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

可以说，地震地震学中最有趣的新现象是间歇性震颤和滑动事件的发现。这种现象由地震记录的震颤事件组成，其特征频率为1-6赫兹，加上全球定位系统和应变仪观测到的慢滑事件，持续时间为几周。震颤和滑动在世界各地的各种环境中都有很好的记录，比如日本和卡斯卡迪亚的俯冲带。尽管地震界继续进行有趣的观察，但对潜在的物理过程仍然知之甚少。已经提出了许多机制，其中一些机制涉及流体，但到目前为止，几乎没有数据来评估这些想法。这个项目正在通过确定地壳和上地幔是否存在与这种现象相关的结构变化来解决这个问题。如果有这样的变化，它们将为理解幕式震颤和滑动现象提供重要的新线索。时变地震学的方法正被应用于这个问题，以确定地震波场在这些事件期间或之后是否有变化。由于板状界面可能在偶发性震颤和滑动过程中起中心作用，因此研究重点是板状界面，以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。通过在卡斯卡迪亚进行随时间变化的地震成像实验，一种可控震源方法被用来寻找这种变化。之所以选择卡斯卡迪亚，是因为众所周知的周期性偶发性震颤和滑动事件允许进行实验部署。可控震源是一种很有前途的方法，因为与其他类型的震源（如重复地震、环境噪声层析成像）相比，可控震源具有更好的时间分辨率和可重复性。该部署旨在向社区展示，可以观察和利用感兴趣的地震相位，即与板界面相互作用的地震相位。这是一个适度的概念验证部署，如果成功，将为2010年的赛事扩大部署。所使用的实验配置允许清晰地观察到ETS最强的板-相互作用阶段。这种源接收器的几何形状对应于穿过奥林匹克半岛的西北-东南方向的线。该源被部署在这条线路的南端，允许使用地球范围公司资助的阵列阵列部署（185个站点）的数据，该部署将在2009年和2010年的事件中部署在北部。通过该项目还部署了另外50个台站，将震源连接到沿地震线的阵列接收器。

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.