Detailed spatiotemporal analysis of the tectonic stress regime near the central Alpine Fault, New Zealand

We construct a new data set of 845 focal mechanisms derived from microseismicity recorded between late 2008 and early 2017, to investigate the state of stress near the central Alpine Fault, a major obliquely convergent plate boundary fault. We obtain an average maximum horizontal compressive stress orientation, SHmax, of 121 ± 11°, which is uniform to first-order along the length of the central Southern Alps. We also make use of the local magnitudes of the earthquakes with focal mechanisms to examine the spatial patterns of seismic moment release and obtain a relatively uniform cumulative seismic moment release distribution adjacent to the central Alpine Fault, which appears to be independent of both the perpendicular and parallel distances to the fault. We observe an average angle between SHmax and the Alpine Fault of 66°, which is consistent with previous observations in the northern and southern sections of the Alpine Fault. This result implies that central Alpine Fault is not optimally oriented for reactivation. Detailed temporal investigations using two different approaches did not show any systematic change of the stress parameters with time. We also examine the distribution of the shear to normal stress ratio on the fault and do not observe any significant spatial variation along the Alpine Fault. This lack of variation combined with the high angle between the fault strike and SHmax implies that the fault is unfavourably oriented for slip.