THE GEOLOGICAL RECORD OF THE EARTHQUAKE CYCLE IN THE LOWER CRUST

了解下地壳的短期和长期力学行为对于理解地震周期和沿活动断裂带的相关危险是至关重要的。在某些地区，约20%的bbbb5级大陆内地震在深度为30-40公里的下地壳中形成核。例如，喜马拉雅地区的大部分地震活动，以及与2001年印度布吉破坏性地震有关的余震，都是在印度地盾的粒状下地壳中成核的。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，彻底了解大陆内背景下的地震周期是至关重要的。这需要了解下地壳的力学行为和强度（地球科学家通常指岩石在变形前所能承受的最大应力）。大陆地壳强度的最常见的概念模型预测了一个强大的、易发生地震的脆性上地壳（其中发震层的底部通常被认为在10-15公里的深度）和一个脆弱的、粘性的、易发生地震的下地壳。这个模型最近受到了质疑，因为发现下地壳可能会发生地震，因此具有机械强度。问题来了，地壳中的发震层有多厚？回答这个问题对于确定大地震造成的潜在危害至关重要，大地震通常也是最深的地震。我们对下地壳力学行为的认识有限，主要是由于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）。世界上只有少数几大块暴露良好的大陆下地壳被发掘出来。其中一个位于罗弗敦群岛（挪威北部），它是在北大西洋开放期间从原始的地壳深处挖掘出来的。我们建议对罗浮敦的脆性-粘性剪切带（即局部强烈变形的地质物质带）网络进行综合的多学科研究，该网络记录了间歇性快速滑动事件（地震）与长期地震蠕变交替发生。这项研究将把构造地质学（分析地质断层和剪切带）、岩石学（分析岩石的组成和质地）、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地震周期期间大陆下地壳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当前活动地震变形的地球物理观测进行检验。这些项目的累积结果将揭示目前在地震周期中较差的下地壳力学行为，从而揭示地震间滑动的顺序（沿断层缓慢积累弹性变形的时期）。同震滑动（沿断层的突然破裂，即地震）和震后滑动（地震后地壳和断层适应地震引起的地壳应力改变状态的直接时期）。这将极大地扩展和补充科学界在理解和解释下地壳记录的地震周期中岩石的力学行为和相关危害方面的现有努力，并将为大陆动力学的数值模型提供关键输入。

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.