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，例如美国西北部的卡斯卡迪亚俯冲带，海洋板块潜入地幔。 2011年发生在日本北方的东北冲9级大地震就是一个明显的例子，也是迄今为止历史上仪器监测最好的大地震。 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，人们知道北方日本俯冲带既表现出缓慢滑动（没有地震的断层蠕动），也表现出地震时的快速滑动。 人们曾错误地认为，蠕动带将限制在大地震中可能滑动的区域，从而不可能发生9级地震。研究人员过去的工作表明，与预期相反，断层的同一部分可以表现出蠕变和地震。 这个新项目开发了一个严格的物理为基础的机械性能，控制如何以及何时发生故障蠕变的理解。 模型将根据精确的GPS测量结果进行测试，这些测量结果量化了应力建立的速度，以及跟踪断层蠕动的小地震。 之前对日本GPS数据的分析表明，在9级地震之前的几十年里，断层蠕动加速;值得注意的是，这一点得到了小型重复地震的独立数据的证实。 我们将检验蠕变区随着应力在板块边界上的积累而随时间扩展的假设。P.I.西格尔将参加Geoscape湾区的初中和高中科学教师专业发展计划。他将开发一个模块，将日本东北地震的科学转化为美国的地震灾害，这与喀斯喀特俯冲带有着特殊的关系。该项目探索模型，以解释日本东北部独特的震间和震后观测，受到重复地震和GPS数据的限制，在2011年9级地震之前和之后。我们的假设加速地震间蠕变以及后滑重叠的历史破裂是，而地震成核的速度减弱区，他们破裂到速度加强区由于强大的动力减弱。在震间阶段，蠕变渗透到速度强化区，随时间侵蚀锁定的粗糙体。模拟包括速率状态摩擦和热加压，以及对GPS和重复地震数据的测试预测。随时间变化的微凸体侵蚀似乎是一个很有前途的机制来解释长时间的应变瞬态。弹性模型锁定凹凸局限于孕震深度不能解释海岸沉降记录在过去的世纪，这些数据需要深耦合（60？100公里）或地幔松弛。全球定位系统和海底大地测量，后Mw 9，需要一些组合的后滑和深地幔流。东北海岸经历了全新世隆起，但目前尚不清楚是否震后过程本身甚至会恢复所观察到的地震间和同震沉降。我们将使用耦合的粘弹性和物理为基础的后滑模型来研究地幔流和界面耦合对震间变形的相对贡献，并进一步测试侵蚀粗糙假设。这些基于物理的断层摩擦和地震周期变形模型能够整合不同的数据集，并提供这个独特板块边界力学行为的综合模型。该项目指导一名SURGE（夏季本科生地球科学与工程研究）学生分析来自东北的GPS垂直时间序列，并使用这些结果来限制板块边界上的地震间滑动率。学生将参加关于准备GRE，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.