Cluster Analysis of Dense GNSS Velocity Field Reveals Characteristics Associated With Regional Tectonics in New Zealand

New Zealand is located in the converging zone between the Australian and Pacific Plates. The Pacific Plate is descending beneath the Australian Plate in the north of New Zealand, while the Australian Plate is descending beneath the Pacific Plate in the south. The contrastive tectonics is connected by the Alpine Fault, the transform fault with NE-SW strike along most of the length of New Zealand. Recently, previous studies showed that a cluster analysis of dense Global Navigation Satellite System (GNSS) velocity field objectively yields major velocity discontinuities, which may reflect an important tectonic boundary. We applied two different types of clustering algorithms: hierarchical agglomerative clustering (HAC) and Euler pole clustering (EPC), to the dense GNSS horizontal velocity data in New Zealand. Both methods agree that the primary kinematic boundary lies from the northern Wairarapa region to the southwestern end of the Alpine Fault. The HAC introduces a cluster boundary along the North Island Dextral Fault Belt whereas EPC emphasizes the importance of the Taupo Volcanic Zone (TVZ). Since EPC fits the data reasonably well, it indicates the TVZ is an important boundary. This result displays the limitation of the flat earth assumption. The Chatham Islands is identified as an independent cluster by the HAC, but not by the EPC. Further cluster splits illustrate that the relative motion between the two plates is accommodated by distributed deformation in the Marlborough Fault Zone in the South Island whereas the North Island is characterized by local block rotations.