Three-dimensional variations in present-day tectonic stress along the Australia-Pacific plate boundary in New Zealand

Using a newly compiled data set of 3424 focal mechanisms, we have estimated tectonic stress parameters at 100 locations throughout central New Zealand in the largest study to date of the tectonic stress field along the Australia-Pacific plate boundary. The results reveal pronounced changes in the azimuth of maximum horizontal compressive stress S-Hmax along the shallow portion (<50 km depths) of the Hikurangi subduction margin, with a marked change from margin-parallel S-Hmax north of Hawke's Bay (latitude c. 40 degrees S) to a more oblique S-Hmax orientation further south. This change appears to coincide with the along-strike variations in subduction thrust coupling inferred from geodetic and seismological observations. In contrast, the orientation of S-Hmax is highly uniform across most of the South Island (averaging c. 115 degrees) and collinear with the axis of relative contractional strain rate. Analysis of focal mechanisms recorded before and after the damaging M(W)6.2 Christchurch earthquake of 22 February 2010 reveals no significant change in S-Hmax orientation or the overall stress regime: this suggests that even the high-stress drop Christchurch earthquake was incapable of substantially modifying the ambient stress field, at least on the scales at which focal mechanism stress estimation can be performed. (C) 2012 Elsevier B.V. All rights reserved.