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Seismological evidence for tearing of the Pacific plate at the northern termination of the Tonga subduction zone

¹ Department of Geological Sciences, Indiana University, Bloomington, Indiana 47405

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 (M_s =8) event and aftershocks suggest tearing of the plate may extend through the entire thickness of the oceanic lithosphere.

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