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

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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.