Maximum depth of seismicity and thermal parameter of the subducting slab: general empirical relation and its application

The maximum depth of seismicity (D-m) in subduction zones is limited by some physical mechanisms related to the rheological strength of slab material, metastability conditions, failure strength etc. The main factors controlling these properties are temperature and pressure within the lithosphere sinking into the mantle. These P-T conditions may be coarsely assessed from thermal models of slab subduction. The observed relationship between D-m, the age of the subducted lithosphere (A) and the convergence rate (V) can be used to test the theoretical models and reveal inconsistencies which may indicate changes in failure mechanisms with depth. We analyze the relation between D, and the thermal parameter of the descending slab (phi), which is a product of the age of the subducted lithosphere and the Vertical component of convergence rate (V perpendicular to). Seismicity profiles across the subduction zones of Mexico, Chile, Kamchatka, Kuriles, Japan, Sumatra, New Hebrides, Aleutians, Tonga and Marianas are used to estimate D-m. The quasi-linear part of the dependence (D-m less than or equal to 240 km, and phi less than or equal to 2000 km) corresponds to relatively young and slowly descending slab and is in general agreement with the 'critical temperature' models of deep earthquakes. For phi > 2000 km, which corresponds to the relatively older lithosphere subducting at a higher rate, the D-m = f(phi) dependence is nonlinear. The flattening of the empirical D-m = f(phi) curve in the range of 2000 km < phi < 3500 km is found to be a direct indication of the influence of the equilibrium Ol-Sp phase transition on D-m. Models invoking the metastable Ol-Sp phase transition as a mechanism which controls the deepest seismicity cannot be definitely constrained by the results of this study. The general dependence D-m = f(phi) may be applied as a standard relation between D-m, A, and V for the analysis of deep earthquakes and seismotectonic studies of the subduction zones.