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日发生的造成2.5万人死亡的东北大地震，是日本近代以来发生的最大地震。长期预测的失败导致了我们对俯冲带断层动力学的科学理解的重新评估。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巨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，这两个方面提出了我们在全球范围内预测未来灾害的能力：1)大地应变累积对地震矩预算的限制；2)板块界面上的力学性质分布如何控制地震和非地震滑动。地震灾害的预测主要依赖于历史记录、古地震学和大地应变累积。虽然严格的时间可预测模型已被证明与某些地区的大地测量数据不一致，但应变累积的速度肯定会强烈影响大地震的复发率和/或规模。先前对大量日本GPS数据集的分析正则化了欠定反问题；由此推断的板耦合是以选择正则化泛函为条件的。此外，大多数假设在沟渠处没有耦合，这一区域实际上位于模型零空间中。相反，研究人员建议估计矩赤字率的严格界限，而不是任何正则化泛函。他们还建议系统地探讨随时间变化的粘弹性效应对滑动缺陷估计的影响。东北地震还提出了关于俯冲板块界面摩擦性质的深刻问题。日本-千岛海沟沿线发生的7兆瓦地震几乎在相同的地点重演。这些地震后的大量余滑表明，这些地震“凹凸体”周围的断层具有不同的摩擦性质，例如，速度减弱的“凹凸体”被速度增强区包围。3月11日的地震似乎在这两个地区都发生了破裂，这一事实引发了重要的问题。粗糙度范式是否存在根本性缺陷？或者，M9是否激活了其他减弱机制(热增压？)这使得它能够穿过速度加强区？为了区分相互竞争的假说，研究人员将使用基于GPS数据的假设检验和基于物理的断层建模相结合的方法。他们将测试是否只有在过去几十年没有经历过地震滑动的地区，地震后GPS数据才能与余滑相吻合。他们将测试可以解释观察到的行为的替代物理模型，将我们的数值代码扩展到三维，这些代码将速率状态摩擦、热压力和膨胀剂强化耦合在一起。这将允许他们测试，例如，膨胀是否可以稳定速度减弱区域的滑动，或者热压是否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.