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

全球定位系统最近的观测显示，到目前为止，在美国太平洋西北部、墨西哥西南海岸的格雷罗地区和日本的南开地区，浅俯冲区发生了短暂但无地震的形变事件。这些似乎涉及幕式滑动，在一个月到几年的时间窗口中，沿着俯冲界面的更深区域累积，预计不会引发常规地震或在此类地震期间明显移动。这些瞬变对它们的起源提出了重大问题，也与断裂带锁定部分(最终将在地震中失效)如何加载的现有概念有关。也就是说，最近的结果暗示了载荷的一个插曲成分，这对提高地震的预报性有影响。这个项目的研究解决了瞬变的物理原因。主要问题包括它们是如何开始的，以及是什么控制了它们在倾角方向和沿走向的迁移。它们发生在俯冲的海底物质有变质流体释放的区域，因此我们特别关注非常高压力下的孔隙水，作为理解瞬变可能的重要组成部分。更广泛地说，我们想要了解它们是如何依赖于特定俯冲带的构造参数(几何形状、收敛速度、热结构、变质反应模式)的。我们工作的一个主要组成部分是建立俯冲断层的3D数值模型，该模型具有依赖温度的、因此依赖深度的摩擦属性，涉及从较冷的、锁定的断裂带部分向稳定的摩擦下倾过渡。值得注意的是，在对沿走向多次地震的长构造加载序列的模拟中，我们发现非地震瞬变滑移幕自发出现，具有一些自然观测中的特征。在计划中的工作中，我们将在模拟中加入对流体释放、运输和孔隙加压的更完整的物理描述，以与某些俯冲带中与之相关的非火山地震的观测结果相一致的方式来描述瞬变。为了检验和约束模型中假设的下倾无地震断裂带的流变性，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约束的拟合情况。

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.