Viscoelastic subduction modelling to understand megathrust earthquake potential

最大的地震，震级可达9.5级，发生在俯冲板块边界。在这些边界处，当两个板块汇聚时，一个板块俯冲到另一个板块下方，两个板块在接触处的摩擦可能会导致非常大的地震，这些地震总共释放了全球80%以上的地震能量预算。这些所谓的巨型断裂带地震及其相关危险，包括海洋范围的海啸，已经造成数十万人死亡，2004年的苏门答腊岛和2011年的日本就是如此。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极大地震的记录，而其他部分似乎没有能力容纳大地震。当应力超过摩擦断层强度时，当俯冲边界的力平衡不断推动收敛时，在锁定的巨型褶皱处积累的弹性应力突然释放，就会发生地震。人们一致认为，地震潜力的变化反映了巨型逆冲带弹性载荷的巨大差异。可变载荷归因于俯冲参数，如板块会聚速度、上板块的强度或板块之间界面上的沉积物厚度。然而，地震重复次数通常比我们的仪器目录的持续时间长得多，因此，俯冲参数与最大地震大小的统计相关性仍然没有定论。板块和界面属性之间的相互作用可能决定了应力的积累，因此需要一种物理模拟方法来了解不同因素是如何影响巨型逆冲地震潜力的。粘弹性板块的局部俯冲模型通过规定几何形状和收敛速度来适应特定环境，有助于了解地震周期的后果，包括决定海啸潜力的地表变形和断层滑动模式。这样的模型不能提供关于俯冲参数如何控制应力载荷的大范围和长期差异的洞察。其他更大尺度的模型让板块的几何形状和运动动态发展，并帮助理解了力平衡和长期应力。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，才能使运行大规模粘弹性俯冲板块模型的系统集合成为可能。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，这些模型使我们能够估计实际俯冲区的板块弯曲的弹性分量。我们发现，更高的弹性弯曲估计与更高的观测到的大地震的相对数量相关(与较小的地震相比)。这说明了这种大规模粘弹性俯冲模型在理解巨型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)，它将首次允许我们运行一套系统的3D粘弹性俯冲模型来描述俯冲系统中弹性能量存储的变化。通过确定板块弯曲(下倾和沿走向)和上板块应力对俯冲板块和上板块性质变化的响应以及它们之间的耦合强度，我们将检验这些性质的什么组合可以解释观察到的俯冲参数与最大地震规模之间的关系。从我们的模型得到的关系将提供一种新的、基于物理的方法来估计全球俯冲部分容纳超大型地震的潜力，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.