Viscoelastic subduction modelling to understand megathrust earthquake potential

最大的地震，震级高达9.5级，发生在俯冲板块边界。在这些边界上，当两个板块汇合时，一个板块俯冲到另一个板块之下，板块在接触处的摩擦会导致非常大的地震，这些地震总共释放了全球地震能量预算的80%以上。这些所谓的大型逆冲地震及其相关危害，包括波及整个海洋的海啸，已造成数十万人死亡，比如2004年的苏门答腊地震和2011年的日本地震。一个长期存在的问题是，为什么只有俯冲边界的某些部分有非常大的地震记录，而其他部分似乎没有能力举办大地震。当应力超过摩擦断层强度时，由于俯冲边界的力平衡不断推动辐合而在锁定的大逆冲构造中积累的弹性应力突然释放，地震就会发生。认为地震电位的变化反映了大逆冲构造弹性载荷的大尺度差异。可变载荷归因于俯冲参数，如板块收敛速度、上板块的强度或板块之间界面上沉积物的厚度。然而，地震重复次数通常比我们仪器目录的持续时间长得多，因此，俯冲参数与最大地震规模的统计相关性仍然不确定。板块和界面特性之间的相互作用可能决定了应力的积累，因此需要一种物理建模方法来了解不同因素如何影响大逆冲地震的潜在发生。局部尺度的粘弹性板块俯冲模型，通过规定几何形状和收敛速度，为特定环境量身定制，有助于理解地震周期的后果，包括决定海啸潜力的地表变形和断层滑动模式。这些模型并没有提供俯冲参数如何控制应力载荷的大规模和长期差异的见解。其他更大规模的模型让板块几何形状和运动动态发展，并有助于理解力平衡和长期应力。然而，这些模型通常将板近似为粘性而忽略了弹性应力。直到现在，建模能力才足够成熟，可以运行系统的大型粘弹性俯冲板块模型集。在我们团队的一项2D初步研究中，我们从模型中推导出关系，使我们能够估计实际俯冲带板块弯曲的弹性成分。我们发现，较高的弹性弯曲估计与较高的观测到的大地震相对数量相关（与较小的事件相比）。这说明了这种大规模粘弹性俯冲模型在理解大逆冲地震潜力方面的前景。在这里提出的项目中，我们将使用最先进的板块建模平台（Underworld），这将使我们能够首次运行一套系统的3D粘弹性俯冲模型，以表征俯冲系统中弹性能量储存的变化。通过确定板块弯曲（下倾和沿走向）和板块上部应力对俯冲板块、板块上部和它们之间耦合强度变化的响应，我们将测试这些属性的哪种组合可以解释观测到的俯冲参数和最大地震规模之间的关系。从我们的模型中得出的关系将提供一种新颖的、基于物理的方法来估计全球俯冲段发生特大地震的可能性，包括在没有历史地震目录的边界。

The largest earthquakes, of magnitudes up to 9.5, occur at subduction plate boundaries. At these boundaries, one plate dives below the other as the two plates converge, and the rubbing of the plates at their contact can lead to very large earthquakes, which together release over 80% of the global budget of earthquake energy. These so-called megathrust earthquakes and their associated hazards, including ocean-wide tsunamis, have caused 100s of thousands of deaths, as in Sumatra in 2004 and in Japan in 2011. A long-standing question is why only some parts of subduction boundaries have a record of very large earthquakes, while others do not appear capable of hosting major earthquakes. Earthquakes occur when elastic stresses, which accumulate at the locked megathrust as the balance of forces at the subduction boundary continuously drives convergence, are suddenly released when frictional fault strength is exceeded by the stress. It is agreed that variations in earthquake potential reflect large-scale differences in elastic loading of the megathrust. Variable loading has been attributed to subduction parameters such as plate convergence velocities, strength of the upper plate, or thickness of sediments on the interface between the plates. However, earthquake repeat times are generally much longer than the duration of our instrumental catalogues, and as a result, statistical correlations of subduction parameters with maximum earthquake size remain inconclusive. An interplay between plate and interface properties probably determines stress build up, and therefore a physical modelling approach is needed to understand how different factors contribute to megathrust earthquake potential.Local-scale subduction models of visco-elastic plates, tailored to a specific setting by prescribing geometry and convergence velocities, have helped to understand consequences of the earthquake cycle, including surface deformation and fault slip patterns that determine tsunami potential. Such models do not provide insight in how subduction parameters control large-scale and long-term differences in stress loading. Other, larger-scale, models let plate geometry and motions develop dynamically and have helped to understand the force balance and long-term stresses. However, these models usually approximate plates as viscous and neglect elastic stresses. Only now have modelling capabilities matured sufficiently to make running systematic sets of large-scale models of visco-elastic subducting plates feasible. In a 2D pilot study by our team, we derived relationships from models that let us estimate the elastic component of plate bending at actual subduction zones. We found that higher estimates of elastic bending correlated with higher observed relative numbers of large earthquakes (compared to smaller events). This illustrates the promise of such large-scale visco-elastic subduction modelling for understanding megathrust seismic potential.In the here-proposed project, we will use a state-of-the-art plate-modelling platform (Underworld) that will allow us to, for the first time, run a systematic set of 3D visco-elastic subduction models to characterise the variation of elastic energy storage in the subduction system. By determining the response of plate bending (downdip and along-strike) and upper-plate stress to variations in properties of the subducting plate, upper plate and coupling strength between them, we will test what combination of these properties can explain observed relations between subduction parameters and maximum earthquake size. The relations derived from our models will provide a novel, physics-based, method to estimate of the potential of subduction segments around the globe to host very large earthquakes, including at boundaries without a historic earthquake catalogue.