Creating a New Seismic Instrument from EarthScope Strainmeter Installations

美国国家科学基金会EarthScope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的板块边界观测站。作为板块边界观测站的一部分，已经安装了74个钻孔应变仪，收集与加利福尼亚、俄勒冈和华盛顿州主要断层系统沿线的自然地震过程有关的地面变形数据。这些系统记录的应变信号取决于井眼周围500英尺或更深深度的岩石状况，以及仪器如何被粘入井眼的工程细节。与其他地球物理仪器不同，井眼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商联安援助团的科学家将开发一种方法，通过使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。这些方法将用于南加州ANZA地震网的应变仪，也将依赖于另外两个地震阵列实验收集的数据。一个安装在加州大学圣迭戈分校管理的皮尼昂平原天文台，另一个由联安援助团安装在ANZA网内选定的应变仪场址。这些仪器的校准对科学界来说是一项宝贵的服务，但更重要的是，校准和同时放置的地震和应变仪器通过最近发展的地震波传播理论，允许地球范围设备的新的科学用途。每种应变仪/地震仪器的组合都可以代替大型的专门的地震仪器阵列，以确定局部地震产生的地震波的方向和速度。这使得井眼应变计网络可以作为地震阵列网络来快速定位和描述正在发生的大地震。这对西海岸的地震预警具有很大的社会效益。孟菲斯大学和UNAVCO之间的这个合作项目将产生分析方法，用于有效校准利用地震数据的板块边界天文台（PBO）格拉德温张量应变仪（GTSMs）。研究人员将扩展孟菲斯大学先前开发的方法，该方法结合了参考计算，使用地震波梯度法计算由ANZA南加州网络记录的大地震的区域应变场。目前的研究将开发一种方法，该方法将一个单一的三分量地震传感器与GTSM结合在一起，使用波浪梯度概念而不是使用大型地震传感器阵列来估计地震观测的应变。联安援助团在ANZA网络的PBO GTSM安装了3个宽带地震仪，以增加现有的宽带/GTSM装置，加州大学圣地亚哥分校在Pinyon Flat设施区域内安装了13个密集的宽带传感器阵列。来自这些新的和现有的共存仪器的地震数据将用于估计参考地震应变，然后将这些参考地震应变与GTSMs测量的测量应变相关联。开发这种方法的一个基本限制是，ANZA gtsm以前是使用阵列方法校准的，因此可以控制评估单站方法的成功。与GTSMs一起安装的短周期地震仪的数据也将纳入这项工作，目的是使用现有的数据集来校准任何其他PBO应变仪，而无需额外的实地工作。该项目的总体科学目标是使用校准良好的GTSM和位于同一位置的地震仪器作为局部？梯度张量?可作为测点的地震仪器。用于分析P-SV波场的地震阵列。这样的仪器，原则上可以发现波的属性，如传播方位角和慢度。因此，ANZA PBO网络是一个点阵列阵列，可用于研究南加州中度事件的区域波传播、波散射和震源参数，这些参数在地震预警中应该是有用的。这将是EarthScope设备的一个新应用。

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.