The vast majority of earthquakes with focal depths exceeding 60 km are associated with convergent plate boundaries (Fig. 1), including a few in relict subduction zones (e.g., beneath southern Spain) and others along current or former plate boundaries presently undergoing continental convergence (Hindu Kush, Greater Tethyan Himalayas, Romania) (Zhu and Helmberger, 1996; Chen and Yang, 2004; Frohlich, 2006; Monsalve et al., 2006). The remaining deeper, rare events within continental interiors are of special interest because their focal depths provide information about the strength and rheology of the crust and upper mantle.

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