition, the seafloor roughness of the subducting plate isusually thought to be a cause of segmentation along subduction zones. But, afterdiscussing the role of inherited structures within the upper plate to the megathrustsegmentation in Ilo area during the Arequipa earthquake, we suggested thatthe continental structure itself may exert some feedback control on the segmentationand thus participate in the rupture pattern of major subduction earthquakes.This hypothesis tends to correlate with similar observations on crustal faultingin Pichelemu area in Chile. Finally, it’s now widely accepted that paleotsunamideposits can be used as the primary sources of information on past large megathrustevents. In southern Ecuador and northern Peru, the low coupling areas suggestrather rare tsunamigenic events and sedimentary evidences represent a usefultool to extend the cataloging of large earthquakes in the past thousand years. Alltogether these multidisciplinary approaches suggest that the spatial variationsof frictional properties along the megathrust dictate the geomorphology of thecoastal zone and can inform on the extent of seismic ruptures along strike