EAGER: Detecting Structural Changes During an ETS Event: Proof of Concept

可以说，地震学中最有趣的新现象是幕式震颤和滑动事件的发现。 这种现象包括地震记录的震颤事件，特征频率为1-6赫兹，加上全球定位系统和应变仪观察到的缓慢滑动事件，持续时间为数周。 地震和滑动在世界范围内的各种环境中都有很好的记录，例如日本和卡斯卡迪亚的俯冲带。虽然地震学界继续进行有趣的观测，但对潜在的物理过程仍然知之甚少。已经提出了许多机制，其中一些调用流体，但到目前为止，几乎没有数据来评估这些想法。 本项目正在解决这一问题，确定地壳和上地幔是否发生了与这一现象有关的结构变化。 如果真的存在这种变化，将为理解阵发性震颤和滑脱现象提供重要的新线索。与时间有关的地震学方法正被应用于这个问题，以确定在这些事件期间或之后地震波场是否有变化。 重点是板界面，由于其可能在幕式震颤和滑动过程中的中心作用，寻找可能反映流体迁移或介质中其他相关变化的结构变化。一种受控源方法正在被用来寻找这种变化，通过在卡斯卡迪亚进行一个随时间变化的地震成像实验。选择卡斯卡迪亚是因为众所周知的间歇性震颤和滑动事件的周期性允许实验部署。受控震源是一种很有前途的方法，因为与其他类型的震源（例如重复地震、环境噪声层析成像）相比，受控震源具有上级时间分辨率和可重复性。这一部署旨在向社会证明，它将有可能观察和利用感兴趣的地震阶段，即那些与板界面相互作用。这是一个适度的概念验证部署，如果成功，将导致2010年活动的扩大部署。 正在使用的实验配置允许清楚地观察板相互作用阶段ETS是最强的。 这种源-接收器几何形状对应于穿过奥林匹克半岛的西北-东南方向的线。 该源正部署在这条线的南端，从而可以使用由地球观测所资助的阵列中的阵列部署（185个台站）的数据，该阵列将部署在2009年和2010年的事件的北部。 通过该项目还部署了另外50个台站，将震源与沿地震测线沿着的阵列接收器阵列连接起来。

Arguably the most interesting new phenomenon in Earthquake seismology is the discovery of Episodic Tremor and Slip events. This phenomenon consists of seismically recorded tremor events with characteristic frequencies of 1-6 Hz, coupled with slow slip events observed by global positioning systems and strainmeters, with durations of order weeks. Both tremor and slip have been well documented in a variety of environments worldwide, such as subduction zones in Japan and Cascadia. Although the seismic community continues to make interesting observations, the underlying physical process remains poorly understood. There have been many proposed mechanisms, several of which invoke fluids, but thus far, there is little data with which to evaluate these ideas. This problem is being addressed in this project by determining whether or not there are structural changes in the crust and upper mantle associated with this phenomenon. If there are such changes, they would provide an important new clue in understanding the Episodic Tremor and Slip phenomenon. The method of time-dependent seismology is being applied to this problem, to determine whether there are changes in the seismic wavefield during or after such events. The focus is on the slab interface, due to its likely central role in the Episodic Tremor and Slip process, to look for structural changes that may reflect the migration of fluids or other related changes in the medium. A controlled-source approach is being used to look for such changes by conducting a time-dependent seismic imaging experiment in Cascadia. Cascadia was chosen because the well-known periodicity of Episodic Tremor and Slip events permits experimental deployments. Controlled sources are a promising approach, because of superior time resolution and repeatability, compared to other types of sources (e.g. repeating earthquakes, ambient noise tomography). This deployment is designed to demonstrate to the community that it will be possible to observe and utilize the seismic phases of interest, namely those that interact with the slab interface. This is a modest proof-of-concept deployment that, if successful, will lead to an expanded deployment for the 2010 event. The experimental configuration being used permits the clear observation of slab-interaction phases where ETS is strongest. This source-receiver geometry corresponds to a Northwest-Southeast-oriented line through the Olympic Peninsula. The source is being deployed at the southern end of this line, permitting the use of data from the Earthscope-funded Array-of-Arrays deployment (185 stations), which will deployed to the north for both the 2009 and 2010 events. An additional 50 stations is also being deployed through this project that connects the source to the Array of Arrays receivers along the seismic line.