Seismic characterization of the DFDP-2 drill hole environment

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The Alpine Fault (South Island, New Zealand) is one of the largest active fault zones on earth, which ruptures every 200-400 years in a magnitude ~7.9 earthquake. The Deep-Fault-Drilling-Project-Alpine-Fault (DFDP-AF) proposes to drill, sample, and monitor the Alpine Fault at depth, to take advantage of excellent surface exposures and the relatively shallow depths of geological transitions, and hence to better understand fundamental processes of rock deformation, seismogenesis (earthquake initiation and rupture), and earthquake deformation. In that context the Alpine Fault constitutes an internationally significant natural laboratory for research into how active plate-bounding continental faults work and, in particular, the nature of earthquake generation. The first phase of the project (DFDP-1) was undertaken in 2011 with drilling of two shallow boreholes through the Alpine Fault at Gaunt Creek. Preparations are now under way for the second phase of the project (DFDP-2) which will be conducted in the Whataroa Valley, and will reach a depth of approximately 1.4 km. The latter borehole (DFDP-2) will be drilled through the Alpine Fault at a depth of about 1.2 km, based on the imaging results of our active-source seismic drill site characterization survey (WhataDUSIE) acquired in 2011.Here it is proposed to use this new borehole (DFDP-2) to conduct an extended walkaway vertical seismic profiling (W-VSP) experiment using fixed borehole sensors as well as surface receivers to record active seismic sources distributed mainly along the previous WhataDUSIE profile line. The result will be a high-resolution image of the geometry and structure of the Alpine Fault and its subsurface environment, especially at those depths which the proposed drilling operation might target. The resulting images will enable to relate borehole logging results as well as core and physical property measurements to the adjacent parts of the Alpine Fault and its larger-scale geometry and structure. This is equally important for a better understanding of the local tectonic setting and for evaluating earthquake rupture scenarios and the related geohazard, thereby providing an important basis for most of the associated DFDP-AF research groups.