From the seismic cycle to long-term deformation: linking seismic coupling and Quaternary coastal geomorphology along the Andean megathrust
Résumé
Measurement of interseismic strain along subduction zones reveals the location of both locked asperities, which might rupture during megathrust earthquakes, and creeping zones, which tend to arrest such seismic ruptures. The heterogeneous pattern of interseismic coupling relates to spatial variations of frictional properties along the subduction interface and may also show up in the fore-arc morphology. To investigate this hypothesis, we compiled information on the extent of earthquake ruptures for the last 500 yrs and uplift rates derived from dated marine terraces along the central Peru to southern Chile coastline. We additionally calculated a new interseismic coupling model for that same area based on a compilation of GPS data. We show that the coastline geometry (i.e. the trench-to-coast distance), the latitudinal variations of long-term uplift rates and the spatial pattern of interseismic coupling are correlated. Zones of faster and long-term permanent coastal uplift, evidenced by uplifted marine terraces, coincide with peninsulas and also with areas of creep on the megathrust where slip is mostly aseismic and tend to arrest seismic ruptures. This correlation suggests that these areas prevent elastic strain buildup and inhibit lateral seismic rupture propagation. Correlation between the location of these regions across- and along-strike of convergence and the long-term morphology of the subduction margin suggests that the barrier effect might be due to rheology, namely, rate-strengthening friction, although geometric effects might also play a secondary role. Peninsulas are the surface expression of large subduction earthquakes segment boundaries and show evidence for their stability over multiple seismic cycles. We propose that the survey of the long-term deformation of the upper plate to better identify multicycles margin segmentation would be an interesting improvement in seismic hazard assessment in subduction zone.