The Kermadec-Tonga Subduction Zone is a convergent plate boundary which stretches from the North Island

North Island

The North Island is one of the two main islands of New Zealand, separated from the much less populous South Island by Cook Strait. The island is in area, making it the world's 14th-largest island...

 of New Zealand

New Zealand

New Zealand is an island country in the south-western Pacific Ocean comprising two main landmasses and numerous smaller islands. The country is situated some east of Australia across the Tasman Sea, and roughly south of the Pacific island nations of New Caledonia, Fiji, and Tonga...

 northward, and includes the Hikurangi Trough, the Kermadec Trench

Kermadec Trench

The Kermadec trench is one of Earth's deepest oceanic trenches, reaching a depth of . Formed by the subduction of the Pacific Plate under the Indo-Australian Plate, it runs over a thousand kilometres parallel with and to the east of the Kermadec Ridge and island arc, from near the northeastern tip...

 and the Tonga Trench

Tonga Trench

The Tonga Trench is located in the South Pacific Ocean and is deep at its deepest point, known as the Horizon Deep.The Tonga Trench is a convergent plate boundary. The trench lies at the northern end of the Kermadec-Tonga Subduction Zone, an active subduction zone where the Pacific Plate is being...

. Along this zone, the Pacific Plate

Pacific Plate

The Pacific Plate is an oceanic tectonic plate that lies beneath the Pacific Ocean. At 103 million square kilometres, it is the largest tectonic plate....

 to the east is subducting beneath the Indo-Australian Plate

Indo-Australian Plate

The Indo-Australian Plate is a major tectonic plate that includes the continent of Australia and surrounding ocean, and extends northwest to include the Indian subcontinent and adjacent waters...

.

The southern end of the subduction zone transitions to a right lateral-moving transform fault

Transform fault

A transform fault or transform boundary, also known as conservative plate boundary since these faults neither create nor destroy lithosphere, is a type of fault whose relative motion is predominantly horizontal in either sinistral or dextral direction. Furthermore, transform faults end abruptly...

 south of the North Island called the Alpine Fault

Alpine Fault

The Alpine Fault is a geological fault, more specifically known as a right-lateral strike-slip fault, that runs almost the entire length of New Zealand's South Island. It forms a transform boundary between the Pacific Plate and the Indo-Australian Plate. Earthquakes along the fault, and the...

.