The May 23, 2013 Mw 7.4 earthquake southwest of Vaini, Tonga, occurred as the result of normal faulting at the depth of approximately 170 km. At the location of this earthquake, the Pacific and Australia plates are converging at the rate of approximately 73 mm/yr in an east-west direction, resulting in the westward subduction of the Pacific plate beneath Tonga at the Tonga-Kermadec trench. The depth and faulting mechanism of the May 23rd earthquake indicate it ruptured a fault within the subducting Pacific lithosphere rather than on the shallower thrust interface between the two plates.
The Tonga-Kermadec arc has frequent moderate-to-large earthquakes, and has hosted over a dozen M 6.5+ earthquakes within 500 km of the May 23rd earthquake over the past 40 years. Most of these also occurred at intermediate depths; the largest was an Mw 7.7 earthquake in October of 1997, approximately 110 km to the north-northeast of the event on May 23, 2013.
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