Thick, strong sediment subduction along south-central Chile and its role in great earthquakes

Abstract

The south-central Chile margin experienced the largest and sixth largest earthquakes ever recorded - the 1960 Mw 9.5 Valdivia and 2010 Mw 8.8 Maule megathrust earthquakes, respectively. In early 2017, we conducted a seismic survey along 1,000 km of south-central Chile to image these rupture zones using a 15.15-km-long multi-channel seismic streamer. We processed these data using pre-stack depth migration, which provides the best look at the shallow part of the south-central Chile margin to date. Relative to other sediment-dominated subduction zones, where sediment is typically accreted at the toe, an unusually large percentage of the thick trench sediments are consistently subducted beneath the slope with little thrust faulting or deformation. Analysis of the sediment P-wave velocities and structure in the trench and outer wedge leads us to conclude that most of south-central Chile contains well-drained, strong sediments. An exception in the vicinity of the subducting Mocha Fracture Zone (MFZ) has trench sediments that appear to experience localized delayed compaction, thus lowering their strength and allowing the development of protothrusts, similar to what is seen in other accretionary subduction zones. The very shallow décollement along the south-central Chile allows more sediment to pass beneath the lower slope than almost all other subduction zone, many of which have much thicker trench sections. We conclude that subduction of the strong, well-drained, thick sediment layer beneath the lower slope is typical for nearly all of the south-central Chile. Comparison to other thick-sedimented subduction zones worldwide reveals that the subduction of such a large fraction of the trench sediment is particularly unusual. This strong, thick, subducting sediment is likely a determining factor for developing a smooth plate interface located well above the subducted crust topography that ultimately becomes a broad megathrust with high, homogeneous frictional properties, and generates particularly large earthquakes along the south central Chile margin.