Collaborative Research: Modeling fault ruptures along bends and stepovers

当断层破裂时就会发生地震，当破裂沿着断层带传播很长一段距离时就会产生大地震。大地震通常在几十到几百英里的断层上破裂，包括断层段之间的间隙和断层的弯曲。这些不规则的断层也可以阻止破裂的扩展。因此，它们在了解地震危险性评估方面发挥着重要作用。地震破裂是否能够通过给定的台阶或弯曲是非常重要的。先前的研究表明，超过5公里宽的台阶可以阻止破裂的传播，但最近的一些大地震已经跳过了更大的裂缝。例如，2016年新西兰7.8级地震涉及断层之间超过15公里宽的间隙的破裂。本项目将利用构造断层模型模拟初始应力场，特别是斜坡和弯曲附近的应力场，然后利用结果改进断层破裂模型。该项目的结果将提高我们对大地震及其危害的认识，特别是在断层系统复杂的大陆内。本研究将培养两名断层与地震多物理场新兴领域的研究生。本科生将参与毕业论文研究，研究成果，包括断层破裂动画，将被纳入德州农工大学和密苏里大学的本科课程，并免费提供给其他教育工作者。在这个项目中开发的计算机代码将与其他研究人员免费共享。该项目将使用数值模型来模拟沿台阶和弯曲的断层破裂和传播，这些断层的不规则性通常会阻止断层破裂，从而限制地震的大小。了解给定的台阶或弯曲是否能阻止断层破裂传播对于危险性评估至关重要，因为大地震，特别是大陆内走滑断层上的地震，通常会通过跳过台阶和沿着断层弯曲传播而破裂多个断层段。先前的数值模拟和一些现场观测表明，超过5公里宽的台阶可以阻止断层破裂；然而，在2016年新西兰凯库拉7.8级地震中，断层之间的断裂幅度超过了15公里。断层几何形状和初始应力是决定破裂行为的最重要因素之一，但在以前的动态破裂模型中，初始应力往往没有得到很好的约束，并被简化为均匀或非均匀。另一方面，众所周知，应力往往集中在台阶、弯曲和其他不规则断层周围。本项目将利用断层构造模型模拟台阶和弯道周围的区域静应力变化，以及由于以前的滑动事件而引起的准静应力变化。得到的应力场将用于动态破裂模型来模拟地震中断层破裂的自发传播。该研究将使用通用断层模型来探索控制台阶和弯曲断裂的关键参数，然后基于2016年凯库拉地震建立一个模型，以深入了解涉及多个断层的复杂断裂。该项目的结果将提高我们对大地震及其危害的理解，特别是在断层系统复杂且大事件经常涉及多个断层或分段破裂的大陆内。这项研究朝着建立一个完整的断层力学模型迈出了重要的一步，该模型可以模拟多个时间尺度上的应力演化和破裂行为。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。

Earthquakes occur when faults rupture, and large earthquakes are produced when ruptures propagate long distances along fault zones. Large earthquakes often rupture across tens to hundreds of miles of faults, including stepovers (gaps) between fault segments and bends in faults. These fault irregularities can also stop rupture propagation. So, they play an important role in understanding earthquake hazard assessment. Whether or not an earthquake rupture is able to pass through a given stepover or a bend is very important. Previous studies have suggested that stepovers more than about 5 km wide would stop rupture propagation, but some recent large earthquakes have jumped wider gaps. The 2016 magnitude 7.8 New Zealand earthquake, for example, involved rupture jumping over a gap more than 15 km wide between faults. This project will use tectonic fault models to simulate the initial stress field, especially around stepovers and bends, and then use the results to improve fault rupture models. Results of this project will improve our understanding of large earthquakes and their hazards, particularly within continents where fault systems are complex. This research will train two graduate students in the emerging field of multiphysics in faulting and earthquakes. Undergraduate students will be involved through senior thesis research, and research results, including animations of fault ruptures, will be incorporated into the undergraduate curriculum at Texas A&M and the University of Missouri, and be freely available to other educators. Computer codes developed in this project will be freely shared with other researchers.This project will use numerical models to simulate fault rupture and propagation along stepovers and bends, the fault irregularities that often stop fault rupture, hence limiting the size of earthquakes. Knowing whether or not a given stepover or bend can stop fault rupture propagation is critical for hazard assessment, because large earthquakes, especially those on intracontinental strike-slip faults, usually rupture multiple fault segments by jumping over stepovers and propagating along fault bends. Previous numerical modeling and some field observations have suggested that stepovers more than ~5 km wide would stop fault rupturing; however, ruptures in the 2016 Mw 7.8 Kaikoura earthquake in New Zealand jumped more than 15 km between faults. Fault geometry and initial stress are among the most important factors dictating rupture behavior, but initial stress is often poorly constrained and simplified as homogeneous or ad hoc heterogeneous in previous dynamic rupture models. On the other hand, it is well known that stress tends to concentrate around stepovers, bends, and other fault irregularities. This project will use fault tectonics models to simulate changes of regional static stress around stepovers and bends, and quasi-static stress changes due to previous slip events. The resulting stress fields will then be used in dynamic rupture models to simulate spontaneous propagation of fault ruptures during earthquakes. The research will use generic fault models to explore key parameters controlling rupture along stepovers and bends, and then a model based on the 2016 Kaikoura earthquake will be developed to gain insights into complex ruptures involving multiple faults. Results of this project will improve our understanding of large earthquakes and their hazards, particularly within continents where fault systems are complex and large events often involve rupture of multiple faults or segments. This research takes an important step toward a fully integrated model of fault mechanics that simulates stress evolution and rupture behaviors over multiple timescales.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.