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

阿尔卑斯断层位于新西兰南岛，是一条主要的斜滑板块边界断裂带，是ICDP钻探项目--深断层钻探项目(DFDP)的目标。与其他主要断裂带的钻探项目相比，DFDP是独一无二的，因为它是在预计不久的将来发生的大地震之前启动的。除了迫在眉睫的地震危险外，以前的研究表明，阿尔卑斯山断裂的两个不寻常的特征是，它可能在地表以下是摩擦不稳定的，并且具有非常高的地温梯度。到目前为止，作为第一阶段钻井，DFDP-1已经钻出了两个先导井。我们目前拥有来自这些钻孔的约40个岩心样品，包括几个全圆岩心，覆盖了阿尔卑斯山断裂的所有主要岩性，包括主滑移带(PSZ)内和附近。我们计划增加样品库存，增加今年秋季即将进行的第二阶段钻探(DFDP-2)的岩心样品。我们建议在DFDP初步工作的基础上，通过在广泛的实验条件(压力、温度、滑动速度)下量化岩石强度和摩擦滑动行为来进行。本项目要解决的主要问题是：(1)阿尔卑斯断裂从几公里深一直到地表的摩擦不稳定？(2)阿尔卑斯断裂地温梯度升高对岩石特征和力学性质的影响？(3)阿尔卑斯断裂的滑动行为与其他主要断裂带相比如何？我们将在代表近地表和几公里(孕震)深度的条件下，使用直接剪切技术来解决这些问题。我们计划利用高温(高达~240C)和大范围的滑动速度，高达同震速率(m/S)来解决上述问题。这项研究的一个关键部分将是测试和比较完整的全圆芯样品和样品粉末。预期的结果将有助于回答DFDP的目标科学问题之一：什么环境条件和岩石性质允许在晚期大陆板块边界断层上形成和传播大地震？

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?