New Zealand Journal of Geology and Geophysics abstracts
Tectonics of the Taranaki region, New Zealand: earthquake focal mechanisms and stress axes
Steven Sherburn*
Robert S. White
Bullard Laboratories
University of Cambridge
Madingley Rd
Cambridge CB3 0EZ, United Kingdom
s.sherburn@gns.cri.nz
*Present address: GNS Science, Wairakei Research Centre, Private Bag 2000, Taupo, New Zealand.
Abstract We present 39 well-determined focal
mechanisms for crustal earthquakes from the Taranaki region in western
North Island, New Zealand. Earthquake locations, azimuths, and take-off
angles were calculated using a 3D velocity model and only those
mechanisms with at least 15 clear first motions were considered.
Principal stress axes were determined by inverting focal mechanisms and
independently by inverting earthquake first motions. Based on misfit
values and differences in seismicity and geology we interpret data east
and west of Mt Taranaki separately. Lower crustal earthquakes in
eastern Taranaki display both strike-slip and normal faulting
mechanisms; σ3 is subhorizontal and aligned northwest–southeast, while the best fit σ1
is aligned northeast–southwest with a dip of 27–38° to
the horizontal. The principal stress directions in eastern Taranaki are
similar to those near the southern Taupo Volcanic Zone, suggesting that
the back-arc extension that characterises the Taupo Volcanic Zone
continues into eastern Taranaki. Swarm earthquakes from the Cape Egmont
Fault Zone, west of Mt Taranaki, have dominantly strike-slip focal
mechanisms. The maximum (σ1) and minimum (σ3) compressive stresses west of Mt Taranaki are both subhorizontal, with σ1 aligned east–west and σ3
north–south. The focal mechanisms and principal stress directions
do not agree with the geologically inferred northwest–southeast
extension direction. We suggest that western Taranaki may be affected
by stresses induced by magmatism beneath Mt Taranaki and that the
normal faulting seen at the surface mainly occurs associated with
significant eruptions from the volcano. The failure angle on faults in
western Taranaki exceeds that expected for Byerlee friction and
hydrostatic fluid pressure and this suggests that these faults have a
relatively low coefficient of friction or relatively high pore fluid
pressure.
Keywords Taranaki; tectonics; focal mechanism; stress axes; volcanism
G05053; Received 2 November 2005; accepted 10 March 2006; Online publication date 17 May 2006
New Zealand Journal of Geology & Geophysics, 2006, Vol. 49: 269–279
0028–8306/06/4902–0269 © The Royal Society of New Zealand 2006
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