Focal mechanisms of large earthquakes in the South Island of New Zealand: implications for the accommodation of Pacific-Australia plate motion

Abstract

b The plate motion model NUVEL‐1 predicts oblique convergence between the Pacific and Australian plates in the South Island of New Zealand. We used P and SH body waveform analysis to constrain the focal mechanisms of the 15 largest earthquakes (Ms > 5.8) that have occurred in this region since 1964, in order to see how the plate motion is accommodated. At the southern end of the Alpine Fault, convergence is achieved by oblique slip movement along a concentrated zone of deformation. In the southern offshore region one event may be related to thrusting of the Australian plate beneath the Pacific plate, and another strike‐slip event probably demonstrates movement on an active strike‐slip fault system parallel to, but offset from, the southern limit of the Alpine Fault. This geometry provides a possible mechanism for the rapid uplift of the Fiordland region. Deformation in the northern South Island is more distributed. In the south‐west Marlborough region partitioning occurs between strike‐slip faulting in the SE and reverse faulting farther NW in the Buller region. We suggest that the partitioning developed as a consequence of an increasing component of shortening that was accommodated by slip on reactivated pre‐existing normal faults in the Buller region. Shortening in the Buller region may have deflected the NE end of the Alpine Fault towards the NW, forming the prominent bend. The Marlborough Fault System, with its youngest and most active faults to the SE, probably developed in an attempt to maintain a through‐going strike‐slip structure as each of the strike‐slip faults was transported towards the north‐west. Partitioning of the opposite polarity (with reverse faulting SE of the strike‐slip faulting) occurs in north‐east Marlborough. The boundary between the two different styles of partitioning in NE and SW Marlborough appears to coincide with a change in the nature of the downgoing slab and a change in strike of faults of the Marlborough Fault System. A normal faulting earthquake on the northern edge of the Chatham rise probably results from a complex interaction of the buoyant continental crust in that region with the subduction zone and the overlying Marlborough Fault System.

The 1990 Lake Tennyson earthquake sequence, Marlborough, New Zealand

“…The orientation of the main fault lineation is consistent with the mainshock focal mechanism determined by Anderson et al (1993) and the direction of relative PacificAustralia plate motion in the region. The focal mechanism determined by Anderson et al (1993) from teleseismic longperiod body waves was a right-lateral strike-slip mechanism with a strike of 55 ± 10° and a dip of 89 ± 5°.…”

“…To the north of the Wairau sector of the Alpine Fault, the dominant structural trend is NNE N. In contrast to the Marlborough faults, recent deformation on the northern side of the Wairau Fault is dominantly reverse (Pettinga & Wise 1994). The contrast in deformation is also evident from focal mechanisms and the slip vectors of large historic earthquakes that have occurred in the region (Anderson et al 1993), and focal mechanisms determined (B, C, D, F, M, N and O) are those of large earthquakes that have occurred in the northern South Island, as determined by Anderson et al (1993). Mechanism M is the focal mechanism for the Lake Tennyson earthquake.…”

“…Aftershocks located by Webb appeared to define an elongated zone c. 12 km long, parallel to the local trend of the Awatere Fault. A focal mechanism determined for the mainshock from teleseismic long-period body waves (Anderson et al 1993) was strike-slip, with the northeaststriking nodal plane consistent with the elongated zone located by Webb, suggesting that right-lateral slip had occurred.…”

“…174" from microseismic surveys (Arabasz & Robinson 1976;Kieckhefer 1977;Eberhart-Phillips & Reyners 1997). Anderson et al (1993) suggested that this contrast in deformation has resulted from a change in plate-motion direction during the late Oligocene to early Miocene, which required a component of shortening to be accommodated.…”

“…New Zealand lies across the boundary between the Pacific and Australian plates, which has resulted in a complex region of active deformation 200-300 km in width (Anderson et al 1993). Present-day convergence at this boundary zone is accommodated in different ways along the length of New Zealand (Fig.…”