Collaborative Research: Evaluating fault creep in California using geodetic and seismic observations

加州在未来30年内发生6.7级或更大地震的可能性为99.7%。地震是一种断层滑动方式，导致地震波辐射到地球上。然而，在地震周期内，断层也可能经历无地震滑动(或蠕动)，即不辐射地震波。地震与非地震滑动的发生取决于断裂带的初始摩擦性质及其作为断层滑动速率的函数的变化。了解断层上蠕变速率变化的原因、时间、地点和方式，对于量化加州断层系统的地震潜力是至关重要的。断层蠕变的空间分布知识可以估计未来地震的位置和大小，而蠕变速率的时间变化可以用来确定断裂带的摩擦性质。在这个项目中，我们将从星载干涉合成孔径雷达和全球定位系统获得的地表变形测量与地震观测相结合，通过数值和分析模型来约束沿圣安德烈亚斯断层中部和南部的空间和时间变化的蠕变速率。我们将分析几颗雷达卫星从1992-2020年间获取的合成孔径雷达图像的大量数据集，以前所未有的分辨率和精度生成地表形变时间序列图。该项目的成果将用于调查活跃的地壳形变，为了解其根本机制和动力学提供新的见解，并使更好地认识和评估加州的地震灾害及其相关风险。特别是，我们将致力于回答重要的问题：圣安德烈亚斯断层附近发生了多少弹性应变与永久性应变？这个比例会随着断层的长度而变化吗？断层滑移率是如何随时间变化和演化的？短期大地测量与长期地质测量如何匹配？地震是如何引发的？断层几何、流变学和历史如何结合在一起来确定地震的传播、大小和位置？在大地震破裂的深度和条件下，断层上的摩擦力是多少？流体在无声滑移事件的发生中扮演了什么角色？该项目还将汇集来自美国和英国大学的年轻的早期职业科学家，其中包括一名女性，并将为他们提供部分支持。它还将为研究生提供宝贵的研究经验。该项目的成果将被纳入本科教学，包括物理地质学以及地壳形变和雷达遥感研究生课程，这两门课程都包括圣安德烈亚斯断层的大量案例。提高对时空变化的地表形变场的认识，以及与地震和无震断层滑动的联系，对于理解活动构造、断层形成和触发大地震的机制至关重要。圣安德烈亚斯断层的独特之处在于结合了丰富的历史数据集、最近部署的EarthScope仪器、故障复杂性和各种自然瞬变现象，使其成为研究断层过程的天然实验室。这个为期3年的研究项目是由来自3所大学的3名早期职业科学家合作完成的，目的是增进对加州地震断层形成过程和潜在机制的了解。这项研究的灵感来自于对圣安德烈亚斯断层中部和南部地震间蠕变速率变化的地震和大地测量观测。通过这项研究，将探索通过EarthScope提供的大量地震、大地测量和地质数据集的全部能力。将把先进的多时相干涉合成孔径雷达(InSAR)算法应用于若干雷达卫星(例如ERS1、2、Envisat、ALOS、TerraSAR-X、CosmoSkyMed和Sentinel-A，B)从1992-2020年期间获取的大量合成孔径雷达图像。与全球定位系统(GPS)观测相结合，这项工作以前所未有的分辨率和精度提供了地表形变时间序列的观测。时变运动学模型将被用来约束断层蠕变的时空分布，将InSAR、蠕变仪、GPS和重复地震结合在一起。以蠕变时间序列和实验室测量为基础的动力学模型允许将断层的瞬时和长期行为与其摩擦性质、有效正应力的演化和地壳岩性联系起来。最后，通过静应力传递来研究蠕变段的速率变化与相邻锁定段的大地震发生之间的联系。

There is a 99.7% chance a magnitude 6.7 earthquake or larger will strike California within the next 30 years. Earthquakes are a mode of fault slip that cause seismic waves to be radiated into the Earth. Within earthquake cycle, however, faults may also undergo aseismic slip (or creep), which radiates no seismic wave. The occurrence of seismic versus aseismic slip depends on the initial frictional properties of the fault zone and their variation as a function of fault slip rate. Understanding, why, when, where and how creep rate varies on a fault is essential for quantifying earthquake potential on California's fault systems. The knowledge of spatial distribution of fault creep allows estimating location and size of future earthquakes, while the temporal variation of creep rate can be used to determine frictional properties of the fault zone. In this project we integrate measurement of ground surface deformation obtained from space-borne Interferometric Synthetic Aperture Radar and Global Positioning System with seismic observations through numerical and analytical models to constrain spatially and temporally variable creep rates along the Central and Southern San Andreas Faults. We will analyze large data sets of Synthetic Aperture Radar images acquired by several radar satellites spanning period 1992 - 2020, to generate maps of surface deformation time series at unprecedented resolution and accuracy. The results from this project will be used to investigate active crustal deformation and provide new insight into the its underlying mechanisms and dynamics, and allows better recognition and assessment of earthquake hazard and its associated risk in California. In particular, we will work to answer important questions: How much elastic vs. permanent strain occurs adjacent to the San Andreas Fault? Does this proportion change along the length of the fault? How do fault slip rates change and evolve over time? How do short-term geodetic measurements match with long-term geological measurements? How do earthquakes initiate? How do fault geometry, rheology, and history combine to determine the propagation, size, and location of earthquakes? What is the friction on a fault at the depths and conditions at which big earthquakes rupture? What role do fluids play in the generation of silent slip events? This project will also bring together young early career scientists, including one female, from American and British universities and will provide them with partial support. It will also provide valuable research experience for a graduate student. The results from this project will be incorporated in undergraduate teaching, including Physical Geology as well as graduate courses Crustal Deformation and Radar Remote Sensing, which both include numerous case examples from the San Andreas Fault.An improved knowledge of the spatially and temporally variable surface deformation field and the link to seismic and aseismic slips on faults are critically important for understanding active tectonics, mechanics of faulting and triggering large earthquakes. Unique to the San Andreas Fault is the combination of rich historic data sets, the recent deployment of EarthScope instrumentation, fault complexities and variety of natural transient phenomena, making it a natural laboratory for studying faulting processes. This 3-year research project is a collaboration between 3 early career scientists from 3 universities to advance understanding of aseismic faulting processes and underlying mechanisms in California. The study is inspired by seismic and geodetic observations of interseismic creep rate variations along the Central and Southern San Andreas Fault. Through this study, the full capacity of vast seismic, geodetic and geologic data sets provided through EarthScope will be explored. An advanced multitemporal interferometric synthetic aperture radar (InSAR) algorithm will be applied to large data sets of SAR images acquired by several radar satellites (e.g., ERS1,2, Envisat, ALOS, TerraSAR-X, CosmoSkyMed and Sentinel-A,B) spanning the period 1992 - 2020. In combination with Global Positioning System (GPS) observations, this effort provides observations of surface deformation time series at unprecedented resolution and accuracy. Time-dependent kinematic models will be applied to constrain spatiotemporal distribution of fault creep, integrating InSAR, Creepmeter, GPS and repeating earthquakes. Dynamic models informed by creep time series and lab measurements allow linking fault transient and long term behaviors to its frictional properties, evolution of effective normal stress and crustal lithology. Lastly, the link between rate changes on creeping segments and occurrence of major earthquakes on the adjacent locked sections will be investigated through static stress transferring.