Dynamic Fragmentation and Earthquake Energy Partitioning

像加州的圣安德烈亚斯断层这样的走滑断层由于反复发生的大地震对生命和财产构成了重大威胁。在地震中释放的能量中，只有不到20%的能量以弹性波的形式从震源辐射出去，从而引起地面震动。很大一部分能量以摩擦热的形式通过各种过程在断层周围散失。这些过程控制着地震破裂的传播方式。在每次地震中，活动断层周围的岩石由于单个矿物颗粒的破裂而破碎。它们形成了所谓的损伤区。在许多地震周期中，该区域的破坏会增加，导致地震波的方向和速度发生变化。了解这些过程对于评估整个孕震带的能量收支至关重要。评估地震灾害也至关重要。在这里，研究人员将基于实地的结果与建模相结合，以提高对地震活跃断层的理解。他们量化了在孕震带较深处引起破碎的过程。他们开发了波速模型，可以用来更好地预测地面震动的方向和强度。该跨学科项目支持一名博士后，以及缅因大学本科生和研究生的培训。开发的代码和分析协议通过公共门户网站公开提供。这些成果可以应用于材料科学与工程的地震学之外，特别是研究陶瓷和先进复合材料的脆性破碎。本项目的目的是：(1)表征两个深挖发震断裂/剪切带中破碎矿物的微裂缝密度和破碎粒度分布；(2)利用碎片大小分布估计碎片消耗的能量；(3)应用基于物理的破碎模型估算与破碎相关的应变速率；(4)更好地约束震源体的能量分配；(5)开发和推广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来测试有关源能量收支，破裂传播方式以及更深发震带中加载条件与岩石微观结构之间关系的假设。这项研究还可以评估脆性矿物的强烈破碎是否可以像在地表一样，在深部用作发震标志。所选择的方法应用的模型和概念主要源于物理和工程社区，并且主要构成了地球科学的新研究。在地球科学和材料工程领域，人们越来越认识到微压裂对宏观行为的重要性，并对其进行定量处理。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。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

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 energy released during an earthquake radiates away from the source as elastic waves that cause ground shaking. A large portion of the energy is dissipated around the fault as frictional heat and through a variety of processes. These processes control how the earthquake rupture propagates. During each earthquake, the rocks surrounding the active fault are fragmented by cracking of individual mineral grains. They form the so-called damage zone. Over many earthquake cycles, damage increases in this zone causing changes in seismic-wave direction and speed. Understanding these processes is critical to evaluate the energy budget through the entire seismogenic zone. It is also critical to assess seismic hazards. Here, the researchers combine field-based results and modeling to improve the understanding of seismically active faults. They quantify processes causing fragmentation in the deeper reaches of the seismogenic zone. They develop wave speed models that can be used to better predict ground shaking directions and intensities. The interdisciplinary project supports a postdoctoral associate, as well as the training of undergraduate and graduate students at University of Maine. The developed codes and analytical protocols are made openly available through public portals. These outcomes can be applied beyond Seismology in Material Sciences and Engineering, notably to investigate brittle fragmentation in ceramics and advanced composite materials. The objectives of this project are: (1) to characterize the microfracture density and fragment size distributions for fragmented minerals in two deeply exhumed seismogenic faults/shear zones; (2) to use the fragment size distributions to estimate the energy that was consumed by fragmentation; (3) to apply physics-based fragmentation modeling to estimate strain rates associated with the fragmentation; (4) to better constrain the energy partitioning in the earthquake source volume; (5) to 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 study also allows evaluating whether intense fragmentation of brittle minerals can be used as a seismogenic signature at depth, as it is at the surface. The chosen approach applies models and concepts that originate largely in the physics and engineering communities, and mostly constitutes novel research in the geosciences. 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. Outcomes of this project will provide a framework for future efforts in both fields, and for collaborations between them.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.