Effects of preexisting and evolving weaknesses on the kinematic evolution of strike-slip restraining bends in the Eastern California shear zone

该项目通过在南加州进行的重点地质研究，促进了对断层之间的屏障如何影响美国大地震的时空发生的理解。太平洋和北美构造板块之间的断层边界直接穿过加利福尼亚州，并通过地震震动、地面破裂和引发山体滑坡对其密集的人口和经济基础设施构成极大威胁。然而，这个边界不是由单个断层组成的。在南加州，大约一半的断层活动发生在圣安德烈亚斯断层沿线，而其余的则分布在更广泛的断层系统中。大约25%的地震活动发生在125公里宽的东加利福尼亚剪切带，该剪切带最近发生了破坏性的（7级）地震，并且似乎处于活动的加剧阶段。这个剪切带是神秘的，因为地震是通过多个断层同时或连续的运动产生的，这些断层切断了原本被认为是障碍的间隙。该区域在测量的运动速率上也存在差异，因为使用空间技术进行的跨越几十年的测量比基于地质偏移的长期测量快2到3倍。这种差异可能是由于测量误差、未记录的变形或运动速率随时间的变化造成的。由于这些问题，该地区未来的地震风险，包括主要的国防设施（例如海军陆战队空中地面作战中心- MCAGCC），以及未来的破裂如何与板块边界的其余部分（例如洛杉矶附近）相互作用甚至触发事件，都没有得到很好的理解。该项目通过记录复杂断层连接的演变和量化发生在莫哈韦沙漠南部主要断层之间的内部构造变形的程度，促进了人们的理解。这项工作包括使用高分辨率地形图进行尖端测绘，使用各种地质计时器测量偏移量的年代，以及测试地壳中先前存在的弱点如何影响神秘的断层模式。最终，该项目将重建断层是如何演变和相互作用的，从而改进对南加州大地震的预测，并改进了解断层屏障如何控制地震大小的框架，该框架可应用于整个美国。该项目还将通过基于互联网的视频推广提高公众科学素养，并加强STEM（科学、技术、工程和数学）劳动力，从而造福社会。该项目是在东加利福尼亚剪切带新生走滑断层的起源和演化背景下测试地壳强度的概念处理。一种观点认为强度是一个不断变化的量，随着断层的平滑，强度可能随着位移的累积而降低，或者随着分布变形的几何复杂性的增加而自生增加。另一种观点考虑了地壳各向异性（即预先存在的结构弱点）如何影响断层阵列的初始几何形状和运动学。东加利福尼亚剪切带的几个特征可能与初始或演化强度有关，如断层走向的可变性，断层段之间明显的、自相似的约束弯曲的普遍性，以及复杂的变形分区，这些都不能与断层的总滑动、年龄、方向或空间位置明确相关。该项目通过记录12个关键的逆压带（例如，引人注意但未被研究的Calico-Hidalgo断层台阶和相关的边界地带）的断层和褶皱的演变，并通过基岩测绘将活动变形与地壳流变变化模式进行比较，从而测试强度在断层发育中的作用。断层弯曲的运动学解释是基于高分辨率地形和新构造测绘、构造分析、地球物理地下成像、断层链之间渗透应变的记录以及使用综合第四纪测年的变形年表。该项目直接有助于理解逆压作用、机械各向异性对大陆变形的影响、走滑断层的分割和整合（包括“地震门”，它可能构成破裂障碍并影响地震灾害）、大地测量和地质确定的滑动率之间差异的原因、当地地震灾害和加利福尼亚构造。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。

This project is advancing understanding of how barriers between faults influence the spatial and temporal occurrence of major earthquakes in the United States via a focused geologic study in southern California. The fault boundary between the Pacific and North American tectonic plates cuts directly through California and poses extreme hazard to its dense population and economic infrastructure via earthquake shaking, ground rupture, and triggered landslides. This boundary does not consist of a single fault, however. In southern California, roughly half of the fault activity occurs along the San Andreas fault, whereas the remainder is distributed across a broader fault system. About 25% of the activity occurs along the 125-kilometer-wide Eastern California Shear Zone, which has produced recent damaging (magnitude7) earthquakes and appears to be in an intensified phase of activity. This shear zone is enigmatic, in that earthquakes are produced via simultaneous or sequential motion on multiple faults that cut across gaps which would have otherwise been expected to be barriers. The zone also exhibits discrepancy in measured rates of motion, as measurements spanning decades using space-based techniques are 2 to 3 times faster than long-term measurements based on geologic offsets. This discrepancy may result from measurement errors, undocumented deformation, or changes in motion rate through time. As a result of these problems, the future earthquake risk of the region, which includes major national defense installations (e.g. Marine Corps Air Ground Combat Center - MCAGCC), as well as how future ruptures may interact with or even trigger events along the remainder of the plate boundary (e.g. near Los Angeles), are not well understood. This project is advancing understanding by documenting the evolution of complex fault connections and quantifying the magnitude of internal tectonic deformation that occurs in-between the major faults of the southern Mojave Desert. This effort involves cutting-edge mapping using high-resolution topography, dating of measured offsets using various geochronometers, and testing of how preexisting weaknesses in the crust may be influencing enigmatic patterns of faulting. Ultimately the project will reconstruct of how faults have evolved and interacted, leading to improved forecasting of what to expect from major earthquakes in southern California as well as an improved framework for understanding how fault barriers control earthquake size that can be applied to the entire United States. This project will also benefit society by improving public scientific literacy via internet-based video outreach and strengthening the STEM (science, technology, engineering and mathematics) workforce.This project is testing conceptual treatments of crustal strength in the context of the origin and evolution of nascent strike-slip faults of the Eastern California Shear Zone. One conceptual view considers strength as an evolving quantity, which may decrease with accumulating displacement as faults smoothen or increase autogenically as geometric complexity grows due to distributed deformation. Another perspective considers how crustal anisotropy (i.e. preexisting structural weaknesses) influences the initial geometry and kinematics of fault arrays. Several characteristics of the Eastern California Shear Zone may relate to initial or evolving strength, such as variability in fault trend, a prevalence of prominent, self-similar restraining bends between fault segments, and complex deformation partitioning, none of which can be clearly related to total fault slip, age, orientation, or spatial position. This project is testing the role of strength in fault development by documenting the evolution of faulting and folding (i.e. integrating or complexifying) through twelve key transpressive zones (e.g. the remarkable but previously unstudied Calico-Hidalgo fault stepover and associated borderlands) and comparing active deformation to patterns of crustal rheological variations via bedrock mapping. Kinematic interpretation of fault bends is based on high-resolution topography and neotectonic mapping, structural analysis, geophysical subsurface imaging, documentation of penetrative strain between fault strands, and chronologies of deformation using comprehensive Quaternary dating. This project is directly contributing to understanding of transpression, the influence of mechanical anisotropy on continental deformation, segmentation and integration of strike-slip faults (including "earthquake gates," which may present barriers to rupture and influence seismic hazards), the cause of the discrepancy between geodetic and geologically determined slip rates, local earthquake hazards, and California tectonics.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.