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

2011年3月11日，日本东北冲县发生的里氏9级地震是日本1400多年有记录以来最大的地震。地震及其引发的海啸对日本人民的影响是严重的，从大约2万人死亡和下落不明到估计10万亿日元（1200亿美元）的经济损失。东北-冲绳县的断裂位于东京以北，这引发了日本社会对首都即将发生什么的强烈关注。具体来说，东北大地震是否会加速下一个更直接影响东京大都市区的大地震。日本政府最近的研究估计，类似于1923年7.9级“关东大地震”的浅层大逆冲地震将导致约11,000人死亡，经济损失高达3万亿美元。对东京地区地震灾害恶化的担忧是有理由的，因为地震科学的最新进展表明：(a)在一个地区减轻应力的地震将在邻近地区增加应力，(b)虽然地震期间应力变化迅速，但地震后的过程将导致地壳应力在大地震后数年至数十年的重新分布。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，以及它对该地区活动断层的影响是这个国家科学基金会支持的项目的主要目标。该项目是与日本科学家的合作，他们寻求改进对东京地区目前和未来几十年地震危害的估计。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这类地震危害研究的能力。该项目旨在了解东京地区在东北大地震之后的几年和几十年里地壳应力的演变和相关的地震灾害。应力变化是由震后两个主要过程引起的，即余震和粘弹性松弛。前者与沿北美/太平洋板块界面在同震滑动区域内及其下方的地震滑动有关，而后者涉及汇聚板块下热弱地幔的松弛。这两种过程都会引起应力向发震上地壳的时变传递。我们的目标是了解这个应力传递过程，以及它是如何在东京地区加载活动断层的。这将通过开发一个观测约束的有限元模型来实现，该模型可以准确计算由于后滑和粘弹性松弛引起的应力变化。观测约束将主要考虑描述该地区构造几何和弹性结构的地震数据，以及能够确定该地区流变学（粘性强度）的大地测量约束。最重要的是，该模型应该使我们能够分离出余震和粘性松弛对震后大地测量数据的相对贡献，这不仅是准确计算应力变化所必需的，而且将提供关于这两个过程的性质的宝贵见解，这将有助于我们对俯冲带构造的总体理解。日本史无前例的震后GPS覆盖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.