Microseismicity, tectonic stress, and exhumation rates near the central Alpine Fault, New Zealand

This thesis documents a detailed examination of the seismic activity and characteristics of crustal deformation along the central Alpine Fault, a major obliquely convergent plate-boundary fault. Paleoseismic evidence has established that the Alpine Fault produces large to great (M7−8) earthquakes every 250−300 years, in a quasi-periodic manner, with the last surface-rupturing earthquake occurring in 1717. This renders the fault late in its typical earthquake cycle, posing substantial seismic risk to southern and central New Zealand. Understanding the seismic and tectonic character of this fault may yield information of both societal and scientific significance regarding seismic hazard and late-interseismic processes leading up to a large earthquake. However, the central Alpine Fault is currently seismically quiescent when compared to adjacent regions, and therefore requires detailed, long-duration observations to study seismotectonic processes. The work in this thesis addresses the need for a greater understanding of along-strike variations in seismic character of the Alpine Fault ahead of an anticipated large earthquake.  To achieve observations with high spatial and temporal resolution across the length of the central Alpine Fault, I use 8.5 years of continuous seismic data from the Southern Alps Microearthquake Borehole Array (SAMBA), and data from four other temporary seismic networks and five local GeoNet permanent sites. Incorporating all of these temporary and permanent seismic sites provides us with a dense composite network of seismometers. Without such a dense network, homogeneous examination of the characteristics of low-magnitude seismicity near the Alpine Fault would be impossible.  Using this dataset, I have constructed the most extensive microearthquake catalog for the central Alpine Fault region to date, containing 9,111 earthquakes and covering the time between late 2008 and early 2017. To construct this catalog I created an objective workflow to ensure catalog uniformity. Overall, 7,719 earthquakes were successfully relocated with location uncertainties generally ≤ 0.5 km in both the horizontal and vertical directions. The majority of the earthquakes were found to occur southeast of the Alpine Fault (i.e. in the hanging-wall). I observed a lack of seismicity beneath Aoraki/Mount Cook that has previously been shown to be associated with locally high uplift rates (6–10 mm/yr) and high geothermal gradients (∼60◦C/km). Seismogenic cut-off depths were observed to significantly vary along the strike of the Alpine Fault, ranging from 8 km beneath the highest topography to 20 km in the adjacent areas.  To quantify the scale of the seismic deformation, a new local magnitude scale was also derived, corrected for geometric spreading, attenuation and site terms based on individually calculated GeoNet moment magnitude (Mw) values. Earthquake local magnitudes range between ML –1.2 and 4.6 and the catalog is complete above ML 1.1.  To examine the stress regime near the central Alpine Fault, I built a new data set of 845 focal mechanisms from earthquakes in our catalog. This was achieved by manually determining P wave arrival polarity picks from all earthquakes larger than ML 1.5. In order to determine the orientations and characteristics of the stress parameters, I grouped these focal mechanisms and performed stress inversion calculations that provided an average maximum horizontal compressive stress orientation, SHmax, of 121±11◦ , which is uniform within uncertainty along the length of the central Southern Alps. I observed an average angle of 65◦ between the SHmax and the strike of the Alpine Fault, which is consistent with results from similar previous studies in the northern and southern sections of the Alpine Fault. This implies that the Alpine Fault is misoriented for reactivation, in the prevailing stress field.  Using a 1-D steady-state thermal structure model constrained by seismicity and thermochronology data, I investigated the crustal thermal structure and vertical kinematics of the central Southern Alps orogen. The short-term seismicity data and longer-term thermochronology data impose complementary constraints on the model. I observed a large variation in exhumation rate estimates (1–8 mm/yr) along the length of the Alpine Fault, with maximum calculated values observed near Aoraki/Mount Cook. I calculated the temperature at the brittle-ductile transition zone, which ranges from 440 to 457◦C in the different models considered. This temperature is slightly hotter than expected for crust composed by quartz-rich rocks, but consistent with the presence of feldspar-rich mafic rocks in parts of the crust.