Recent Canterbury Earthquakes
Background: NZ's position on the Australia-Pacific plate boundary
Due to the unique position of of the New Zealand microcontinent New Zealand experiences large numbers of small earthquakes in a well-defined belt stretching from Fiordland to East Cape and the Bay of Plenty. This pattern is part of the ‘Ring of Fire’, the almost continuous belt of volcanoes and earthquakes rimming the Pacific Ocean.
The largely submerged New Zealand micro-continent straddles the Australia-Pacific plate boundary zone, and the relative motion of these plates has controlled the Upper Cenozoic (approximately the last 25 million years) evolution and present shape of the emergent New Zealand landmass. The major features of the South Island plate boundary zone include the Alpine Fault and the Marlborough Fault Zone, these together have long been recognised as a trench-trench transform, linking opposite dipping obliquely convergent subduction zones (see Figure).
The Alpine Fault forms a linear feature extending for about 420 km along the west central portion of South Island. Geodetic and geologic data show that at ~70% of the plate boundary motion is accommodated along the narrow high strain zone associated with the Alpine Fault. The remainder of the oblique plate motion is distributed across the
150-200 km wide Southern Alps into the Canterbury region.
Much of the north Canterbury region is located within this wide zone of active earth deformation associated with the oblique collision between the Australian and Pacific tectonic plates east of the Alpine Fault. The present day tectonic tempo of active earth deformation is greatest along the narrow zone adjacent to the Alpine Fault, and where the plate boundary zone transfers across South Island, through the Marlborough and north Canterbury areas to link with the offshore trench and subduction zone from near Kaikoura.
In the north Canterbury region the southward transition from subduction to continental collision is associated with tectonic shortening, crustal thickening and uplift. Landforms reflect the ongoing nature of this active earth deformation, and also reveal that the Australia-Pacific plate boundary zone has progressively widened here, and continues to do so, during the Late Quaternary. East of the main divide of the Southern Alps, in central and south Canterbury, the tempo of tectonic deformation progressively diminishes to the southeast.