Abstract
Introduction: In New Zealand, the Pacific plate is subducting beneath the Australian plate in the North Island and the north part of the South Island. The northwestern part of the South Island is under a compressional stress field with ESE-WNW direction. Many reverse faults active in the present-day tectonic regime are inherited Late Cretaceous-Paleocene normal faults formed during opening of the Tasman Sea that were subsequently inverted (tectonic inversion fault). In the northwestern part of South Island, earthquakes occur less frequently than in many other parts of New Zealand. However, historical large events (e.g., the 1929 Buller earthquake, Mw 7.3; the 1968 Inangahua earthquake, Mw 7.2; and the 1991 Hawks Crag earthquake, Mw 6.0) were caused by active reverse faults in this area. To investigate relationship between stress regime and fault activities in the northwestern part of South Island, we estimate the present-day stress field and evaluate the likelihood of the fault slip. Data and methods: We use moment tensor data (2003-2022) derived from GNS Science, which were determined from routinely operated stations (GeoNet), as well as focal mechanisms estimated from P-wave initial motions (2013-2018) at temporary stations deployed across the region (Okada et al., 2019). We estimate the stress field using the stress tensor inversion method developed by Michael (1984, 1987). To evaluate the likelihood of slip, we use the Slip Tendency (ST) method (Morris et al., 1996), whereby large ST values indicate that the fault slips easily, and small values indicate that the fault is not well-oriented for slip. Result - Stress field: Reverse and strike-slip fault types are characterizing most earthquakes that occurred in the study area. The stress field is intermediate of reverse and strike-slip fault type. Result - Slip Tendency (ST): First, we evaluate the ST value for overall fault geometry using data Ghisetti et al. (2014) and the stress field that we determined. The N-S strikes with east-dipping fault planes show larger ST values than west-dipping planes. Also, fault planes with N-S strikes with dip 66 degrees -76 degrees or E-W strikes shows low ST values (ST <0.6). There are no significant differences between the ST of the present (new) reverse faults and tectonic inversion fault.
Poster presentation.