Collaborative Research: Reorganization of stresses beneath greater Tokyo after the 2011 Tohoku-Oki M9 earthquake

2011年3月11日，东北冲地震9级是日本1400多年历史记录中最大的地震。地震和由此引发的海啸对日本人民的影响是严重的，约有20 000人死亡和下落不明，经济损失估计达10万亿日元（1 200亿美元）。东北-隐岐断裂位于东京以北，导致日本国内对他们的首都城市有着强烈的社会兴趣。具体而言，东北冲地震是否可能加速下一次重大地震，更直接地影响东京首都圈。日本政府最近的研究估计，类似于1923年M7.9“大关东”地震的浅层巨型逆冲断层地震将导致约11，000人死亡和高达3万亿美元的经济损失。根据地震科学的最新进展，对东京地区地震灾害恶化的担忧是有道理的，这些进展表明：（a）在一个地区释放应力的地震将在邻近地区建立应力;（B）虽然地震期间应力变化迅速，但震后过程将导致大地震后数年至数十年的地壳应力重新分布。 本项目的主要目的是了解2011年东北冲断裂附近东京地区应力积累的持续时间演变及其对该地区活动断层的影响。该项目是与日本科学家合作进行的，他们寻求改善对东京地区目前和未来几十年地震危险的估计。该项目将为美国学生提供研究培训，以及国际研究经验，并寻求帮助我们的日本同事提高他们进行此类地震灾害研究的能力。本项目旨在了解东京地区在东北冲地震后数年和数十年内地壳应力的演变及其相关的地震灾害。应力变化的发生主要是由于震后的两个主要过程，即震后滑移和粘弹性松弛。前者与同震滑动区内和同震滑动区下方沿着北美/太平洋板块交界面的无震滑动有关，后者则与会聚板块下方热弱地幔的松弛有关。这两个过程都将导致应力随时间转移到孕震上地壳。我们的目标是了解这种应力转移过程以及它如何加载东京地区的活动断层。这将是通过开发一个观测约束有限元模型，可以准确地计算应力变化，由于后滑和粘弹性松弛。观测方面的制约因素将主要考虑描述该区域构造几何和弹性结构的地震学数据，以及能够确定该区域流变学（粘性强度）的大地测量方面的制约因素。最重要的是，该模型应使我们能够分离出的相对贡献的余震和粘性松弛地震后的大地测量数据，这不仅是需要一个精确的计算应力变化，但将提供宝贵的见解，这两个过程的性质，将有利于我们的俯冲带构造的一般理解。正是日本境内前所未有的震后GPS覆盖范围将使这些见解得以实现。

The March 11, 2011, Tohoku-Oki earthquake at magnitude 9 was the largest event in Japan's 1400+ years of recorded history. The impact of the earthquake and resulting tsunami on the people of Japan was severe, from the approximately 20,000 fatalities and unaccounted persons to the estimated ¥10 trillion (US$ 120 billion) of economic loss. The Tohoku-Oki rupture was located north of Tokyo, leading to a strong societal interest within Japan as to what is in store for their capital city. Specifically, whether the Tohoku-Oki earthquake may have accelerated the next significant earthquake to more directly impact the Tokyo metropolitan region. Recent Japan government studies estimate that a shallow megathrust earthquake similar to the 1923 M7.9 "Great Kanto" earthquake would result in ~11,000 fatalities and up to US$ 3 trillion in economic damage. Concern for a worsening of seismic hazards in Tokyo region is warranted based on recent advances in earthquake science that indicate (a) that an earthquake that relieves stress in one area will build up stress in adjacent areas, and (b) while stress changes occur rapidly during the earthquake, postseismic processes will lead to the redistribution of crustal stresses for years to many decades after a large earthquake. Understanding the ongoing time-evolution of stress buildup in the Tokyo region adjacent to the 2011 Tohoku-Oki rupture, and its influence on active faults in that region is the primary objective of this NSF-supported project. This project is a collaboration with Japanese scientists who seek improved estimates of seismic hazards in the Tokyo region both currently and for decades to come. This project will provide research training to U.S. students, as well as international research experience, and also seeks to help our Japanese colleagues improve their ability to conduct this type of seismic hazard research. This project seeks to understand the evolution of crustal stresses and associated seismic hazards in the Tokyo region in the years and decades to following the Tohoku-Oki earthquake. Stress changes will occur due to two primary postseismic processes, afterslip and viscoelastic relaxation. The former is associated with aseismic slip along the North America/Pacific plate interface within and below the region of coseismic slip, while the latter involves the relaxation of hot weak mantle beneath the converging plates. Both processes will cause a time-dependent transfer of stress to the seismogenic upper crust. It is our goal to understand this process of stress transfer and how it works to load active faults in the Tokyo region. This will be achieved by developing an observationally constrained finite element model that can accurately calculate stress changes due to afterslip and viscoelastic relaxation. Observational constraints will primarily consider seismological data that describes the tectonic geometry and elastic structure of the region, and geodetic constraints that will enable a determination of the rheology (viscous strength) of the region. Most importantly, the model should enable us to separate out the relative contributions of afterslip and viscous relaxation to postseismic geodetic data, which is not only required for an accurate calculation of stress changes, but will provide invaluable insights as to the nature of these two processes that will benefit our general understanding of subduction zone tectonics. It is the unprecedented postseismic GPS coverage within Japan that will enable these insights to be achieved.