Plate coupling and the hazard of large subduction thrust earthquakes at the Hikurangi subduction zone, New Zealand

Abstract

Recent dense deployments of portable seismographs along the Hikurangi subduction zone have provided insights into the structure and seismic strain regime of the subducted and overlying plates, and the nature of plate coupling at the shallow part of the plate interface. Beneath Marlborough, the plates appear to be permanently locked, and large subduction thrust events are not expected. In the Wellington and Wairarapa regions, the plates appear to be strongly coupled over a downdip width of the plate interface of c. 70 km. Subduction thrust earthquakes of about M W 8.0 are estimated for this region. Farther to the northeast, the downdip width of the inferred locked portion of the plate interface progressively decreases, and subduction thrust events of about M W 6.9 are estimated for the northern part of the Raukumara Peninsula. In the south of the subduction zone, changes in coupling arise principally from changes in the thickness of the subducted plate, whereas in the north they are mainly due to changes in thickness of the overlying plate. Tectonic rotations within the overlying plate observed paleomagnetically appear to be a natural consequence of changes in plate coupling along the subduction zone.

Contemporary horizontal velocity and strain rate fields of the Pacific‐Australian plate boundary zone through New Zealand

“…In the northern part of the North Island the subduction interface exhibits low, and perhaps temporally variable, seismic coupling as evidenced by measurements of strain rates in the overlying plate [Walcott, 1978;Arnadottit et al, 1999] and by seismic observations [Reyners, 1998]. This part of the plate interface also exhibits a subaerial region of back-arc rifting, the Taupo Volcanic Zone (TVZ), which lies in the Australian plate west of the margin.…”

Investigating subduction earthquake geology along the southern Hikurangi margin using palaeoenvironmental histories of intertidal inlets

“…GPS data shows the plate interface is presently strongly interseismically coupled ( Fig. 1B; Wallace et al 2004) and seismicity delineates a c. 70 km wide locked zone on the plate interface across the southern North Island (Reyners 1998). Long-term geologic data indicate permanent shortening in the upper plate over the past 5 Ma accounting for only 20% of the margin-normal plate convergence; the remaining 80% is assumed to be accommodated by slip on the subduction interface (Nicol & Beavan 2003).…”

Evidence from GPS measurements for contemporary interplate coupling on the southern Hikurangi subduction thrust and for partitioning of strain in the upper plate

“…Geological and geodetic data have been interpreted to show that the subduction interface thrust between the two plates is permanently locked in that region and that the boundary has moved onto upper crustal faults within the overriding plate [Bibby, 1981;Reyners and Cowan, 1993;Holt and Haines, 1995]. It has been argued that contemporary coupling, which we define later in this section, diminishes farther northeast beneath the North Island, reducing from high levels near Wellington to low levels near East Cape [Walcott, 1978;Beanland, 1995;Reyners, 1998]. While the existence of coupling is revealed by earthquake activity and active faults and folds in the overriding plate, little is known quantitatively about how this coupling varies spatially.…”