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 conditions prevail within the interiors of active faults is crucial for elucidating the mechanisms governing long-term fault evolution and, in particular, the earthquake rupture process. This project involves a detailed study of microearthquakes and crustal structure within and surrounding the Alpine Fault, South Island, New Zealand, around the Whataroa fault zone drilling site. It will be carried out close collaboration with a team of New Zealand scientists. The key scientific issues to be addressed by this research project include: (1) determining the geometry of the AF to mid-crustal depths and the depth to the base of the seismogenic zone, (2) determining focal mechanisms and estimating principal stress orientations, (3) seismic imaging of crustal structure using multiple methods, and (4) searching for possible nonvolcanic tremor (NVT). Our project is designed to test a number of hypotheses, including: (1) that the complexity of the surface fault trace does not reflect the fault trace at depth, rather the partitioning seen at the surface merges into a throughgoing fault with oblique slip in the upper crust; (2) that the depth to the base of the seismogenic zone is elevated to 15 km, or perhaps shallower, on the AF beneath the Whataroa area; (3) that the AF dip remains close to 50° to the base of the seismogenic zone; (4) that principal stress orientations do not vary with distance from the AF; (5) that the relatively low velocity zone imaged at large scale through much of the crust along the SIGHT transects is actually a narrower zone closer to the AF, similar to the resistivity model; (6) that NVT is not present beneath the AF.
The goal of this project is to examine the structure and microearthquake characteristics of a major active continental fault, the Alpine Fault, South Island, New Zealand. The Alpine Fault is late in its earthquake cycle and is the focus of a long-term, multidisciplinary scientific investigation. In this project, we will carry out seismic field work and subsequent analyses of microearthquakes associated with the Alpine Fault. The research will be conducted in conjunction with an ambitious fault zone drilling project focused on the mechanical properties of the Alpine Fault and the adjacent crust. Their work will help (1) define the geometry of the fault at depth, (2) determine the depth range of seismic activity (the "seismogenic zone"), (3) establish a 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.
