Geomechanical Investigation of Frictional Stability in DFDP Drilling Samples from the Alpine Fault, New Zealand

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The Alpine Fault, located on South Island of New Zealand, is a major oblique-slip plate boundary fault zone that is the target of an ICDP drilling project, the Deep Fault Drilling Project (DFDP). DFDP is unique compared to other drilling projects on major fault zones in that it has been initiated in advance of a major earthquake, which is expected to occur in the near future. In addition to its imminent earthquake hazard, previous research suggests that two unusual features of the Alpine Fault are that it may be frictionally unstable up to the earth surface, and have a very high geothermal gradient. Thus far, two pilot boreholes have been drilled as the first phase of drilling, DFDP-1. We currently hold ~40 core samples from these boreholes, including several whole-round cores, covering all major lithologies of the Alpine Fault including within and near the Principal Slip Zones (PSZ). We plan on adding to the sample inventory with core samples from the upcoming second stage of drilling (DFDP-2) scheduled this fall. We propose here to build on preliminary DFDP work by quantifying rock strength and frictional slip behavior under a wide range of experimental conditions (pressure, temperature, slip velocity). Main questions to be addressed in the proposed project are: (1) Is the Alpine Fault frictionally unstable from several km depth all the way to the earth surface? (2) What is the effect of an elevated Alpine Fault geothermal gradient on rock characteristics and mechanical properties? (3) How does the slip behavior of the Alpine Fault compare with other major fault zones? We will tackle these questions using direct shearing techniques under conditions representative of both the near surface and at several km (seismogenic) depths. We plan on utilizing elevated temperatures (up to ~240 C) and a wide range of slip velocities, up to coseismic rates (m/s) in order to address the above questions. A key part of the research will be to test and compare both intact whole-round core samples and sample powders. The expected results will help answering one of the target scientific questions of DFDP: what are the ambient conditions and rock properties that allow the nucleation and propagation of large earthquakes on a late-stage continental plate-boundary fault?