Evolution of the physical, geochemical and mechanical properties of the Alpine Fault Zone: A journey through an active plate boundary

该提案是英国一项重大国际运动的组成部分，该运动名为“深断层钻探项目”（DFDP），旨在在新西兰的阿尔卑斯断层上钻一系列洞。DFDP的总体目标是通过采集岩心和观察主要大陆断层在大地震事件发生期间的形成过程，更好地了解导致大地震的过程。该项目的下一阶段将是在阿尔卑斯断层上钻一个1.5公里的洞并进行仪器测量。地震是主要的地质灾害。尽管科学家可以预测地球表面地震最有可能发生的地方，主要是沿着板块边界，但我们的知识并不完善。我们也不知道地震何时会发生。最近在新西兰南岛发生的事件很好地说明了这一点。2010年9月和2011年2月在基督城发生的两次地震造成181人死亡和70 - 100亿英镑的损失（约占新西兰GDP的10%）。然而，克赖斯特彻奇此前被认为地震风险相对较低。相比之下，南岛的西侧是由南阿尔卑斯山界定的，南阿尔卑斯山是沿澳大利亚-太平洋板块边界形成的一条主要山脉（约3700米）。直到几百万年前，这个板块边界还是一个走滑断层，就像加州的圣安德烈亚斯断层一样，但板块运动的微妙变化导致了太平洋板块和澳大利亚板块的碰撞。这导致了山脉的隆起，由于非常高的降雨和侵蚀率，快速挖掘出直到最近还在地球深处的岩石。尽管这些板块以30毫米/年的速度相互移动，而南阿尔卑斯山的隆升速度接近10毫米/年，但在新西兰的历史上，阿尔卑斯断层并没有发生过大地震，尽管历史很短。然而，有古地震证据表明，沿阿尔卑斯断裂带确实发生大地震，每200-400年发生一次~8级地震，最近一次发生在公元1717年。地震的发生是因为应力在相对较强、较脆的上地壳内部积聚。在更深的深度（大约15公里），岩石可以塑性流动，板块可以相互移动，而不会产生危险的压力。在一些断层上，脆弱的地壳在许多小的微地震事件中“蠕动”，这抑制了应力的积累。不幸的是，沿着阿尔卑斯断层甚至很少有微地震事件或地表变形的证据，这表明自1717年以来，沿着这块板块边界的应力一直在积累——如果这种应力在一次地震中释放出来，它将导致横跨断层的水平偏移bbbb8米！地震研究的一个主要障碍是缺乏来自地震前应力积聚的深处的断层岩石样本。暴露在地表的断层岩往往受到强烈的蚀变。断层岩石的强度取决于许多因素，包括压力、温度和材料的性质，但也取决于是否存在地热流体。阿尔卑斯断层的几何形状是特别的，因为最近在地壳深处变形的断层岩石暴露在接近表面的地方。此外，由于快速的隆升和侵蚀，当地的地热梯度很高，地表附近有相对热的岩石。这导致从脆性到塑性行为转变的相对较浅的深度（5-8公里）。这为钻入断裂带以恢复断层岩心、对井内地层进行测试以及在井内安装测量温度、流体压力和地震活动的仪器提供了独特的机会。一旦获得岩心样本，我们将对断层岩石进行地球化学和微观结构分析，以了解它们变形的条件。我们将对它们进行地质力学测试，看看环境的变化如何影响岩石的强度和它们在破裂前适应应力的能力。

This proposal is the UK component of a major international campaign, the Deep Fault Drilling Project (DFDP) to drill a series of holes into the Alpine Fault, New Zealand. The overarching aim of the DFDP to understand better the processes that lead to major earthquakes by taking cores and observing a major continental fault during its build up to a large seismic event. The next stage of this project will be to drill and instrument a 1.5 km hole into the Alpine Fault.Earthquakes are major geohazards. Although scientists can predict where on the Earth's surface earthquakes are most likely to occur, principally along plate boundaries, we have only imperfect knowledge. We also don't know when earthquakes will occur. This is well illustrated by recent events on the South Island of NZ. Two earthquakes in Christchurch in Sept 2010 and Feb 2011 caused 181 deaths and £7-10 billion of damage (~10% NZ GDP). Yet Christchurch had previously been considered of relatively low seismic risk. In contrast, the western side of the South Island is defined by the Southern Alps, a major mountain chain (>3700 m) formed along the Australian-Pacific Plate boundary. Until a few million years ago this plate boundary was a strike-slip fault like the San Andreas Fault in California, but subtle changes in plate motion has led to the collision of the Pacific and Australian Plates. This caused uplift of the mountains and due to very high rates of rainfall and erosion, rapid exhumation of rocks that until recently had been deep within the Earth. Although these plates are moving past each other at ~30 mm/y and the uplift rate in the Southern Alps approaches 10 mm/y, there has not been a major earthquake along the Alpine Fault in NZ's, albeit short, written history. However, there is palaeo-seismic evidence that major earthquakes do occur along the Alpine Fault with magnitude ~8 earthquakes occurring every 200-400 years, with the latest event in 1717 AD.Earthquake occur because stresses build-up within the relatively strong brittle upper crust. At greater depths (>15 km) rocks can flow plastically and plates can move past each other without building up dangerous stresses. On some faults, the brittle crust "creeps" in numerous small micro-earthquake events and this inhibits the build up of stress. Unfortunately there are few even micro-earthquake events along the Alpine Fault or surface evidence for deformation, suggesting that the stresses along this plate boundary have been building up since 1717 - if that stress was released in a single earthquake it would result in a horizontal offset across the fault of >8 m!A major hindrance to earthquake research is a lack of fault rock samples from the depths where stresses build up before an earthquake. Fault rocks exposed at the surface tend to be strongly altered. The strength of fault rocks will depend on a number of factors include pressure, temperature and the nature of the materials, but also whether there are geothermal fluids present. The geometry of the Alpine Fault is special in that the fault rocks that were recently deforming at depth within the crust are exposed close to the surface. Also because of rapid uplift and erosion the local geothermal gradients are high and relatively hot rocks are near the surface. This results in a relatively shallow depth (5-8 km) for the transition from brittle to plastic behaviour. This provides a unique opportunity to drill into the fault zone to recover cores of the fault, to undertake tests of the borehole strata, and to install within the borehole instruments to measure temperature, fluid pressures, and seismic activity. Once core samples are recovered we will perform geochemical and microstructural analyses on the fault rocks to understand the conditions at which they were deformed. We will subject them to geomechanical testing to see how changes in their environment affects the strength of the rocks and their ability to accommodate stresses before breaking.