The Benioff zone, also called the Wadati–Benioff zone, is a dipping plane of earthquakes along the slab of an oceanic plate that is being subducted into the mantle beneath an overriding plate. It reaches from shallow depths near the trench to depths of about 700 kilometers. Studying it reveals the structure of subducting plates, how earthquakes are generated deep in the mantle, and the forces that drive plate tectonics.
Source Isacks, B., Oliver, J., & Sykes, L. R. (1968). “Seismology and the New Global Tectonics.” Journal of Geophysical Research, 73(18), 5855-5899; Frohlich, C. (2006). “Deep Earthquakes.” Cambridge University Press.