M 7.4 - 292 km SW of Vaini, Tonga

Tectonic Summary

The May 23, 2013, M 7.4 earthquake southwest of Vaini, Tonga, occurred as a result of normal faulting at an intermediate depth within subducted Pacific lithosphere, approximately 170 km beneath the Pacific Ocean. Focal mechanism solutions indicate that rupture occurred on either a near-vertical, north-striking normal fault, or on a shallow, south-striking normal fault. At the location of this earthquake, the Pacific and Australia plates are converging at a velocity of about 73 mm/yr in an east-west direction, resulting in the westward subduction of the Pacific plate beneath Tonga at the Tonga-Kermadec Trench, 200 km west of the earthquake. The depth and faulting mechanism of the May 23rd earthquake indicate that it ruptured a fault within the subducting Pacific lithosphere rather than on the shallower thrust interface between the two plates. Slip on a fault aligned with either nodal plane of the focal mechanism solution is consistent with this intraplate setting.

The Tonga-Kermadec Arc has frequent moderate to large earthquakes and has hosted more than 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 a M 7.7 earthquake in October 1997, approximately 110 km to the north-northeast of the May 23, 2013, event. None are known to have caused significant damage.

Earthquakes like this event, with focal depths between 70 and 300 km, are commonly termed “intermediate-depth” earthquakes. Intermediate-depth earthquakes represent deformation within subducted slabs rather than at the shallow plate interface between subducting and overriding tectonic plates. They typically cause less damage on the ground surface above their foci than is the case with similar-magnitude shallow-focus earthquakes, but large intermediate-depth earthquakes may be felt at great distance from their epicenters. “Deep-focus” earthquakes, those with focal depths greater than 300 km, also occur in the subducted Pacific plate beneath the Lau Ridge and South Fiji Basin to the west. Earthquakes have been reliably located to depths of about 650 km in this region.

Hayes et al. (2016) Tectonic summaries of magnitude 7 and greater earthquakes from 2000 to 2015, USGS Open-File Report 2016-1192. (5.2 MB PDF)