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A zone where the oceanic or continental crust subducts below the continental crust or arc at an angle of nearly 45 degrees and seismicity occurs if the rate of subduction suddenly changes owing to the movement of subducting plate. The zone can dip at gentle angle or a steep one depending on the rate at which subduction occurs and the thickness of the crust at the point where the slab subducts. The earthquakes are shallow near the trench and deep away from it a fact that helps to determine the dip of the subducting slab.

Lower figure after online Tectonics book by P Rey