Crustal Deformation across the U.S. from Harmonic Analysis of Receiver Functions

EarthScope移动式阵列正在以前所未有的全大陆规模收集地震数据，台站位置相距约70公里。由于美国各地的地壳厚度在~20 - 55 km之间变化，因此，当使用地震波（台站下方具有陡峭的入射角）对结构进行成像时，这种密集的台站间距仍然主要提供每个台站下方地壳的点照明。在这样的照明下，当使用地震体波和环境噪声以及地震面波时，地壳结构通常被假定为局部一维（局部水平界面）。地壳物质通常也被假定为地震各向同性（方向无关的波传播）。接收器函数技术照亮地震台站下面的界面，在那里入射的压缩波部分地转换成剪切波。如果界面不是水平的，或者如果存在地震各向异性的对比，则转换波显示出随入射波到达方位角而变化的系统的和潜在的高振幅信号。在整个可移动阵列中，大约20%的径向分量和40%的横向分量总接收器函数信号幅度由具有与倾斜或各向异性界面匹配的系统变化的到达组成，并且信号强度与构造省有很好的相关性。地壳变形导致地壳界面倾斜和地壳各向异性。因此，接收器函数中的方位角变化信号可用于绘制地壳变形图。该项目不是试图通过改变界面倾角和/或各向异性来精确拟合波形，这是一个高度非唯一的过程，而是映射界面和各向异性层的信号强度，深度和方向（有点类似于映射分裂SKS波中的延迟时间和快速方向，除了接收器函数方法还提供深度信息）。将由此产生的地图与地表地质和地壳变形的模型或地质历史进行比较，从而为检验与这些模型或历史有关的假设提供了机会。该项目基于地震学家和地质学家之间的密切合作，并通过RESESS（学生固体地球科学研究经验，一个通过与导师配对为少数民族本科生实习生提供研究机会的计划）吸引本科生研究助理。

The EarthScope Transportable Array is collecting seismic data on a previously unprecedented continent-wide scale with station locations ~70 km apart. Since the thickness of the crust across the U.S. varies from ~20 - 55 km, such dense station spacing still offers mostly spot illumination of the crust under each station when teleseismic earthquake waves (with steep incidence angles under the station) are used for imaging structure. With such illumination, the crustal structure is typically assumed to be locally one-dimensional (locally horizontal interfaces) when teleseismic body waves and ambient noise and teleseismic surface waves are used. Crustal material is also typically assumed to be seismically isotropic (direction-independent wave propagation).The receiver function technique illuminates interfaces under a seismic station where an incident compressional wave partly converts to a shear wave. If an interface is not horizontal, or if a contrast in seismic anisotropy is present, the converted waves show a systematic and potentially high-amplitude signal that varies with the azimuth of arrival of the incident wave. Across the Transportable Array, roughly 20% of the radial and 40% of the transverse component total receiver function signal amplitude consists of arrivals with a systematic variation matching dipping or anisotropic interfaces, and the signal strength is well correlated to tectonic provinces. Dipping crustal interfaces as well as crustal anisotropy result from deformation of the crust. The azimuthally varying signal in receiver functions can therefore be used to map crustal deformation. Rather than attempting to fit the waveform exactly by varying interface dip and/or anisotropy, which is a highly non-unique process, this project maps the signal strength, depth, and orientation of interfaces and anisotropic layers (somewhat akin to mapping delay time and fast orientation in split SKS waves, except that the receiver function method also offers depth information). The resulting maps are compared to surface geology and models or geological histories of crustal deformation and thus offer the chance for hypothesis testing related to those models or histories. The project is based on close collaboration between a seismologist and a geologist and engages undergraduate research assistants through RESESS (Research Experiences in Solid Earth Sciences for Students, a program providing research opportunities for minority undergraduate interns by pairing them with mentors).