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

合作研究：利用 GPS、地震活动和基于物理的模型探索东北巨型逆冲断层的摩擦行为

• 批准号: 1620496
• 负责人: Paul Segall
• 金额: $ 39.51万
• 依托单位: Stanford University
• 依托单位国家: 美国
• 项目类别: Continuing Grant
• 财政年份: 2016
• 资助国家: 美国
• 起止时间: 2016-08-01 至 2020-07-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1620496&HistoricalAwards=false
• 关键词: Collaborative; Research; Probing; frictional; behavior

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.

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，那里的海洋板块潜入地幔。 2011 年日本北部发生的毁灭性 9 级地震就是一个著名的例子，也是迄今为止历史上仪器仪表最齐全的大地震。 这次地震的数据提供了一个独特的机会来了解控制俯冲带地震频率和规模的因素。 2011年之前，人们知道日本北部俯冲带既表现出缓和应力的慢滑动（无地震的断层蠕变），也表现出地震时的快速滑动。 人们错误地认为蠕变带会限制大地震中可能发生滑动的区域，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同时表现出蠕变和地震。 这个新项目对控制断层蠕变发生方式和时间的机械特性进行了严格的基于物理学的理解。 模型将根据精确的 GPS 测量结果进行测试，这些测量结果可以量化应力增加的速度，以及跟踪断层蠕变的小地震。 之前对日本记录的 GPS 数据的分析表明，断层蠕变在震级为 9 级之前的几十年中加速；值得注意的是，来自小型重复地震的独立数据证实了这一点。 我们将检验以下假设：随着板边界应力的增加，蠕变区域会随着时间的推移而扩大。 P.I.西格尔将参加针对初中和高中科学教师的 Geoscape 湾区专业发展计划。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。我们关于加速震间蠕变以及后滑重叠历史破裂的假设是，虽然地震在速度减弱区域成核，但由于强烈的动力减弱，它们破裂成速度加强区域。在地震间期，蠕变渗透到速度加强区，随着时间的推移侵蚀锁定的粗糙体。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。与时间相关的粗糙侵蚀似乎是解释长期应变瞬变的一种有前途的机制。具有仅限于地震发生深度的锁定粗糙体的弹性模型无法解释上个世纪记录的海岸沉降；这些数据需要深度耦合（60？100公里）或地幔松弛。 Mw 9 之后的 GPS 和海底大地测量需要将后滑和深部地幔流结合起来。东北海岸经历了全新世隆升，但目前尚不清楚仅震后过程是否能够恢复观测到的震间和同震沉降。我们将使用耦合粘弹性和基于物理的后滑模型来检查地幔流和界面耦合对震间变形的相对贡献，并进一步检验侵蚀粗糙假设。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，并提供这种独特板块边界的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，并利用这些结果来限制板块边界上的震间滑移率。学生将参加有关准备 GRE、申请研究生院以及了解地球科学职业的研讨会。