Maximum seismic depth versus thermal parameter of subducted slab: application to deep earthquakes in Chile and Bolivia

Abstract

The maximum seismic depth (Dm) depends on the thermal parameter of the descending slab (cp), which is a product of the age of the subducted lithosphere (A) and the vertical component of convergence rate (V 1.). We analyze seismicity profiles across the subduction zones of Mexico, Chile, Kamchatka, Kuriles, Japan, Sumatra, New Hebrides and Aleutians. The quasilinear part of the dependence (Dm ~ 240 km, and cp~20x102 km) corresponds to relatively young and slowly descending slab and is in general agreement with the "critical temperature" models of deep earthquakes. For cp >20x 102 km, which corresponds to the relatively older lithosphere subducting at a higher rate, the D m = f ( cp) dependence is essentially nonlinear. The flattening of the empirical Dm = f(cp) curve in the range of 20x102 km < cp < 35xl02 km is found to be a direct indication of the influence of the equilibrium 0!-Sp phase transition on Dm. Models invoking the metastable 01-Sp phase transition as a mechanism which controls the deepest seismicity can not be constrained by the results of this study. Deep focus earthquakes of the Chilean subduction zone and the strong event of June 9, 1994 in Bolivia (Mw = 8.2), were analyzed using ~he general standard dependence Dm = f (cp). A group of Chilean deep events south of 26°S has occurred inside the detached fragment of the subducted slab. D m values corresponding to these deep events lie outside D m = f (cp) curve. All other events, including the Bolivian earthquake, fit the curve within the uncertainty estimate. Thus the northern group of Chilean events does not necessarily pertain to the detached slab, and the Bolivian earthquake probably occurred at the northernmost deep edge of the oldest segment of the subducted Nazca plate.