EFFECTS OF HYDROTHERMAL CIRCULATION ON TEMPERATURES IN RUPTURE ZONE OF THE GREAT 1960 CHILEAN EARTHQUAKE

Temperatures along the megathrust fault in subduction zones are a major control on the updip and downdip limits of failure in great earthquakes, which affect the extent of strong ground motions. In southern Chile, ~44°S, existing thermal models without hydrothermal circulation yield megathrust fault temperatures of 200°C and 400°C for the updip and downdip limits of the rupture zone of the great 1960 Chilean earthquake. These temperatures differ from the commonly inferred seismogenic zone thermal limits of ~150 to 350°C. Fluid flow in the basaltic basement aquifer of subducting ocean crust may redistribute heat and influence megathrust temperatures in the Chile subduction zone. This fluid circulation is consistent with anomalously high heat flux observed at the trench. Downdip of the trench, fluid circulation mines heat from the subducting slab, cooling it and shifting the location of 350°C on the megathrust fault landward. The subducting Nazca plate is 0 to ~20 m.y. old in south Chile, and multiple fracture zones north of the subducting Chile Rise at 46°S partition Nazca plate age and therefore the thermal state of the subducting plate along the trench. We use a steady state finite element model including the effect of vigorous fluid and heat transport in an ocean crust aquifer to find a suite of conditions consistent with observed surface heat flux. Results of these thermal models yield new estimates for temperatures in the megathrust rupture area.