- © 2015 by the Seismological Society of America
Online Material: Figures showing broadband station locations, waveform fit, S‐wave velocity models, and Love‐wave fundamental mode velocities.
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.
There has been a controversy concerning whether earthquakes can occur only within the crust or also within the upper mantle. Within both regions, the strength decreases as temperature increases and thus decreases rapidly with depth. However, because crustal and mantle composition differ, the strength in the uppermost mantle exceeds the strength at the base of the crust, and it is plausible that earthquakes can …