Thermal Models of Flat Subduction and the Rupture Zone of Great Subduction Earthquakes

In subduction zones, the size of the seismogenic zone that ruptures during a great thrust earthquake may be thermally controlled. We have constructed finite-element models of the thermal structure of six subduction zones, incorporating complex slab geometries in order to determine the temperature distribution along the plate interface and to predict the size of the seismogenic zone. We focus on the rupture zones of the great earthquakes of Nankai (SW Japan) 1946 M8.3 and Alaska 1964 M9.2 as well as on the Cascadia margin. Steeper dipping slab segments exhibit rupture zones of 100-150 km downdip width, consistent with earlier elastic dislocation models and thermal models. Our models predict larger locked zones, of 150-250 km width, for shallow dipping regions corresponding to flat slab segments. Our values are in good agreement with the analysis of aftershock distribution following the Nankai and Alaska great earthquakes as well as with modern geodetic studies of SW Japan, Alaska, and Cascadia. The wider seismogenic zone predicted for flat slab segments results from an uncommonly wide, cold, forearc region, as corroborated by available heat flow data from the upper plate. A global analysis of great M8 interplate earthquakes of the 20th century reveals that more than a third of these events occured in flat slab segments, whereas these segments represent only 10% of modern convergent margins. This implies substantially higher interplate coupling for flat slab segments, likely due to the increased downdip extent of the seismogenic zone and suggests the seismic risk near such regions may be higher than previously thought.