The thermal structure of subduction zones predicted by plate cooling models with variable thermal properties

Previous modelling studies have investigated the effects of experimentally constrained thermal properties (i.e. thermal conductivity, specific heat and the thermal expansion coefficient) on the thermal structure of subduction zones. However, these studies have not carefully considered whether the assumed thermal structure of the slab before subduction is consistent with geophysical observations. This study investigates the effects of thermal properties on the thermal structure of the Tohoku subduction zone, northeast Japan, by using the slab temperature at the trench determined from plate cooling models. Three types of thermal properties were tested: constant, temperature-dependent and temperature- and lithology-dependent types. For each case, the parameters for the plate cooling models were inferred based on the observed surface heat flow and seafloor depth using Bayes' theorem. It was found that the predicted temperature and location of phase boundaries in the slab, which are possibly related to intermediate-depth earthquakes, are similar for the three cases. This suggests that, in the Tohoku subduction zone, constant thermal properties can be used in modelling to examine phenomena related to slab dehydration. The depth uncertainties for isotherms in the oceanic plate and slab increase with temperature, and are about +/- 10 and +/- 20 km for the 600 and 1200 degrees C isotherms, respectively. When this uncertainty is considered, the location of the serpentinite-out boundary matches that of the lower plane of double seismic zone, suggesting that dehydration may be important in triggering intermediate-depth seismicity. However, the large uncertainty makes it difficult to discuss in detail the origins of intraplate earthquakes, the lithosphere-asthenosphere boundary, and the lower boundary of the slab in terms of temperature.