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 SHmax along the shallow portion (<50km depths) of the Hikurangi subduction margin, with a marked change from margin-parallel SHmax north of Hawke's Bay (latitude c. 40°S) to a more oblique SHmax 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 SHmax is highly uniform across most of the South Island (averaging c. 115°) and collinear with the axis of relative contractional strain rate. Analysis of focal mechanisms recorded before and after the damaging MW6.2 Christchurch earthquake of 22 February 2010 reveals no significant change in SHmax 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.

► We examine the tectonic stress field in New Zealand using >3400 focal mechanisms. ► This is the first New Zealand-wide study of tectonic stress. ► We determine the axis of maximum horizontal compressive stress at 100 locations. ► The stress field in the South Island is uniform and consistent with plate motions. ► Stress varies along the Hikurangi subduction margin, reflecting changes in locking.