Fault lubrication during earthquake propagation in thermally unstable rocks

在碳酸盐岩等热不稳定岩石的滑移过程中，三种热激活机制被认为是特别相关的：1)在高应力摩擦微接触（凹凸不平）处的闪热；2)对释放和/或被困在滑移区的加热流体进行热加压；3)热诱导化学分解反应（脱碳）产生的纳米颗粒的润滑效果。为了研究碳酸盐断裂带中的这种化学和物理反应是否会使断层变得极其脆弱，并有利于地震破裂的持续传播，我们在这里提出了一个多学科的研究计划，其中综合了从现场和实验室研究中获得的力学、矿物学、微观结构、流体流动特性和建模数据。实地研究将在意大利亚平宁山脉的碳酸盐岩中进行，以重建自然断裂带的几何形状，确定不同的构造域及其相关的断层岩石组合。野外观测和微观结构/矿物学分析的结合将为分析和解释在实验变形样品中观察到的主要变形过程提供重要的地质限制。这些结果将用于产生一种新的碳酸盐岩地震断层岩分类方案，该分类方案基于断层岩和作为地震滑动事件指标的微观结构的诊断关联的识别和描述。这种分类将有助于识别沿暴露断层段的化石地震，因此可用于解释世界其他地区的古地震断层记录，有助于风险/危害评估。高速摩擦实验将在固体和粒状碳酸盐岩上进行，剪切速度与大地震相似（1.3m/s），以评估从研究区域的活动断裂带收集的断层岩石材料可能的动态摩擦强度Tf。通过在炉中热分解碳酸盐获得的合成纳米粉末也将进行测试。将实验室摩擦试验结果与实验断层和自然断层的微观结构研究相结合，应该能够识别出主要的削弱机制，并约束它们在自然环境中的运行条件。颗粒材料的渗透率受粒度分布、颗粒形状、固体体积分数和孔隙连通性等因素的控制。所有这些几何参数在断裂带中都是不同的。渗透率实验室测量将对沿着与断裂带主要滑动面正交平行的横断面收集的现场样品进行。这些数据可以提供已知/受控条件下（摩擦、位移、流体排放时间）滑移带渗透率演变的有用“快照”信息，可用于校准/测试流体流动建模结果。我们将使用最先进的数值模型（名称）来确定从定量微观结构分析中获得的几何参数范围的渗透率张量。这种数值方法将使我们能够以一种仅通过实验室实验无法实现的方式探索系统渗透率变化。碳酸盐在滑动过程中热分解产生的流体的耗散速率将被量化，从而确定它们在断裂带润滑中的作用。

Three thermally activated mechanisms are considered to be of particular relevance during slip in thermally unstable rocks such as carbonates: 1) flash heating at highly stressed frictional micro-contacts (asperities); 2) thermal pressurization of heated fluids released and/or trapped in the slip zone; and 3) the lubrication effects of nanoparticles produced by thermally-induced chemical decomposition reactions (decarbonation). In order to investigate whether such chemical and physical reactions in carbonate fault zones can make faults extremely weak and favour the continued propagation of earthquake ruptures, we propose here a multidisciplinary research program where mechanical, mineralogical, microstructural, fluid flow properties and modelling data, obtained from both field and laboratory studies are integrated. Fieldwork studies will be carried out in carbonate rocks from the Italian Apennines to reconstruct the natural fault zone geometries, identify the different structural domains and their associated fault rock assemblages. The integration of field observations and microstructural/mineralogical analyses will provide important geological constraints in the analysis and interpretation of the dominant deformation processes observed in experimentally deformed samples. These results will be used to produce a new classification scheme for seismic fault rocks in carbonates, based on the identification and description of diagnostic associations of fault rocks and microstructures which are indicators of earthquake slip events. This classification will aid in the recognition of fossil earthquakes along exposed fault segments and, therefore, can be used to interpret records of palaeo-seismic faulting in other parts of the world, aiding in risk/hazard assessment. High velocity friction experiments will be performed on solid and granular carbonate rocks, sheared at speeds similar to that seen in large earthquakes (1.3m/s), in order to assess the likely dynamic frictional strength Tf of fault rock materials collected from active fault zones in the study areas. Synthetic nano-powders obtained by thermal decomposition of carbonates in a furnace will also be tested. The integration of laboratory friction test results and microstructural studies from both experimental and natural faults should allow the identification of the dominant weakening mechanisms and constrain their operational conditions in natural environments. The permeability of granular materials is controlled by grain size distribution, grain shape, solid volume fraction and pore connectivity. All of these geometric parameters vary across a fault zone. Permeability laboratory measurements will be performed on field samples collected along transects oriented orthogonal and parallel to principal slip surfaces in the fault zones. These data can provide useful 'snapshot' information on the evolution of permeability of slip zones under known/controlled conditions (friction, displacement, timing of fluid emissions), which can be used to calibrate/test fluid flow modelling results. We will use a state-of-the-art numerical model (name) to determine the permeability tensor for the range of geometric parameters obtained from quantitative microstructaral analyses. This numerical approach will allow us to explore systematic permeability variations in a way that cannot be achieved through laboratory experiments alone. The rate of dissipation of the fluids generated by thermal decomposition of the carbonate during slip will be quantified, allowing the role they play in fault zone lubrication to be determined.