Creating a New Seismic Instrument from EarthScope Strainmeter Installations

美国国家科学基金会地球观测项目设施的一个主要部分是位于美国大陆和阿拉斯加太平洋沿岸沿着的板块边界观测站。 作为板块边界观测站的一部分，已安装了74个钻孔应变仪，收集与加州、俄勒冈州和华盛顿州的主要断层系统沿着的自然地震过程有关的地面变形数据。 由这些系统记录的应变信号取决于在500英尺或更深的深度处直接围绕钻孔的岩石的条件，以及仪器如何被胶结到钻孔中的工程细节。 与其他地球物理仪器不同的是，钻孔应变仪必须在安装后进行校准，以使结果数据有意义。 来自孟菲斯大学和主要设施运营方联合国驻阿富汗大使馆的科学家将开发一种方法，利用几乎所有应变仪都安装的地震仪器记录的地震信号来校准这些应变仪。 这些方法将用于加州南部ANZA地震网络内的应变仪，并将依赖于两个额外的地震阵列实验收集的数据。 其中一个安装在圣地亚哥加州大学管理的Pinyon Flat天文台，另一个安装在安新局网络内选定的应变仪地点。 这些仪器的校准对科学界是一项宝贵的服务，但更重要的是，经过校准并位于同一地点的地震和应变仪器使人们能够通过最近发展的地震波传播理论，对这一EarthScope设施进行新的科学利用。 每个应变仪/地震仪器组合都可以取代大量专门的地震仪器阵列，以确定当地地震产生的地震波的方向和速度。 这可以允许钻孔应变仪网络用作地震阵列网络，以快速定位和表征正在进行的大地震。 孟菲斯大学和联合国非洲志愿人员组织之间的这一合作项目将导致使用地震数据有效校准板块边界观测站（PBO）Gladwin张量应变仪（GTSM）的分析方法。 研究人员将扩展以前由孟菲斯大学开发的方法，该方法包括使用ANZA南加州网络记录的大地震的地震波梯度测量法计算参考区域应变场。本研究将开发一种方法，采用一个单一的三分量地震传感器与GTSM共同定位，以估计应变从地震观测使用波梯度的概念，而不是使用一个大的地震传感器阵列。 联刚特派团已在澳新局网络的PBO全球技术监测机制安装了3个宽带地震仪，以加强现有的宽带/全球技术监测机制装置，而加州大学圣地亚哥分校已在Pinyon Flat设施地区安装了一个密集的宽带阵列，由13个传感器组成。 来自这些新的和现有的共址仪器的地震数据将用于估计参考地震应变，然后将其与GTSM测量的应变计应变相关联。开发这种方法的一个重要限制是，ANZA GTSM先前已使用阵列方法进行校准，从而控制对单站方法成功与否的评估。与GTSM一起安装的短周期地震仪的数据也将被纳入这项工作，目的是使用现有的数据集来校准任何其他PBO应变仪，而无需额外的现场工作。 该项目的主要科学目标是使用校准良好的GTSM和共址地震仪器作为部分？梯度张量地震仪器，可以作为一个？重点？地震台阵分析P-SV波场。 这种仪器原则上可以找到波的属性，如传播方位角和慢度。 因此，ANZA PBO网络是一组点阵列，可用于研究区域波传播、波散射和加州南部中等事件的震源参数，这些参数在地震预警中应该是有用的。这将是EarthScope设施的一个新应用。

A major part of the National Science Foundation's EarthScope program facility is the Plate Boundary Observatory located along the Pacific coasts of the continental U.S. and Alaska. 74 borehole strainmeters have been installed as part of the Plate Boundary Observatory and collect ground deformation data that pertain to natural earthquake processes along the major fault systems of California, Oregon, and Washington states. Strain signals recorded by these systems depend on the conditions of the rock that immediately surrounds the borehole at depths of 500 feet or more in addition to the engineering details of how the instrument was cemented into the borehole. Unlike other geophysical instruments, borehole strainmeters must be calibrated after they have been installed to make sense of the resulting data. Scientists from the University of Memphis and UNAVCO, the major facilities operator, will be developing a method to calibrate these strainmeters by using seismic signals recorded by seismic instruments that have been installed with nearly all strainmeters. These methods will be developed for strainmeters within the ANZA seismic network of southern California and will also rely on data collected by two additional seismic array experiments. One has been installed at the Pinyon Flat Observatory run by the University of California, San Diego and at other by UNAVCO at selected strainmeter sites within the ANZA network. Calibration of these instruments is an invaluable service to the scientific community but, more importantly, calibrated and co-located seismic and strain instruments allow a new scientific use of this EarthScope facility through recently developed theory of seismic wave propagation. Each strainmeter/seismic instrument combination can take the place of a large specialized array of seismic instruments to determine the direction and speed of seismic waves generated by local earthquakes. This can allow a network of borehole strainmeters to be used as a network of seismic arrays to quickly locate and characterize an ongoing large earthquake. This can have great societal benefit for Earthquake Early Warning on the west coast.This collaborative project between the University of Memphis and UNAVCO will result in analytic methods for efficiently calibrating Plate Boundary Observatory (PBO) Gladwin Tensor Strain Meters (GTSMs) using seismic data from earthquakes. The investigators will extend methods previously developed by the University of Memphis that incorporate computation of reference, regional strain fields using seismic wave gradiometry for large earthquakes recorded by the ANZA southern California network. The present research will develop a method that incorporates a single three-component seismic sensor co-located with a GTSM to estimate strains from earthquake observations using wave gradiometry concepts rather than using a large array of seismic sensors. UNAVCO has installed 3 broadband seismometers at PBO GTSMs in the ANZA network to augment existing broadband/GTSM installations and UCSD has installed a dense, broadband array of 13 sensors within the area of the Pinyon Flat facility. Earthquake data from these new and existing co-located instruments will be used to estimate reference seismic strains that are then related to measured gauge strains by the GTSMs. An essential constraint in developing this method is that the ANZA GTSMs have been previously calibrated using the array method giving control on evaluating the success of the single station method. Data from the short-period seismometers installed with the GTSMs will also be incorporated in this work with the goal of using this existing data set to calibrate any other PBO strain meter without additional fieldwork. The over arching scientific goal of this project is to use a well-calibrated GTSM and co-located seismic instrument as a partial ?gradient tensor? seismic instrument that can serve as a ?point? seismic array for analyzing the P-SV wave field. Such an instrument can, in principle, find wave attributes such as propagation azimuth and slowness. The ANZA PBO network is, therefore, an array of point arrays that can be used to study regional wave propagation, wave scattering, and source parameters from moderate events in southern California that should be useful in Earthquake Early Warning. This will be a new application of EarthScope facilities.