Control of forearc density structure on megathrust shear strength along the Chilean subduction zone

Unveiling how the interaction of friction (mu), pore pressure (p) and normal stress (sigma(n)) controls the shear strength distribution along megathrust faults at convergent plate boundaries is crucial to our understanding of processes leading to the occurrence of devastating earthquake and the construction of mountains. An existing density model of the Chilean margin (18 degrees-45 degrees S) allows the computation of the so-called Vertical Stress Anomaly (VSA) that accounts for the component of normal stress due to the weight of the overlying crustal column. A spatial correlation between VSA, surface geology and several earthquake parameters suggests a significant control exerted by the geologically-dominated density structure of the crust on megathrust shear strength. The very seismogenic forearc in front of the high Central Andes (18 degrees-34 degrees S) has positive VSA values (up to 60 MPa) and a mafic-dominated crust, which suggest high values of shear strength at the megathrust that are partially controlled by the weight of a dense forearc crustal column, in addition to a likely dry subduction channel characterized by high-mu and low-p. In contrast, the Southern Andean forearc (34 degrees-45 degrees S) shows neutral-to-negative VSA (down to 30 MPa) correlated with a felsic-dominated crust, which added to a presumably wet subduction channel (low-mu and high-p) configures a rather weak megathrust below a light forearc. Earthquakes here are concentrated around the Arauco peninsula, a first-order morpho-structural anomaly characterized by comparatively higher values of VSA than the rest of this segment, which is the nest of the giant (Mw 9.5) Valdivia 1960 earthquake and the southern rupture termination of the recent Mw 8.8 Maule earthquake. The ultimately giant size of the 1960 earthquake resulted from rupture propagation along a 1000 km long region characterized by negative VSA values. The correlation between VSA and the along-strike segmentation of the Andean orogen indicates that the geologically-inherited density structure of the forearc also rules long-term mechanical coupling between converging plates and prescribes the possible structure of the orogen. Because Central and Southern Andes represent two extremes in terms of orogenic structure and seismogenic behavior, results presented here have significant implications for the general comprehension of processes occurring at short and long time scale at convergent margins. (C) 2010 Elsevier B.V. All rights reserved.