Collaborative Research: Seismic and Geodetic Imaging of Subducting Terranes Under North America

增厚的地壳与俯冲带的碰撞显著地改变了俯冲作用。这些增生事件导致大陆的净增长，并推动了许多与俯冲有关的构造运动。地面碰撞也可能对大型板内地震的大小、耦合和破裂特征产生深远的影响。目前阿拉斯加俯冲系统的外来地体增生是目前活跃的这一过程的少数例子之一。在这个地区，一个厚地壳区域，即雅库塔地体的碰撞，发生在已知的最大的破裂幅度上，这是1964年阿拉斯加Mw9.2地震的一部分。这次碰撞在阿拉斯加海岸和内陆形成了山脉，并可能推动阿拉斯加远端地区向西挤压。最近，利用bearar pascal实验的接收函数，在阿拉斯加中部俯冲板块顶部从70到150公里的深度拍摄到了一个不寻常的层，可能是增厚的地壳。如果与浅层结构连续，这将是迄今为止观测到的最大的深俯冲增厚地壳碎片。这种厚地壳的俯冲作用可能有助于解释1964年粗糙体的大小。然而，深层和浅层结构之间缺乏连续性使其难以区分；这些信号是拍到了地球上最大的厚俯冲地壳，还是别的什么？在任何情况下，俯冲地体对冲断带的力学作用是什么？该项目拍摄了阿拉斯加海岸线和BEAAR之间地区的俯冲板块、上板块和中间变形。这里俯冲穿过1964年的破裂带。宽频地震仪成像下行板块的顶部穿过并低于逆冲带。与之前的研究相结合，提供了迄今为止最长的俯冲带连续样带，横跨700多公里，沿着从海沟到海岸的板块，直到150公里深处最后一次看到的地方。与此同时，大地测量学和地震活动的结合被用来成像目前与板块界面有关的变形，板块界面在地球上已知的第二大地震中破裂。变形模拟与成像相结合，阐明了锁断带的性质、最大凸起的起源以及构造对板间逆冲作用的控制。通过对冲断带下倾端地震活动性、应变和结构的高分辨率采样，这些结果被用来测试中深度地震起源的想法。该实验包括在密集的间隔部署30个宽带地震仪，在高分辨率地震活动可以提供大部分信息的地方配备短周期地震仪，以及对该区域地表变形的GPS测量。稀疏的永久地震和大地测量（PBO）站提供区域控制。许多地震活动和GPS站点是同时配置的，因此同时进行大地测量和地震现场工作可以节省成本。这些数据综合起来，将提供一幅全面的地表增生及其对大地震产生的影响的图景。

The collision of thickened crust with subduction zones significantly modifies subduction. These accretion events lead to net growth of continents and drive much of the subduction-related tectonism. Terrane collision may also have a profound effect on the size, coupling, and rupture characteristics of large intraplate earthquakes. The present accretion of exotic terranes with the Alaska subduction system represents one of the few examples of this process currently active. In this region, the collision of a region of thickened crust, the Yakutat terrane, occurs at the largest rupture asperity known, part of the 1964 Mw9.2 Alaska earthquake. The collision produces mountains along the Alaska coast and perhaps far inland, and may drive westward extrusion of distal parts of Alaska. Recently, an unusual layer, perhaps thickened crust, has been imaged at the top of the subducting plate beneath central Alaska from 70 to 150 km depth, using receiver functions from the BEAAR PASSCAL experiment. If continuous with the shallow structure, this would represent the largest deeply-subducted fragment of thickened crust yet observed. Subduction of such thick crust may help explain the size of the 1964 asperity. However, the lack of continuity between deep and shallow structures makes it difficult to tell; have these signals imaged the largest piece of thick subducted crust on the planet, or something else? In any case, what is the effect of subducting terranes on mechanics of the thrust zone?This project images the subducted plate, upper plate, and intervening deformation in the region between the Alaska coastline and BEAAR. Here subduction passes through and past the 1964 rupture zone. Broadband seismographs image the top of the downgoing plate through and below the thrust zone. Integration with previous studies providse the longest continuous transect of a subduction zone yet available, over 700 km across strike, following a slab from the trench to coast to where last seen at 150 km depth. In parallel, a combination of geodesy and seismicity is used to image deformation currently associated with the plate interface, where it ruptured in the planet's second largest known earthquake. Modeling of deformation, when integrated with the imaging, elucidates the nature of the locked zone, the origin of the largest asperity, and the structural controls on interplate thrust processes. These results are used to test ideas for the origins of intermediate-depth earthquakes, by sampling at high resolution the transition at the down-dip end of the thrust zone in seismicity, strain, and structure.The experiment consists of a deployment of 30 broadband seismographs at dense spacing, supplemented by short-period seismographs in places where higher-resolution seismicity would provide most information, and GPS measurements of surface deformation across this zone. Sparse permanent seismic and geodetic (PBO) stations provide regional control. Many of the seismicity and GPS sites are collocated, so there are cost savings to simultaneously conducting geodetic and seismic field work. These data, when integrated, will provide a thorough picture of terrane accretion and its impact on the generation of great earthquakes.