We present seismological evidence for tearing of the Pacific plate at the northern termination of the Tonga subduction zone. Seismicity cross sections indicate (1) abrupt termination of intermediate-depth seismicity along an east-west–trending zone coincident with the northern termination of the subduction zone and (2) shoaling of the subducted slab at its northern termination. Earthquake distribution and source-mechanism determinations indicate progressive downwarping and tearing of the Pacific plate as it enters the northernmost segment of the Tonga subduction zone. Source mechanisms for 57 events located along a narrow belt of high seismicity in northernmost Tonga indicate dip-slip faulting along near-vertical planes oriented ∼285°, coinciding with the observed direction of plate convergence. Precisely determined depths for 21 events (18–57 km) suggest that the plate is downwarped >15 km prior to initiation of tearing; depths of the 7 April 1995 (Ms =8) event and aftershocks suggest tearing of the plate may extend through the entire thickness of the oceanic lithosphere.

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