Effects of regional and local stresses on fault slip tendency in the southern Taranaki Basin, New Zealand

Determining the potential for faults to slip is widely employed for evaluating fault slip potential and associated earthquake hazards, and characterising reservoir properties. Here we use borehole and 3D seismic reflection data to estimate stress orientations and magnitudes, fault geometries and slip tendency in the southern Taranaki Basin, New Zealand. We highlight uncertainties in maximum horizontal stress (SHmax) magnitude calculations from borehole breakout width and rock strength. As in other settings, breakout width is uncertain on resistivity images because one of the breakout edges often lies in-between the resistivity imager pads, so only a subset of borehole breakouts can be used with confidence. The main uncertainty on SHmax magnitude is the rock strength at the borehole depth at which breakouts form. Given the rarity of basin-specific rock mechanical data, we rely on equations used to convert downhole acoustic compressional wave slowness into rock strength defined in sandstone and mudstones. However, lithologies in the southern Taranaki Basin are commonly muddy sandstones and sandy mudstone that can be interlayered. In addition, we show an example where breakouts are confined to moderately cemented carbonate units without change in acoustic compressional wave slowness. Using a range of rock strength equations based on sandstones and mudstones provides a possible SHmax magnitude range. With only one focal mechanism available in the study area, constraints on SHmax magnitudes from borehole data remain valuable and inform on stresses in the shallow crust.

Although the southern Taranaki basin is undergoing active deformation at plate tectonic scales, the stress magnitudes appear insufficiently high to reactivate the faults assuming a classic coefficient of friction. SHmax azimuths and SHmax:Sv magnitude ratios vary locally between boreholes and with depth. A borehole that intersects an inactive seismic-scale fault and borehole-scale faults over a 150-m interval shows SHmax to rotate by up to 30° proximal to the faults, which are favourably orientated for slip in both strike-slip and normal regimes. The small borehole-scale faults may, however, be active within the inactive seismic scale fault's damage zone. We highlight changes of slip tendency along faults resulting from local variations in the stress field and non-planar fault geometries that could not be resolved using only seismic reflection data and regional stress tensor.