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，是一个主要的斜滑板块边界断裂带，是ICDP钻井项目——深断层钻井项目（DFDP）的目标。与主要断层带的其他钻井项目相比，DFDP的独特之处在于，它是在预计不久的将来发生的大地震之前启动的。除了它迫在眉睫的地震危险之外，先前的研究表明，阿尔卑斯断层的两个不寻常的特征是，它可能在地球表面摩擦不稳定，并且具有非常高的地热梯度。到目前为止，已经钻了两个试点井眼，作为钻井的第一阶段，DFDP-1。目前，我们从这些钻孔中获得了约40个岩心样本，包括几个完整的岩心，覆盖了阿尔卑斯断层的所有主要岩性，包括主滑动带（PSZ）内部和附近。我们计划将今年秋季即将进行的第二阶段钻井（DFDP-2）的岩心样品添加到样品库存中。我们在此建议通过在广泛的实验条件（压力、温度、滑移速度）下量化岩石强度和摩擦滑移行为来建立初步的DFDP工作。在拟建项目中需要解决的主要问题是：(1)从几公里深处一直到地表，高山断层是否存在摩擦不稳定？(2)高寒断层地温梯度对岩石特征和力学性质的影响是什么？(3)与其他主要断裂带相比，高寒断裂带的滑动行为如何？我们将在具有代表性的近地表和几公里（发震）深度的条件下使用直接剪切技术来解决这些问题。为了解决上述问题，我们计划利用高温（高达~240℃）和大范围的滑移速度（高达同震速率（m/s））。研究的一个关键部分将是测试和比较完整的全圆岩心样品和样品粉末。预期的结果将有助于回答DFDP的一个目标科学问题：在晚期大陆板块边界断层上，什么样的环境条件和岩石性质允许大地震的成核和传播？

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?