Materials physics of rapidly sheared faults and consequences for earthquake rupture dynamics

在发生重大事件的成熟和高度滑动的断裂带上发生的地震似乎是在总体剪应力水平上发生的，这明显低于启动断裂壁之间缓慢摩擦滑动所需的“静摩擦”应力水平。如果这些静摩擦应力在地震滑动期间占优势，它们将沿着断层产生可察觉的局部热外流，并留下大量熔化和再凝固的迹象，即使在地壳浅层也是如此。这两种情况通常都没有。此外，最近的野外和实验室观察表明，快速剪切过程中的大部分变形通常局限在沿着断层的一个非常薄的主剪切带上，通常不到一毫米到一厘米宽，这一特征形成在一个更宽的、比如一到一百米宽的颗粒状和受损岩石带内。在计划中的研究中，我们的目的是了解导致断裂带响应特征的材料和热物理，并确定它们对大地震中滑动破裂沿着断层传播的方式的一些影响。人们希望，这种对地震物理学的基本理解最终可能会在改善地震现象和影响的可预测性方面有所收获。我们提出了这样的概念，即在剪切过程中，地下水饱和断层泥的热加热会导致应变在实际狭窄的区域内强烈局部化。这集中于进一步的加热和温度上升，但不是直接导致融化，而是触发了足以限制强度的减弱机制，从而继续加热，从而使断裂带的整体融化变得罕见，至少在地壳浅层。一种相对普遍的减弱形式是，地下水的热膨胀远远超过其矿物质宿主，导致矿物质成分相互之间的推力较小，因此具有较低的摩擦强度。这一过程的一个变种是，碳酸盐和水化粘土等常见断层成分的热分解发生在远低于熔融的温度，并产生一种高度压力的挥发性产物相(分别为CO2或H2O)，这同样会降低强度。进一步的弱化过程与固体分解和磨损产物的纳米尺寸范围有关，其物理细节尚不清楚。我们将模拟这些减弱过程如何影响传播中的地震破裂的特征(例如，裂缝与滑动脉冲、破裂速度、应力降、总滑移)，破裂如何与断层矿物学和深度相关，以及如何在地震观测中识别不同的动态减弱过程。有待检验的假设是，热分解与断层矿物学的变化相结合，可以解释破裂是如何在孕震带底部停止的，热分解可以为通常经历较小事件的断层上偶尔发生的极端地震提供一种机制。我们将把位于狭窄的高变形断层核之外的材料建模为弹性或弹脆塑性固体，并使用我们对变形断层核内局部剪切过程的分析作为更大分析中沿断层表面施加边界条件的基础。这项研究应有助于对地震过程有一个统一的整体理解。它将从细微尺度的材料物理/化学理论、地质断层核心研究、岩石力学实验室摩擦实验、自发破裂模拟、地震滑动模式和地震破裂程度的地震观测，以及通过热流、地形支持和相关研究对发生大地震的应力状态的大尺度约束。

Earthquakes on the well-established and highly slipped fault zones which host major events seem to occur at overall levels of shearing stress which are notably lower than "static friction" stress levels required to initiate slow frictional sliding between the fault walls. If those static friction stresses prevailed during earthquake slip, they would produce perceptible localized heat outflows along faults and leave abundant signs of melting and re-solidification, even at shallow crustal depths. Neither are generally found. Also, recent field and lab observations show that the majority of deformation during rapid shear is generally localized to a remarkably thin principal shear zone along the fault, often less than a millimeter to a centimeter wide, with that feature forming within a much broader, say, one to a hundred meters wide, zone of granulated and damaged rock. Our aim in the planned study is to understand the materials and thermal physics responsible for those features of fault zone response, and to establish some of their consequences for the manner by which slip-ruptures propagate along faults in major earthquakes. It is hoped that such basic understanding of the physics of earthquakes may ultimately have payoffs in the improved predictability of seismic phenomena and effects. We have developed the concept that thermal heating of groundwater-saturated fault gouge during shear leads to strong localization of strain into realistically narrow zones. That focuses further heating and temperature rise, but rather than leading directly to melting, weakening mechanisms are triggered that sufficiently limit strength, and hence continued heating, so as to make bulk melting of the fault zone rare, at least at shallow crustal depths. A relatively universal form of weakening is that groundwater thermally expands much more than its mineral host, causing the mineral constituents to push less strongly against one another, and hence to have low frictional strength. A variant of this process is that thermal decomposition of common fault constituents such as carbonates and hydrated clays occurs, at temperatures far below melting, and creates a highly pressurized volatile product phase (CO2 or H2O, respectively) which similarly reduces strength. Further weakening processes, of which the physical details are still unclear, relate to the nanometer size range of the solid decomposition and wear products. We will model how such weakening processes influence features of propagating earthquake ruptures (e.g., crack vs. slip pulse, rupture velocity, stress drop, total slip), how rupture relates to the fault mineralogy and depth, and how different dynamic weakening processes might be identified in seismic observations. Hypotheses to be tested are that thermal decomposition combined with variation in fault mineralogy could explain how rupture stops at the base of the seismogenic zone, and that thermal decomposition could provide a mechanism for occasional extreme earthquakes on faults that generally experience smaller events. We will model the material lying outside the narrow highly-deforming fault core as an elastic or an elastic-brittle-plastic solid, and use our analyses of the localized shearing processes within the deforming fault core as the basis for imposing boundary conditions along the fault surfaces in the larger analysis. The study should contribute towards a unified overall understanding of seismic processes. It will have inputs from fine scale materials physical/chemical theory, geologic fault core studies, rock mechanics lab friction experiments, spontaneous rupture simulations, seismic observations of the slip mode and extent of seismic ruptures, and large scale constraints, by heat flow, topography support and related studies, of the stress regimes under which major earthquakes occur.