Collaborative Research: Probing the frictional behavior of the Tohoku megathrust using GPS, seismicity, and physics-based models

世界上最大的地震发生在俯冲带，比如美国西北部的卡斯卡迪亚俯冲带，在那里海洋板块潜入地幔。2011年发生在日本北部的9级大地震就是一个显著的例子，也是迄今为止历史上仪器测量最好的大地震。这次地震的数据提供了一个独特的机会来了解是什么控制了俯冲带地震的频率和规模。在2011年之前，人们知道日本北部的俯冲带既表现为缓慢滑动（无地震的断层蠕变），使应力松弛，也表现为地震时的快速滑动。人们曾经错误地认为，蠕动带会限制大地震发生时可能滑动的区域，从而使9级地震不可能发生。研究人员过去的工作表明，与预期相反，断层的同一部分既可以表现出蠕变，也可以表现出地震。这个新项目对控制断层蠕变发生的方式和时间的机械特性进行了严格的物理理解。模型将通过精确的GPS测量进行测试，GPS测量可以量化应力积累的速度，还可以追踪断层蠕变的小地震。先前对日本GPS记录数据的分析表明，断层蠕变在9级地震前的几十年里加速了；值得注意的是，小型重复地震的独立数据证实了这一点。我们将检验这个假设，即爬行区域随着时间的推移而扩大，因为应力在板块边界上积聚。P.I.西格尔将参加为初中和高中科学教师举办的地理景观湾区专业发展项目。他将开发一个模块，将日本东北地震的科学知识转化为美国的地震灾害，这与喀斯喀特俯冲带特别相关。该项目探索了解释日本东北部独特的震间和震后观测的模型，这些观测受到2011年Mw 9地震前后重复地震和GPS数据的限制。我们对加速震间蠕变和余震重叠历史破裂的假设是，地震在速度减弱区成核，但由于强烈的动力减弱，它们破裂成速度增强区。在震间期，蠕变渗透到速度强化带，随着时间的推移侵蚀锁定的岩石。模拟包括速率状态摩擦和热加压，以及针对GPS和重复地震数据的测试预测。时间相关的磨粒侵蚀似乎是解释长时间应变瞬变的一个有希望的机制。局限于发震深度的锁定岩石的弹性模型不能解释上个世纪记录的沿海沉降；这些数据需要深度耦合(60？100公里)或地幔弛豫。mw9之后的GPS和海底大地测量需要余震和深部地幔流动的某种组合。东北海岸经历了全新世的隆升，但目前尚不清楚震后过程是否会恢复观测到的地震间和同震沉降。我们将使用耦合粘弹性和基于物理的余震模型来检验地幔流动和界面耦合对震间变形的相对贡献，并进一步验证侵蚀粗糙度假设。这些基于物理的断层摩擦和地震旋回变形模型能够整合不同的数据集，并提供这一独特板块边界力学行为的综合模型。该项目指导一名SURGE（地球科学与工程暑期本科生研究）学生分析来自东北的GPS垂直时间序列，并利用这些结果来约束板块边界的地震间滑动率。学生将参加准备GRE考试、申请研究生院和了解地球科学职业的研讨会。

The largest earthquakes in the world occur in subduction zones, such as the Cascadia Subduction Zone in the northwestern US, where oceanic plates dive into Earth's mantle. The devastating magnitude 9, 2011 Tohoku-oki earthquake in northern Japan is a notable example, and by far the best instrumented giant earthquake in history. The data from this quake presents a unique opportunity to understand what controls the frequency and size of subduction zone earthquakes. Prior to 2011 it was known that the northern Japan subduction zone exhibited both slow sliding (fault creep without earthquakes) which relaxes stress, as well as rapid slip in earthquakes. It had been, erroneously, thought that the creeping zones would limit the area that could slip in large earthquakes such that magnitude 9 events were not possible. The researchers past work has shown that, contrary to expectation, the same part of the fault can exhibit both creep and earthquakes. This new project develops a rigorous physics-based understanding of the mechanical properties that control how and when fault creep occurs. Models will be tested against precise GPS measurements, which quantify how rapidly stress builds up, as well as small earthquakes which track fault creep. Previous analysis of GPS data recorded in Japan showed that fault creep accelerated in the decades leading up to the magnitude 9; remarkably this was confirmed by independent data from small repeating earthquakes. We will test the hypothesis that creeping areas expand with time as stress builds up on the plate boundary. P.I. Segall will participate in the Geoscape Bay Area Professional Development program for middle and high school science teachers. He will develop a module to translate science from the Tohoku earthquake in Japan to earthquake hazards in the U.S., which has particular relevance to the Cascade subduction zone. This project explores models to explain unique interseismic and post-seismic observations from northeast Japan, constrained by repeating earthquakes and GPS data, both before and after the 2011 Mw 9 earthquake. Our hypothesis for accelerating interseismic creep as well as afterslip overlapping historical ruptures is that while earthquakes nucleate in velocity weakening regions, they rupture into velocity strengthening regions due to strong dynamic weakening. In the interseismic period, creep penetrates into velocity strengthening zones, eroding locked asperities with time. The simulations include rate-state friction and thermal pressurization, and test predictions against GPS and repeating-earthquake data. Time dependent asperity erosion appears to be a promising mechanism to explain the long-duration strain transient. Elastic models with locked asperities restricted to seismogenic depths cannot explain coastal subsidence documented over the last century; these data require either deep coupling (60? 100 km) or mantle relaxation. GPS and seafloor geodetic measurements, post-Mw 9, require some combination of afterslip and deep mantle flow. The Tohoku coast underwent Holocene uplift, yet at present it is unknown whether postseismic processes alone will even recover the observed inter- and co-seismic subsidence. We will use coupled viscoelastic and physics-based afterslip models to examine relative contributions of mantle flow and interface coupling to interseismic deformation and further test the eroding asperity hypothesis. These physics-based models of fault friction and earthquake-cycle deformation are able to integrate diverse datasets and provide a comprehensive model of the mechanical behavior of this unique plate boundary. This project mentors a SURGE (Summer Undergraduate Research in Geoscience and Engineering) student to analyze GPS vertical time series from Tohoku and use these results to constrain interseismic slip rate on the plate boundary. The student will participate in workshops on preparing for the GRE, applying to graduate school, and understanding geoscience careers.