Collaborative Research: Geodetic Constraints on Moment Deficit and Physics-based Earthquake Cycle Models in the Source Region of the M 9 Tohoku, Japan Earthquake

2011年3月11日，日本东北地区发生的Mw 9级地震造成2.5万人死亡，是日本近代发生的最大地震。长期预测无法预测这种规模的事件，导致我们对俯冲带断层动力学的科学认识进行重新评估。与此同时，日本在地震和大地测量监测方面的巨大投资为研究导致大地震的过程提供了前所未有的机会。研究人员将集中在地震的两个方面，这两个方面为我们预测全球未来灾害的能力提出了核心问题：1)大地应变积累对地震力矩预算的限制，以及2)板块界面上的力学特性分布如何控制地震与地震滑动。地震灾害的预测主要依靠历史记录、古地震学和大地应变积累。虽然在某些地区，严格的时间预测模型已被证明与大地测量数据不一致，但应变积累的速度必须强烈地影响大地震的复发率和/或大小。先前对大量日本egps数据集的分析使欠定逆问题正则化；因此，推断的板耦合取决于正则化泛函的选择。此外，大多数假设在海沟处没有耦合，该区域实际上位于模型的零空间。研究人员建议在矩亏率上估计严格的界限，独立于任何正则化泛函。他们还建议系统地探索随时间变化的粘弹性效应对滑移亏损估计的影响。东北地震也对俯冲板块界面的摩擦特性提出了深刻的问题。沿着日本千岛海沟的7级地震在几乎相同的地点重复发生。这些事件之后的大量余震表明，这些地震“凸起”周围的断层具有不同的摩擦特性，例如，速度减弱的“凸起”周围有速度增强的区域。3月11日的地震似乎在这两个地区都发生了破裂，这一事实提出了重要的问题。严酷范式是否存在根本缺陷？还是m9激活了其他削弱机制（热加压？），使其能够穿过速度增强区域破裂？为了区分相互竞争的假设，研究人员将使用基于GPS数据的假设测试和基于物理的故障建模相结合的方法。他们将测试仅在过去几十年没有经历过地震滑动的地区，地震后的gps数据是否可以与余震相匹配。他们将测试可以解释观察到的行为的替代物理模型，通过扩展我们的数值代码，将速率状态摩擦，热加压和膨胀强化耦合到三维。这将允许他们测试，例如，是否膨胀可以稳定滑移，否则速度减弱区域，或者热加压可以允许破裂延伸到速度增强区域。

The Mw 9 March 11, 2011 Tohoku earthquake, resulting in 25,000 fatalities, was the largestearthquake to strike Japan in modern times. The failure of long-term forecasts to anticipate anevent of this magnitude is leading to a reevaluation of our scienti c understanding of subductionzone fault dynamics. At the same time, the enormous Japanese investment in seismic andgeodetic monitoring provides an unprecedented opportunity to study processes leading to giantearthquakes. The investigators will focus here on two aspects of the quake that raise central issues for our abilityto forecast future hazards globally: 1) the constraints that geodetic strain accumulation placeon the seismic moment budget, and 2) how the distribution of mechanical properties on theplate interface control seismic versus aseismic slip.Forecasts of seismic hazard rely primarily on the historical record, paleoseismology, andgeodetic strain accumulation. While a strict time-predictable model has proven inconsistentwith geodetic data in some areas, the rate of strain accumulation must strongly inuence the recurrence rate and/or size of large earthquakes. Previous analyses of the extensive JapaneseGPS data set regularize the underdetermined inverse problem; the inferred plate coupling isthus conditional on the choice of regularizing functional. In addition, most assumed no coupling at the trench, an area that effectively lies in the model null-space. The researchers propose rather to estimate rigorous bounds on the moment deficit rate, independent of any regularizing functional. They also propose to systematically explore the effects of time-dependent viscoelastic effects on estimates of slip deficit.The Tohoku earthquake also raises profound questions about the frictional properties ofthe subducting plate interface. Mw 7 events along the Japan-Kurile trench have repeatedin nearly the same locations. Substantial afterslip following these events suggested that thefault surrounding these seismic `asperities' have different frictional properties, e.g., velocityweakening `asperities' surrounded by velocity strengthening regions. The fact that the March11 earthquake seems to have ruptured through both areas raises important questions. Is theasperity paradigm fundamentally flawed? Or did the M 9 activate other weakening mechanisms(thermal pressurization?) that allowed it to rupture through velocity strengthening regions?To distinguish between competing hypotheses the researchers will use a combination of GPS data basedhypothesis testing and physics based fault modeling. They will test whether or not post-seismicGPS data can be fit with afterslip only in regions that have not experienced seismic slip inthe last several decades. They will test alternative physical models that could accountfor the observed behavior, by extending our numerical codes that couple rate-state friction,thermal pressurization, and dilatant strengthening to three dimensions. This will allow them totest for example whether dilatancy could stabilize slip in otherwise velocity weakening regions,or thermal pressurization could allow ruptures to extend into velocity strengthening areas.