Stress directions in the shallow part of the Hikurangi subduction zone, New Zealand, from the inversion of earthquake first motions

Details Stress directions in the shallow part of the Hikurangi subduction zone, New Zealand, from the inversion of earthquake first motions Stress directions in the shallow part of the Hikurangi subduction zone, New Zealand, from the inversion of earthquake first motions

Aug 2000

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2000geoji.142..339m&link_type=abstract

Geophysical Journal International, Volume 142, Issue 2, pp. 339-350.

Astronomy and Astrophysics

Astronomy

11

Earthquake, New Zealand, Stress Distribution, Subduction

Scientific paper

The numerous earthquakes recorded during recent dense seismograph deployments in the northern South Island and southernmost North Island, and in the Raukumara Peninsula of New Zealand, provide an opportunity to investigate stress directions in the shallow part of the Hikurangi subduction zone. Here we invert for the stress tensor orientation for regions within both the subducted and overlying plates, using a new method that invokes the Coulomb failure criterion and considers all P-wave first motions together, regardless of whether or not they are sufficient to define single-event focal mechanisms. The inversion also provides information on which of the two planes with maximum Coulomb failure stress is the preferred fault plane. In the crust of the subducted plate, the least compressive stress (σ3) is closely aligned with the dip of the plate. The subducted plate is acting as an efficient stress guide, with slab pull from the deeper part of the plate being transmitted to shallow depths. Normal faulting on steeply dipping fault planes is favoured, suggesting that bending of the crust of the subducted plate is accomplished through bulk simple shear (akin to shuffling a vertical deck of cards). In the overlying plate in the northeastern South Island, the greatest and least compressive stresses (σ1 and σ3) are horizontal, thus favouring strike-slip faulting. The orientation of σ1 is close to that expected if the overlying plate is contracting in response to coupling at the underlying plate interface. However, sinistral motion on NW faults is favoured, whereas motion on the major NE-ENE surface faults is predominantly dextral. In the Raukumara Peninsula, thrust faulting on steeply dipping planes is favoured in the lower part of the overlying plate and near the plate interface. The stress regime at the plate interface does not favour interplate thrusting, and a weak interface is required for this to occur. It appears that strain in the overlying plate in both the northeastern South Island and Raukumara Peninsula is partitioned in time-conjugate faults may be active in taking up strain in the interseismic period between large events on the major faults.

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