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

快速剪切断层的材料物理及其对地震破裂动力学的影响

• 批准号: 1315447
• 负责人: James Rice
• 金额: $ 40万
• 依托单位: Harvard University
• 依托单位国家: 美国
• 项目类别: Continuing Grant
• 财政年份: 2013
• 资助国家: 美国
• 起止时间: 2013-07-01 至 2019-06-30
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1315447&HistoricalAwards=false
• 关键词: Materials; physics; rapidly; sheared; faults

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.

在剪切应力的总体水平上发生了良好且高度滑动的断层区域的地震，该地震显着低于“静态摩擦”应力水平，以启动在断层壁之间启动缓慢的摩擦滑动所需的压力水平。 如果这些静态摩擦在地震滑动过程中占据了压力，它们将沿断层产生可感知的局部热流出，并留下大量融化和重新固定的迹象，即使在浅层地壳深度也是如此。通常都没有找到。 同样，最近的田间和实验室观察结果表明，快速剪切过程中的大多数变形通常位于沿断层的明显薄的主剪切区域，通常小于毫米至毫米宽，在宽度上形成，例如在更广泛的（例如，一个到一百米）宽，一个宽度为一百米的岩石区域。 我们在计划的研究中的目的是了解负责断层区响应特征的材料和热物理，并为沿主要地震中断层散开的方式建立一些后果。希望对地震物理学的这种基本理解最终可以在改善地震现象和影响的可预测性方面带来回报。我们已经开发了这样一个概念，即剪切过程中地下水饱和断层的热加热会导致应变的强烈定位到现实的狭窄区域中。 这将集中在进一步的加热和温度上升，但触发的机制却没有直接导致融化，而是触发了足够限制强度的弱化机制，因此持续加热，以使断层区域罕见，至少在浅层地壳深度下进行大量熔化。 弱化的一种相对普遍的形式是，地下水的热膨胀远比其矿物宿主多得多，导致矿物成分彼此之间的强烈推动力降低，因此摩擦强度较低。 该过程的一个变体是，在较低的熔化的温度下发生了常见断层成分（例如碳酸盐和水合粘土）的热分解，并产生高度加压的挥发产物阶段（CO2或H2O），从而降低了强度。 进一步削弱了过程，其物理细节仍不清楚，这与实心分解和磨损产品的纳米尺寸范围有关。 我们将建模这种弱化过程如何影响传播地震破裂的特征（例如，裂纹与滑移脉冲，破裂速度，压力下降，完全滑移），破裂如何与断层矿物学和深度有关，以及如何在地震观察中鉴定出不同的动态弱过程。要测试的假设是，热分解与断层矿物学的变化相结合可以解释地震生成区底部的破裂停止，并且热分解可以为通常经历较小事件的断层上偶尔发生极端地震提供机制。 我们将在狭窄的高度变形断层核心外面的材料中建模为弹性或弹性塑料固体，并使用对变形断层核心内的局部剪切过程的分析，作为在较大分析中沿断层表面施加边界条件的基础。该研究应有助于对地震过程的整体理解。 它将提供精细材料物理/化学理论，地质断层核心研究，岩石力学实验室摩擦实验，自发性破裂模拟，对地震破裂的滑移模式和范围的地震观察以及通过热流量和相关研究的大规模限制，在发生主要地震下发生的压力状态。