Late quarternary deformation and paleoseismicity on the easternmost part of the Awatere Fault, South Island, New Zealand

Abstract

The results of a comprehensive investigation of Late Quaternary deformation and paleoseismicity on a 27 km long portion of the oblique dextral strike-slip Awatere Fault are presented. Detailed mapping of the active fault trace and measurement of the offset of fault displaced geomorphic features provides evidence for significant spatial and temporal patterns in the development of the Awatere fault near the transition between onshore and offshore parts of the plate boundary zone. There is good evidence indicating that on some parts of the Awatere Fault, the vertical component of oblique slip is accommodated off-fault on adjacent structures. A model is offered that invokes topographic perturbation of near-surface stresses as the mechanism controlling the development of a distinctive small-scale segmentation along portions of the Awatere Fault trace. Important new slip-rate data show that: (1) Southwest of Dumgree Station, ~12 km inland from the coast, the rate of strike-slip on the Awatere Fault has been 5-6 mm.yr-1 for at least the past 65 ka. (2) At a given location along the fault, there has been no significant Late Quaternary change in either the sense or magnitude of dip-slip. (3) Northeast of Dumgree Station, the rate of strike-slip on the Awatere Fault decelerates rapidly to <1.5 mm.yr-1 near the coast. The recurrence and magnitude of paleoearthquakes on the eastern section of the Awatere Fault is estimated from stratigraphic information in a fault trench and from the size and distribution of small (<15 m) coseismic fault displacements. Evidence for at least six surface rupturing events since 8,330-8,610 cal yr B.P. and for characteristic earthquake behaviour is described. The last two events were probably surface ruptures of the entire eastern section (~Mw7.5). A method for determining the offset between two topographic profiles by least squares, and for estimating the uncertainty in that offset, is presented. The geodetic strain-rate measured over a small (<10 km) survey network straddling the Awatere Fault is shown to be indistinguishable from strain rates determined from repeated GPS surveys of regional scale networks (>50km).