Interseismic Coupling and Quaternary Coastal Tectonics along the Andean Megathrust
Résumé
The Andean margin is one of the most active subduction zone worldwide (e.g., the
1960 Mw 9.5 Chile or the Mw 8.8 1906 Ecuador earthquakes). We here propose
that the interseismic coupling along the Andean megathrust relates to spatial
variations along the interface as well as the fore-arc geology and inherited fault
zones. For such matter, we compiled information on the extent of earthquake
ruptures for the last 500 years, geodetic data and uplift rates derived from marine
terraces from Ecuador to Chile. First, we show that 1) the coastline geometry,
characterized by the distance between the coast and the trench, 2) the latitudinal
variations of long-term uplift rates, 3) and the spatial pattern of interseismic coupling
are correlated. In addition, the seafloor roughness of the subducting plate is
usually thought to be a cause of segmentation along subduction zones. But, after
discussing the role of inherited structures within the upper plate to the megathrust
segmentation in Ilo area during the Arequipa earthquake, we suggested that
the continental structure itself may exert some feedback control on the segmentation
and thus participate in the rupture pattern of major subduction earthquakes.
This hypothesis tends to correlate with similar observations on crustal faulting
in Pichelemu area in Chile. Finally, it’s now widely accepted that paleotsunami
deposits can be used as the primary sources of information on past large megathrust
events. In southern Ecuador and northern Peru, the low coupling areas suggest
rather rare tsunamigenic events and sedimentary evidences represent a useful
tool to extend the cataloging of large earthquakes in the past thousand years. All
together these multidisciplinary approaches suggest that the spatial variations
of frictional properties along the megathrust dictate the geomorphology of the
coastal zone and can inform on the extent of seismic ruptures along strike.