Tectonic control on sediment accretion and subduction off south central Chile: Implications for coseismic rupture processes of the 1960 and 2010 megathrust earthquakes

Based on a compilation of published and new seismic refraction and multichannel seismic reflection data along the south central Chile margin (33-46S), we study the processes of sediment accretion and subduction and their implications on megathrust seismicity. In terms of the frontal accretionary prism (FAP) size, the marine south central Chile fore arc can be divided in two main segments: (1) the Maule segment (south of the Juan Fernndez Ridge and north of the Mocha block) characterized by a relative large FAP (20-40 km wide) and ( In addition, the Maule and Chilo segments correlate with a thin (<1 km thick) and thick (1.5 km thick) subduction channel, respectively. The Mocha block lies between 37.5and 40S and is configured by the Chile trench, Mocha and Valdivia fracture zones. This region separates young (0-25 Ma) oceanic lithosphere in the south from old (30-35 Ma) oceanic lithosphere in the north, and it represents a fundamental tectonic boundary separating two different styles of sediment accretion and subduction, respectively. A process responsible for this segmentation could be related to differences in initial angles of subduction which in turn depend on the amplitude of the down-deflected oceanic lithosphere under trench sediment loading. On the other hand, a small FAP along the Chilo segment is coincident with the rupture area of the trans-Pacific tsunamigenic 1960 earthquake (M w = 9.5), while a relatively large FAP along the Maule segment is coincident with the rupture area of the 2010 earthquake (M w = 8.8). Differences in earthquake and tsunami magnitudes between these events can be explained in terms of the FAP size along the Chilo and Maule segments that control the location of the updip limit of the seismogenic zone. The rupture area of the 1960 event also correlates with a thick subduction channel (Chilo segment) that may provide enough smoothness at the subduction interface allowing long lateral earthquake rupture propagation.