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

先前存在和演变的弱点对东加州剪切带走滑限制弯曲运动学演化的影响

基本信息

批准号：
1802026
负责人：
James Spotila
金额：
$ 29.79万
依托单位：
Virginia Polytechnic Institute and State University
依托单位国家：
美国
项目类别：
Standard Grant
财政年份：
2018
资助国家：
美国
起止时间：
2018-08-01 至 2023-07-31
项目状态：
已结题

来源：
https://www.nsf.gov/awardsearch/showAward?AWD_ID=1802026&HistoricalAwards=false
关键词：
Effects preexisting evolving weaknesses kinematic

项目摘要

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.

该项目通过在南加州进行的重点地质研究，加深了对断层之间的屏障如何影响美国大地震的空间和时间发生的了解。太平洋和北美板块之间的断层边界直接穿过加利福尼亚州，通过地震、地面破裂和引发的山体滑坡对其稠密的人口和经济基础设施造成极大的危害。然而，该边界并不由单个断层组成。在加利福尼亚州南部，大约一半的断层活动沿着圣安德烈亚斯断层发生，而其余的则分布在更广泛的断层系统中。大约 25% 的活动发生在 125 公里宽的东加州剪切带沿线，该地区最近发生了破坏性（7 级）地震，并且似乎正处于活动加剧阶段。这个剪切带是神秘的，因为地震是通过多个断层上的同时或顺序运动产生的，这些断层跨越了原本被认为是屏障的间隙。该区域的运动速率测量结果也存在差异，因为使用天基技术进行的数十年测量比基于地质偏移量的长期测量快 2 至 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，包括主要国防设施（例如海军陆战队空地作战中心 - MCAGCC），以及未来的破裂可能如何与其余地区相互作用，甚至触发事件板块边界（例如洛杉矶附近）尚不清楚。该项目通过记录复杂断层连接的演变并量化莫哈韦沙漠南部主要断层之间发生的内部构造变形的程度来增进理解。这项工作涉及使用高分辨率地形进行尖端测绘，使用各种地质计时器对测量的偏移量进行测年，以及测试地壳中预先存在的弱点如何影响神秘的断层模式。最终，该项目将重建断层如何演化和相互作用，从而改进对南加州大地震的预测，并改进框架以了解断层屏障如何控制地震规模，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宣传提高公众科学素养并加强 STEM（科学、技术、工程和数学）劳动力队伍，从而造福社会。该项目正在测试地壳强度在地壳起源和演化背景下的概念处理。东加州剪切带的新生走滑断层。一种概念观点认为强度是一个不断变化的量，随着断层平滑，强度可能会随着位移的累积而减小，或者随着分布变形导致几何复杂性的增加而自生增加。另一种观点考虑地壳各向异性（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。东加州剪切带的几个特征可能与初始或演变的强度有关，例如断层趋势的变化、断层段之间普遍存在突出的自相似约束弯曲以及复杂的变形分区，这些特征都不能与总断层滑移、年龄、方向或空间位置。该项目正在测试强度在断层发育中的作用，通过记录十二个关键挤压带（例如值得注意但以前未研究的卡利科-伊达尔戈断层阶跃和相关边界）的断层和褶皱（即整合或复杂化）的演变并比较活动变形通过基岩测绘了解地壳流变变化的模式。断层弯曲的运动学解释基于高分辨率地形和新构造测绘、结构分析、地球物理地下成像、断层带之间的穿透应变记录以及使用综合第四纪测年的变形年代。该项目直接有助于了解压裂、机械各向异性对大陆变形的影响、走滑断层的分段和整合（包括“地震门”，可能构成破裂障碍并影响地震灾害）、大地测量学和地质学确定的滑移率、当地地震灾害和加州构造之间的差异。该奖项反映了 NSF 的法定使命，并通过使用基金会的智力价值和更广泛的影响进行评估，被认为值得支持审查标准。