Episodic Slow Slip Events and Tremors in Subduction Zones and Relation to Megathrust Earthquakes

最近在多个俯冲带中观测到的幕式震颤和滑动（ETS）事件构成了活动板块边界的一种新的变形模式。ETS包括低频非脉冲地震辐射（"震颤”），加上大地测量观测到的持续时间为数周的慢滑动事件（SSE）。ETS事件的发生对它们的起源提出了重要的问题，也与现有的锁定孕震区的震间载荷概念有关。例如，ETS的物理原因是什么？是什么控制了观察到的特征，如复发期和迁移模式？在上倾孕震带上，在稳定的应力积累中施加台阶，会产生什么样的地震后果？以前的数值研究的框架中的速率和状态相关的摩擦可以产生缓慢的滑动事件的功能，如滑动速度，复发周期和滑动传播速度，这是定性类似的自然事件推断。然而，滑动被假定为发生在一个平面故障嵌入在一个半空间具有均匀的弹性模量，在时间无关的流体压力。这样的模型简化，使我们无法使用日益丰富的SSE大地测量数据来约束下倾断层带流变学，并调查其对孕震区强度的影响。本研究的目标是构建一个更现实的俯冲断层模型，封装的基本物理的SSE和震颤，以及从实验室和现场观测的约束，并研究它们的关系，巨型逆冲断层地震。PI将开发流体输送、孔隙扩张和加压的更完整的物理描述，特别是将渗透性增强和剪切加热诱导的流体加压纳入速率和状态建模。这将使研究人员能够研究SSE的发生如何影响巨型逆冲断层地震的成核和地震破裂的空间范围。其次，为了约束断层带的假设流变性，他们将应用不同类型岩石的实验室测量的摩擦特性，并研究3D模型结果与卡斯卡迪亚边缘SSE的大地测量数据的拟合。他们将使用有限元法（FEM）构建一个非平面断层模型，更真实地代表卡斯卡迪亚俯冲界面。最后，他们将通过识别潮汐或潮汐应力触发的震颤激发的有利条件来研究慢滑动事件和震颤（包括低频地震）之间的关系。在俯冲带中发现ETS事件带来了重大的难题，并改变了我们对地震间载荷和活动板块边界整体机械预算的思考方式。更好地了解它们的物理基础及其对巨型逆冲断层上倾和下倾极限的影响，可能会增加我们对地震预报和地震危险性评估的知识。三维卡斯卡迪亚俯冲断层模型的速度和状态摩擦的发展将有助于解释EarthScope大地测量数据集。有限元方法将允许该模型的普遍适用性，以模拟在其他俯冲带和大陆断层的SSE和地震破裂过程。该地区，因为它将被解决是丰富的，涉及到几个学科的投入，包括实验室岩石物理学，大地测量学，岩石学，地震学，力学和水力建模概念。这个拟议的项目也有助于WHOI/MIT联合项目研究生的教育。

Recent observations of episodic tremor and slip (ETS) events in multiple subduction zones constitute a new deformation mode at active plate boundaries. ETS consists of low-frequency non-impulsive seismic radiation (``tremor"), coupled with geodetically observed slow slip events (SSE) with durations of order weeks. The occurrence of ETS events poses significant questions as to their origin, and also relative to existing concepts of interseismic loading of the locked seismogenic regions. For example, what are the physical causes of ETS? What controls the observed features such as the recurrence period and migration pattern? What are the seismic consequences of imposing steps in the otherwise steady stress accumulation on the updip seismogenic zone?Previous numerical studies in the framework of rate and state-dependent friction can produce slow slip events with features, such as the slip velocity, recurrence period and slip propagation speed, that are qualitatively similar to those inferred for natural events. However, slip is assumed to take place on a planar fault embedded in a half space with uniform elastic moduli, under time-independent fluid pressure. Such model simplifications have prevented us from using the increasingly rich SSE geodetic data to constrain the downdip fault zone rheology and to investigate its effects on the strength of the seismogenic zone. The goal of this study is to construct a more realistic subduction fault model that encapsulates the essential physics of SSEs and tremors as well as constraints from laboratory and field observations, and to study their relation to megathrust earthquakes. The PI will develop a more complete physical description of fluid transport, pore dilation and pressurization, specifically, to incorporate the dilatancy-strengthening and shear heating induced fluid pressurization into the rate and state modeling. This will allow the researchers to investigate how the occurrence of SSEs could affect the nucleation of megathrust earthquakes and the spatial extent of earthquake ruptures. Second, to constrain the assumed rheology of the fault zone, they will apply frictional properties from lab measurements of different types of rocks, and investigate the fitting of 3D model results to geodetic data on SSEs in the Cascadia margin. They will use finite element method (FEM) in the construction of a non-planar fault model that more realistically represents the Cascadia subduction interface. Finally, they will study the relation between slow slip events and tremors (including low-frequency earthquakes) by identifying the favorable conditions for tremor excitation due to tidal or teleseismic stress triggering.The discovery of ETS events in subduction zones poses significant puzzles and changes the way we think about the interseismic loading and overall mechanical budget on active plate boundaries. Improved understanding of their physical basis and their implications for the updip and downdip limits of megathrust ruptures may increase our knowledge for earthquake forecast and seismic hazard assessment. The development of a 3D Cascadia subduction fault model with rate and state friction will help interpret EarthScope geodetic data sets. The FEM approach will allow general applicability of this model to simulate SSEs and earthquakes rupture processes in other subduction zones and continental faults. The area as it will be addressed is rich, involving inputs from several disciplines, including laboratory rock physics, geodesy, petrology, seismology, and mechanical and hydraulic modeling concepts. This proposed project also contributes to the education of a WHOI/MIT Joint Program graduate student.