How does lithospheric strength, mantle hydration and slab flexure relate to seismicity in the southern Central Andes?

The southern Central Andes (SCA, 29°S—39°S) orogen is one of the seismically most active regions along the length of the South-American convergent margin, where past earthquakes (e.g., San Juan in 1944, Valdivia M9.5 in 1960 and M8.8 Maule in 2010) have had devastating effects on the population. Past research has extensively focused on linking the occurrence of seismic activity with the stress regime on individual faults at a local scale.  In order to more systematically address the relationship between the long-term rheological configuration of the whole lithosphere and the spatial patterns of seismic deformation in the SCA, we computed a 3D model of the expected mechanical strength and rheology (brittle, ductile) of the SCA and adjacent forearc and foreland regions based on an existing 3D model describing the first-order variations of thickness, composition and temperature of geological units forming the upper and subducting plates. We found that the spatial variation in the predicted rheology correlates well with the distribution of seismic deformation in the upper plate, with seismicity bounded to the modelled brittle deformation domain. Moreover, seismic events localize at the transition between mechanically strong and weak domains. This ultimately indicates that the strength of the lithosphere exerts a first-order control on the mechanical stability of the region.

In contrast, the results from the rheological model fail to reconcile the observed slab seismicity at depths > 50—70 km, where ductile rheological conditions are expected. In this case, we evaluated possible additional mechanisms triggering these earthquakes, including compaction of sediments at the interface, metamorphic reactions within the oceanic crust and mantle, and slab flexural stresses. To characterize the state of hydration of the mantle related to dehydration reactions and/or sediment compaction, we made use of the Vp/Vs ratio from a seismic tomography model. The majority of the slab seismicity was found to spatially correlate with hydrated areas of the slab and overlying continental mantle, apart from a cluster where the slab attains a sub-horizontal angle. In this region, the correlation between the focal mechanisms of these earthquakes and the slab orientation, suggests that seismicity here is driven by enhanced flexural stresses within the oceanic plate.

This contribution showcases the importance of a quantitative characterization of the rheological state of the lithosphere to elucidate the causative dynamics of the spatial distribution of seismicity in the area.