Stress field and kinematics for diffuse microseismicity in a zone of continental transpression, South Island, New Zealand

Abstract

We analyze shallow (0–20 km) microseismicity adjacent to the Alpine Fault in New Zealand, where there is oblique convergence of the Australian and Pacific plates. Focal mechanisms for 155 earthquakes (June 2012 to October 2013) are inverted to determine the orientation of the stress field. This yields a principal horizontal axis of compression, SHmax = 114° ± 10°, which cannot be explained in terms of the sum of stress from tectonic loading due to plate convergence, indicated by GPS observations, and gravitational stresses. The azimuth of slip vectors for individual focal mechanisms cluster perpendicular and parallel to the plate convergence vector. These faults, however, strike at ~45° to SHmax from the stress inversion, suggesting a very low coefficient of friction. The earthquake slip directions may be kinematically controlled, accommodating the plate convergence on a limited set of fractures, similar to the segmentation for neotectonic faulting along the Alpine Fault, which is partitioned into strike‐slip and thrust segments at a 1–10 km scale. We suggest two possible controls on our calculated SHmax azimuths. First, there may be a slight clockwise bias in the estimates of SHmax from earthquakes; slip may be occurring on a more limited range of fractures than assumed by the stress inversion method, although this effect is likely to be relatively small (±5°). More importantly, the components of the stress field may be relieved at different timescales during big earthquakes, resulting in a residual stress field that varies significantly (±15°) on timescales of several large earthquakes.

Fault zone architecture of a large plate-bounding strike-slip fault: a case study from the Alpine Fault, New Zealand

“…Faults control the strength of the Earth's lithosphere (Townend and Zoback, 2000;Bürgman and Dresen, 2008) and govern substantially fluid flow (Caine et al, 1996;Wibberley et al, 2008). Hydrocarbon production from faultcompartmentalized reservoirs (Van Eijs et al, 2006), exploitation of fault-hosted mineral deposits (Cox et al, 1986) and long-term integrity of potential nuclear waste repositories (Laurich et al, 2018) are practical examples demonstrating how important it is to understand fault zone properties and their spatial and temporal evolution.…”

“…However, other faults show a more complex structure with changing properties along strike or down dip (e.g., Wibberley et al, 2008;Faulkner et al, 2010). For example, detailed studies of the Carboneras Fault, Spain, yielded a conceptual model that is suitable for broader, typically phyllosilicaterich fault zones, which tend to contain multiple high-strain zones (Fig.…”

Microseismicity in Southern South Island, New Zealand: Implications for the Mechanism of Crustal Deformation Adjacent to a Major Continental Transform

“…Our new focal mechanisms indicate that strike-slip faulting dominates microseismic deformation in the Southern Lakes (Figure 7b), with a strong alignment to the relative plate convergence direction (see Warren-Smith, Lamb & Stern, 2017, for further discussion and analysis). Reverse faulting (commonly oblique) is more common for deeper events in the subducting Australian slab in northern Fiordland.…”

“…Interestingly, our analysis shows that the principal axis of contraction accommodated by microseismicity is orientated 15–20° clockwise of that for both the accumulation of strain indicated by GPS data, and also what we would anticipate from distributed plate convergence (Figures c–e). However, the microseismicity contraction axes are consistent with principal horizontal axis of compressional stress (azS Hmax ), based on an inversion of focal mechanisms for stress orientations (Warren‐Smith, Lamb & Stern, ). This suggests that the orientation of the stress field may show secular variation in the interseismic period, depending on the stress relief in large earthquakes, presumably rupture on the Alpine Fault itself.…”

Fault zone architecture of a large plate-bounding strike-slip fault: a case study from the Alpine Fault, New Zealand

“…In addition, orientation of the fault (Figs. 2a & b), magnitude of stress and the stress field itself (Boese et al, 2012;Warren-Smith et al, 2017) are fairly constant along the central segment of the Alpine Fault, thus it is unlikely that these parameters are responsible for observed variations of fault gouge thickness. Biegel and Sammis (2004) suggested to explain along-strike variations of gouge thickness as record of rupture arrest.…”