Seismic characterization of the DFDP-2 drill hole environment

新西兰南岛阿尔卑斯断裂是地球上最大的活动断裂带之一，每200-400年一次7.9级地震破裂。深部断层钻探项目阿尔卑斯山断层(DFDP-AF)建议对阿尔卑斯山断层进行深部钻探、取样和监测，以利用良好的地表暴露和相对较浅的地质过渡深度，从而更好地了解岩石变形、地震成因(地震起始和破裂)和地震变形的基本过程。在这方面，阿尔卑斯山断裂构成了一个具有国际意义的天然实验室，用于研究活动的板块边界大陆断裂如何工作，特别是地震孕育的性质。该项目的第一阶段(DFDP-1)于2011年进行，钻探了两个穿过冈特克里克阿尔卑斯断层的浅井。目前正在筹备该项目的第二阶段(DFDP-2)，该项目将在瓦塔罗亚河谷进行，深度约为1.4公里。第二个钻孔(DFDP-2)将钻过阿尔卑斯断层，深度约1.2公里，基于我们在2011年获得的有源地震钻探场地特征调查(WhataDUSIE)的成像结果。这里建议使用这个新的钻孔(DFDP-2)进行扩展的走式垂直地震剖面(W-VSP)实验，使用固定的钻孔传感器和地面接收器来记录主要分布在先前WhataDUSIE剖面线上的活跃震源。其结果将是阿尔卑斯山断层及其地下环境的几何和结构的高分辨率图像，特别是在拟议的钻探作业可能目标的深度。由此产生的图像将能够将钻孔测井结果以及岩心和物性测量与阿尔卑斯山断层及其更大规模的几何和结构的相邻部分联系起来。这对于更好地了解当地的构造环境以及评估地震破裂情景和相关的地质灾害同样重要，从而为大多数相关的DFDP-AF研究小组提供了重要的基础。

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.