The August 12, 2016 M 7.2 earthquake east of New Caledonia in the southwest Pacific,
and the North Fiji Basin, occurred as the result of shallow strike-slip faulting on
or near the complex plate boundary between the Australia and Pacific plates.
At the location of this earthquake the regional plate boundary is transitional,
evolving from east-northeastward-oriented subduction of Australia beneath Pacific
at the New Hebrides Trench to the north and west, to southwest-northeast oriented
left-lateral transform faulting between the two plates just south of this earthquake.
The location of the August 12 event also lies just south and west of the Central
Spreading Ridge, a zone of back-arc spreading accommodating some of the divergence
between the New Hebrides and Tonga subduction zones. The focal mechanism solution of
the August 12 event implies faulting occurred as the result of either right-lateral
slip on a fault oriented east-southeast, or left-lateral slip oriented south-southwest.
Such a mechanism is inconsistent with the expected motions between the Australia and
Pacific plates, and may instead indicate the association with the development of a STEP
(Subduction-Transform-Edge-Propagator) fault, a style of faulting that develops at many
subduction terminations globally. Similar strike-slip events, also apparently inconsistent
with local plate-motion, have occurred recently (2015) along this plate margin to the
northwest at the transition from the Solomon Island subduction zone into the San Cristobal
Trough (transform) plate boundary. At the location of this earthquake, the Australia plate
moves towards the east-northeast with respect to the Pacific at a rate of approximately 75 mm/yr.
Spreading rates along the Central Spreading Ridge are also around 75-80 mm/yr.