Abstract:
The advantages and disadvantages of the 'displacement' approach and the 'strain' approach to the analysis of repeated geodetic surveys for crustal deformation are discussed and two methods of geodetic strain analysis are described in detail.
Repeated geodetic surveys in the central North Island show
i) secular widening of the Taupo Volcanic Zone (TVZ) at 7 mm y-1 without significant transcurrent motion
ii) north-south dextral motion at 14 mm y-1 and east-west narrowing at 4 mm y-1 across the northern end of the North Island Shear Belt
iii) 3.1 m extension at 135° across a 15 km-wide region north of Lake Taupo, and adjacent zones of compressive rebound all associated with the 1922 Taupo Earthquakes.
From the epicentral distribution and horizontal strain pattern a 15 km-square fault dipping 40° and striking parallel to the TVZ is inferred for the 1922 earthquakes. The seismic moment, 1.3 x 10 26 dyne cm, and the stress drop, 134 bars, are abnormally high for the TVZ.
Widening of the TVZ is considered to be back-arc spreading. The spreading axis is postulated to extend northeast into the Havre Trough via a north-south dextral transform; and southwest into the Waverley Fault Zone and Waimea Depression via the sinistral reverse Raetihi Transform.
Deformation of the North Island is not homogeneous. Fault zones are idealized as line plate boundaries and four plates -Indian, Central, Kermadec and Pacific - are postulated to account for the deformation. The Indian-Pacific macroplate pole is adopted and non-unique positions and rotation rates for the remaining poles are determined from geodetic strain data and the geometry of plate interactions. The Central Plate is moving away from the Indian Plate at the back-arc spreading axis; the Kermadec Plate is moving dextrally with respect to the Central Plate at the North Island Shear Belt which accommodates most of the transcurrent component of motion between the Indian and Pacific plates in the North Island and gives almost pure subduction of the Pacific Plate under the Kermadec Plate at the Hikurangi Margin.
