Collaborative Research: Seismic characterization of microearthquakes and crustal velocity structure around the Whataroa fault zone drilling site, Alpine Fault, New Zealand

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The mechanical behavior of large faults is controlled by thermal, chemical, and hydraulic factors,as well as each fault's intrinsic structure and the ambient stresses. Understanding what conditionsprevail within the interiors of active faults is crucial for elucidating the mechanisms governinglong-term fault evolution and, in particular, the earthquake rupture process. This project involvesa detailed study of microearthquakes and crustal structure within and surrounding the AlpineFault, South Island, New Zealand, around the Whataroa fault zone drilling site. It will be carriedout close collaboration with a team of New Zealand scientists. The key scientific issues to beaddressed by this research project include: (1) determining the geometry of the AF to mid-crustaldepths and the depth to the base of the seismogenic zone, (2) determining focal mechanisms andestimating principal stress orientations, (3) seismic imaging of crustal structure using multiplemethods, and (4) searching for possible nonvolcanic tremor (NVT). Our project is designed totest a number of hypotheses, including: (1) that the complexity of the surface fault trace does notreflect the fault trace at depth, rather the partitioning seen at the surface merges into a throughgoingfault with oblique slip in the upper crust; (2) that the depth to the base of the seismogeniczone is elevated to 15 km, or perhaps shallower, on the AF beneath the Whataroa area; (3) thatthe AF dip remains close to 50° to the base of the seismogenic zone; (4) that principal stressorientations do not vary with distance from the AF; (5) that the relatively low velocity zoneimaged at large scale through much of the crust along the SIGHT transects is actually a narrowerzone closer to the AF, similar to the resistivity model; (6) that NVT is not present beneath theAF.The goal of this project is to examine the structure and microearthquake characteristics of amajor active continental fault, the Alpine Fault, South Island, New Zealand. The Alpine Fault islate in its earthquake cycle and is the focus of a long-term, multidisciplinary scientificinvestigation. In this project, we will carry out seismic field work and subsequent analyses ofmicroearthquakes associated with the Alpine Fault. The research will be conducted inconjunction with an ambitious fault zone drilling project focused on the mechanical properties ofthe Alpine Fault and the adjacent crust. Their work will help (1) define the geometry of the faultat depth, (2) determine the depth range of seismic activity (the "seismogenic zone"), (3) establisha three-dimensional structural context for interpreting borehole observations and core, and (4)potentially help to define rupture zones of shallow earthquakes as future drilling targets.