Seismicity and rheology of subducted slabs

Abstract

Since the incorporation of the inclined zones of intermediate and deep seismic activity into the theory of plate tectonics^1,2 it has been generally accepted that earthquakes with focal depths ≳100 km occur in subducted slabs of (predominantly oceanic) lithosphere. Yet our understanding of the processes which generate intermediate and, in particular, deep earthquakes is still incomplete. This is largely due to our limited knowledge of the rheological behaviour of subducted oceanic lithosphere at depths between ∼100 and 700 km. However, recently considerable insight has been gained into the rheology of oceanic lithosphere in near-surface conditions and I investigate here whether the seismic activity at depths >100 km can be understood on the basis of this information. I first present results of temperature calculations performed for all subduction zones for which adequate data on seismicity, relative plate motion and age of the descending oceanic lithosphere exist. Sub-sequently, I adapt the temperature conditions governing the rheology near the surface by including depth-dependence. This leads to a depth-dependent critical temperature T_cr(z) above which the subducted lithosphere cannot sustain the stresses necessary to generate seismic events. Seismicity data and calculated temperature versus depth relationships are in good agreement with the rheological model predictions. This implies that the absence of seismic activity at depths ≳700 km should not be interpreted as direct evidence for the hypothesis that the 650-km discontinuity in the mantle acts as a barrier to vertical motion of subducted slabs.