Aseismic Transients in Subduction Zones: Physical Basis and Possible Seismic Consequences

最近的GPS观测显示，到目前为止，在美国太平洋西北地区、墨西哥西南海岸的格雷罗地区和日本南开地区，浅层俯冲带都有短暂的地震变形事件。这些似乎包括阶段性的滑动，在一个月到几年的时间窗口中，沿着俯冲界面的较深区域积累，这些区域预计不会形成常规地震，也不会在此类地震中明显移动。这些瞬变对它们的起源提出了重要的问题，也对现有的断层带的锁定部分（在地震中最终会失效）是如何加载的概念提出了重要的问题。也就是说，最近的结果暗示了荷载的偶然性成分，这对提高地震的可预测性有影响。这个项目的研究解决了瞬变的物理原因。主要的问题包括它们是如何开始的，是什么控制了它们在倾斜方向和沿走向的迁移。它们发生在有变质流体从俯冲的海底物质释放的区域，因此我们特别关注孔隙水，在非常高的压力下，作为理解瞬变的可能的重要组成部分。更一般地说，我们想了解它们如何依赖于特定俯冲带的构造参数特征（几何形状、收敛速度、热结构、变质反应模式）。我们工作的一个主要组成部分是开发一个与温度相关的俯冲断层的三维数值模型，因此与深度相关，摩擦特性，包括从断裂带较冷的锁定部分过渡到稳定的下倾摩擦。值得注意的是，在沿走向多次地震的长构造加载序列的模拟中，我们发现地震瞬变滑动事件自发出现，其特征与一些自然观测结果相似。在计划的工作中，我们将把流体释放、输送和孔隙加压的更完整的物理描述纳入建模，以一种与在某些俯冲带中与之相关的非火山地震观测相一致的方式来描述瞬变。为了检验和约束模型中假定的下倾地震断裂带流变，我们计划研究该模型对智利北部俯冲带地震后滑动的GPS和InSAR约束的拟合。

Recent observations by GPS have revealed transient but aseismic deformation episodes in shallow subduction zones, thus far in the Pacific Northwest of the US, the Guerrero region along the southwestern coast of Mexico, and the Nankai region in Japan. Those seem to involve episodic slips, accumulating over time windows of a month to a few years, along deeper regions of the subduction interface that would not be expected to nucleate regular earthquakes or to move appreciably during such earthquakes.Those transients pose significant questions as to their origin, and also relative to existing concepts of how locked portions of the fault zone (which will ultimately fail in earthquakes) are loaded. That is, the recent results imply an episodic component of the loading, which has consequences for improved predictability of earthquakes.The research in this project addresses the physical causes of transients. Major questions involve how they start and what controls their migration, in the dip direction and along strike. They occur in regions where there is metamorphic fluid release from the subducted seafloor materials, and thus we focus especially on pore water, at very high pressure, as a possibly essential component of understanding transients. More generally we want to understand how they depend on tectonic parameters characteristic of a particular subduction zone (geometry, convergence rate, thermal structure, pattern of metamorphic reactions).A major component of our work is the development of a 3D numerical model of a subduction fault with temperature-dependent, hence depth-dependent, frictional properties, involving a transition to stable friction downdip from the cooler, locked part of the fault zone. Remarkably, in simulations of long tectonic loading sequences with multiple earthquakes along strike, we found that aseismic transient slip episodes emerged spontaneously, with features like in some of the natural observations.In the planned work, we will incorporate into the modeling a more complete physical description of fluid release, transport and pore pressurization, to develop descriptions of transients in a manner that is also consistent with observations of non-volcanic tremors associated with them in some subduction zones. As a way to check and constrain the assumed rheology of the downdip aseismic fault zone in the model, we plan to investigate the fitting of the model to GPS and InSAR constraints on post-seismic slip from the northern Chile subduction zone.