THE GEOLOGICAL RECORD OF THE EARTHQUAKE CYCLE IN THE LOWER CRUST

在试图了解活动断裂带的地震周期和相关危险时，了解下地壳的短期和长期力学行为是至关重要的。在一些地区，约20%的5级陆内地震发生在30-40公里深的下地壳中。例如，喜马拉雅地区的很大一部分地震活动，以及与2001年印度布吉破坏性地震有关的余震，都是在印度地幔的花岗岩下地壳中孕育的。大陆内部的地震往往是毁灭性的，在过去的一个世纪里，造成的死亡人数远远超过发生在板块边界的地震。因此，对大陆内部背景下的地震周期有一个透彻的了解是至关重要的。这需要了解下地壳的力学行为和强度(地球科学家通常指的是岩石在变形前所能承受的最大应力)。最常见的大陆地壳强度概念模型预测了一个强大、易碎的孕震上地壳(孕震层的底部通常被认为在10-15公里深处)和一个弱的、粘滞的、无地震的下地壳。这一模型最近受到了质疑，因为发现下地壳可能是地震的，因此机械强度也很强。问题来了，地壳中的孕震层有多厚？回答这个问题对于确定大地震造成的潜在危险至关重要，大地震通常也是最严重的。我们对下地壳力学行为的有限了解很大程度上是因为下地壳本身很难进行直接的地质观测，因此我们的大部分知识来自间接的地球物理测量(如地震的分布)。世界上只有少数几个暴露良好的大陆下地壳大段出土。其中之一位于罗福滕群岛(挪威北部)，它是在北大西洋开放期间从原始深部地壳位置挖掘出来的。我们建议对罗福滕的脆性-粘性剪切带(即地质物质的局部强烈变形带)网络进行综合的、多学科的研究，该网络记录了交替发生的幕式快速滑动事件(地震)和长期的无震蠕变。这项研究将把构造地质学(地质断层和剪切带的分析)、岩石学(岩石的成分和结构分析)、地球化学(岩石和矿物的详细化学特征)和实验岩石变形(在实验室中在受控条件下再现地壳深处的变形过程)联系起来。这一综合数据集将为大陆下地壳在地震周期中的力学行为提供一幅新颖、清晰的图景。我们的直接地质和实验观测将与目前活跃的地震形变的地球物理观测相对照。这些项目的累积结果将揭示地震周期期间下地壳目前缺乏约束的力学行为，从而揭示地震间滑动(沿着断层的弹性变形缓慢累积的时期)、同震滑动(沿着地震的断层的突然破裂)和震后滑动(地震后紧随其后的时期，此时地壳和断层调整到地震引起的地壳应力的修改状态)的顺序。这将极大地扩展和补充科学界在了解和解释记录在下地壳的地震周期中岩石的力学行为和相关灾害方面的现有努力，并将为大陆动力学的数值模型提供关键资料。

Understanding the short- and long-term mechanical behaviour of the lower crust is of fundamental importance when trying to understand the earthquake cycle and related hazard along active fault zones. In some regions some 20% of intracontinental earthquakes of magnitude > 5 nucleates in the lower crust at depth of 30-40 km. For example, a significant proportion of seismicity in the Himalaya, as well as aftershocks associated with the destructive 2001 Bhuj earthquake in India, nucleated in the granulitic lower crust of the Indian shield. Earthquakes in the continental interiors are often devastating and, over the past century, have killed significantly more people than earthquakes that occurred at plate boundaries. Thus, a thorough understanding of the earthquake cycle in intracontinental settings is essential. This requires knowledge of the mechanical behaviour and of the strength (by which Earth scientists commonly mean the maximum stress that rocks can sustain before deforming) of the lower crust.The most common conceptual model of the strength of the continental crust predicts a strong, seismogenic brittle upper crust (where the base of the seismogenic layer is typically considered to be at depth of 10-15 km), and a weak, viscous, aseismic lower crust. This model has been recently questioned by the finding that the lower crust can be seismic and, therefore, mechanically strong. The question arises, how thick is the seismogenic layer in the crust? Answering this question is crucial to determine the potential hazard caused by large earthquakes, which are also generally the deepest.Our limited knowledge of the mechanical behaviour of the lower crust is largely due to the lower crust itself being poorly accessible for direct geological observations, so that most of our knowledge derives from indirect geophysical measurements (like the distribution of earthquakes). There are only a few well-exposed large sections of exhumed continental lower crust in the world. One of these is located in the Lofoten islands (northern Norway), which were exhumed from their original deep crustal position during the opening of the North Atlantic Ocean.We propose an integrated, multi-disciplinary study of a network of brittle-viscous shear zones (i.e. zones of localized intense deformation of geological materials) from Lofoten, which records episodic rapid slip events (earthquakes) alternating with long-lasting aseismic creep. The study will link structural geology (analysis of geological faults and shear zones), petrology (analysis of the composition and textures of rocks), geochemistry (detailed chemical characterization of rocks and minerals) and experimental rock deformation (to reproduce in the lab under controlled conditions the deformation processes operative in the deep Earth's crust). This integrated dataset will provide a novel, clear picture of the mechanical behaviour of the continental lower crust during the earthquake cycle. Our direct geological and experimental observations will be tested against geophysical observations of currently active seismic deformation. The cumulative results of the projects will shed light on the currently poorly constrained mechanical behaviour of the lower crust during the earthquake cycle, and therefore on the sequence of inter-seismic slip (the period of slow accumulation of elastic deformation along a fault), co-seismic slip (the sudden rupture along a fault that is the earthquake) and post-seismic slip (the immediate period after an earthquake when the crust and the fault adjust to the modified state of crustal stress caused by an earthquake). This will greatly extend and complement existing efforts by the scientific community to understand and interpret the mechanical behaviour of rocks during the earthquake cycle recorded in the lower crust and the related hazard, and will provide key input for numerical models of continental dynamics.