Dynamic fragmentation and inelastic energy partitioning at the base of the seismogenic zone

由于大地震的反复发生，像加州的圣安德烈亚斯断层这样的走滑断层对生命和财产构成了重大威胁。地震释放的总能量中，只有不到20%的能量以地震波的形式从震源辐射出去，引起地面震动。因此，很大一部分能量以摩擦热的形式在断层周围消散，并通过导致岩石破坏的各种过程消散。在每次地震中，断层破裂段周围的岩石因个别矿物颗粒的破裂而破碎。在许多地震周期中，这些岩石变得如此受损，以至于它们的弹性特性发生变化，这导致地震波的速度和首选传播方向发生变化。因此，这些所谓的破坏区对于了解地震灾害非常重要。尽管它们很重要，但几乎没有关于像圣安德烈亚斯这样的活动断层内破坏区垂直范围的直接信息，但这些信息对于我们了解整个孕震区的能量收支至关重要。这项工作的建模和实地结果相结合，将有助于更好地描述孕震区较深区域的地震活动断层，包括可用于更好地预测地面震动方向和强度的概念和波速模型。该项目开发的开源代码和分析协议将通过现有的公共门户网站提供，并将有许多社区应用，包括微裂纹方向和应力状态之间的关系，地震数据的解释，以及陶瓷和先进复合材料的脆性损伤和破碎分析。其他期望的社会成果包括妇女充分参与STEM，并通过研究生和本科生培训培养具有全球竞争力的STEM劳动力。（1）综合表征从孕震带底部附近挖出的横跨孕震断层/剪切带的沿着三条断面的石榴石和长石颗粒的微裂缝密度和碎片尺寸分布（缅因州诺伦贝加断层系统），（2）应用基于物理学的碎裂模拟来估计与石榴石碎裂相关联的碎裂能量，（3）确定碎片尺寸分布分析是否可用于指示出露出来的断层或剪切带具有孕震历史，（4）追求结果对于震源中的非弹性能量分配的含义，（5）记录剪切带核心周围的不对称破坏，定量描述岩石的弹性和地震特性，以描述双材料效应，以及（6）开发和传播使用电子背散射衍射技术分析矿物和岩石破碎的协议。解决上述目标提供了一个机会，以测试有关的源能量预算，破裂传播的风格，以及加载条件和由此产生的岩石微观结构在更深的孕震区之间的关系的假设。研究小组还将评估脆性矿物的强烈碎裂是否可以像在地表那样用作深度的地震信号。在地球科学和材料工程中，人们越来越认识到微破裂对宏观行为的重要性，并越来越有能力对其进行定量处理。该项目的成果将为这两个领域的未来努力以及它们之间的合作提供一个框架。

Strike-slip faults like the San Andreas Fault in California represent major threats to life and property owing to the repeated generation of large earthquakes. Less than 20% of the total energy released during an earthquake radiates away from the source as seismic waves that cause ground shaking. Thus, a large portion of the energy is dissipated around the fault as frictional heat and by a variety of processes that lead to rock damage. During each earthquake, the rocks surrounding the ruptured segment of the fault are fragmented by cracking of individual mineral grains. Over many earthquake cycles, these rocks become so damaged that their elastic properties change, and this causes changes in the speed and preferred propagation direction of seismic waves. These so-called damage zones are therefore of great importance for understanding seismic hazards. Despite their importance, there is almost no direct information on the vertical extent of damage zones within active faults like the San Andreas, but this information is critical for our understanding of the energy budget through the entire seismogenic zone. The combination of modeling and field-based results from this work will allow improved characterization of seismically active faults in the deeper reaches of the seismogenic zone, including conceptual and wave speed models that can be used to better predict ground shaking directions and intensities. The open-source codes and analytical protocols developed in this project will be made available through existing public portals, and will have many community applications including the relationships between microcrack orientation and state of stress, interpretation of seismological data, and analysis of brittle damage and fragmentation of ceramics and advanced composite materials. Additional desired societal outcomes include full participation of women in STEM and development of a globally competitive STEM workforce through graduate and undergraduate student training.The objectives of this project are to: (1) comprehensively characterize the microfracture density and fragment size distributions for garnet and feldspar grains along three transects across a seismogenic fault/shear zone exhumed from near the base of the seismogenic zone (the Norumbega fault system, Maine), (2) apply physics-based fragmentation modeling to estimate fracture energies associated with the garnet fragmentation, (3) determine whether fragment size distribution analysis can be used to indicate that an exhumed fault or shear zone had a seismogenic history, (4) pursue the implications of the results for inelastic energy partitioning in the seismic source, (5) document asymmetrical damage around the shear zone core and quantitatively characterize the elastic and seismic properties of the rocks to characterize the bi-material effect, and (6) develop and disseminate protocols for using electron backscatter diffraction techniques to analyze mineral and rock fragmentation. Addressing the above objectives provides an opportunity to test hypotheses related to the source energy budget, styles of rupture propagation, and the relations between loading conditions and resulting rock microstructures in the deeper seismogenic zone. The research team will also assess whether intense fragmentation of brittle minerals can be used as a seismogenic signature at depth, as it is at the surface. Recognition of the importance of microfracturing for macroscopic behavior and the ability to treat it quantitatively is growing in both Earth sciences and materials engineering. The results of this project will provide a framework for future efforts in both fields, and for collaborations between them.