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

最近对多个俯冲带的偶发性震颤和滑动（ETS）事件的观测，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数周的慢滑事件（SSE）。ETS事件的发生对其起源提出了重要的问题，也与现有的锁定发震区的震间载荷概念有关。例如，产生ETS的物理原因是什么？是什么控制着观察到的特征，如复发期和迁移模式？在上倾发震带上施加台阶会产生怎样的地震后果？先前在速率和状态相关摩擦框架下的数值研究可以产生具有滑移速度、复发周期和滑移传播速度等特征的慢滑移事件，这些特征与自然事件的推断在性质上相似。然而，滑移假定发生在具有均匀弹性模量的嵌入半空间的平面断层上，且发生在与时间无关的流体压力下。这种模型的简化使我们无法利用日益丰富的SSE大地测量数据来约束下倾断裂带流变学，并研究其对孕震带强度的影响。本研究的目的是建立一个更真实的俯冲断层模型，该模型包含了sse和地震的基本物理特性以及实验室和现场观测的约束条件，并研究它们与特大逆冲地震的关系。PI将对流体输运、孔隙膨胀和加压进行更完整的物理描述，特别是将膨胀强化和剪切加热引起的流体加压纳入速率和状态模型。这将使研究人员能够研究sse的发生如何影响特大逆冲地震的成核和地震破裂的空间范围。其次，为了约束假定的断裂带流变学，他们将应用不同类型岩石的实验室测量结果的摩擦特性，并研究将3D模型结果与卡斯卡迪亚边缘sse的大地测量数据进行拟合。他们将使用有限元法（FEM）来构建非平面断层模型，以更真实地代表卡斯卡迪亚俯冲界面。最后，他们将通过确定潮汐或远震应力触发引发地震的有利条件，研究慢滑事件与地震（包括低频地震）之间的关系。在俯冲带发现的ETS事件提出了重大的难题，并改变了我们对活动板块边界的地震间载荷和整体力学预算的思考方式。更好地了解它们的物理基础及其对大逆冲断裂上倾和下倾极限的影响，可以增加我们对地震预报和地震危险性评估的认识。具有速率和状态摩擦的三维Cascadia俯冲断层模型的开发将有助于解释EarthScope大地测量数据集。FEM方法将使该模型普遍适用于模拟其他俯冲带和大陆断层的地震破裂过程。该领域内容丰富，涉及多个学科的投入，包括实验室岩石物理学、大地测量学、岩石学、地震学以及机械和水力建模概念。这个提议的项目也有助于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.