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。这些吸积事件导致大陆的净增长，并驱动了大部分与俯冲相关的构造作用。地体碰撞也可能对板内大地震的大小、耦合和破裂特征产生深远影响。目前外来地体与阿拉斯加俯冲系统的吸积是目前活跃的这一过程的少数例子之一。在该地区，一个增厚的地壳区域雅库塔特地体的碰撞发生在已知的最大的破裂粗糙处，这是1964年阿拉斯加9.2 Mw9.2地震的一部分。碰撞产生了沿着阿拉斯加海岸的山脉，可能还会沿着遥远的内陆形成山脉，并可能推动阿拉斯加远端部分向西挤压。最近，利用BEAAR PASSCAL实验的接收器功能，在阿拉斯加中部俯冲板块顶部70至150公里处拍摄到了一层不同寻常的层，可能是增厚的地壳。如果与浅层结构连续，这将是迄今观察到的最大的深俯冲增厚地壳碎片。如此厚的地壳俯冲可能有助于解释1964年粗糙度的大小。然而，深部和浅部构造之间缺乏连续性，这让人很难区分；这些信号是拍摄到了地球上最大的一块厚俯冲地壳，还是其他什么？无论如何，俯冲地体对冲断带的力学有什么影响？这个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。这里俯冲经过1964年的断裂带。宽带地震仪通过逆冲带和逆冲带下方对下行板块的顶部进行成像。与以前的研究相结合，提供了迄今可用的最长的俯冲带连续断面，横跨走向超过700公里，从海沟到海岸一直到最后看到的150公里深的地方。同时，大地测量和地震活动的组合被用来成像目前与板块界面相关的变形，在地球上已知的第二大地震中板块界面破裂。形变模拟与成像相结合，阐明了闭锁带的性质、最大粗糙度的来源以及对板间推覆过程的构造控制。这些结果被用来检验中深度地震的起源，通过高分辨率采样逆冲带下倾端的地震活动、应变和构造的转变。实验包括部署30台密集分布的宽带地震仪，并在高分辨率地震活动提供最多信息的地方部署短周期地震仪，以及整个地区地表形变的GPS测量。稀疏的永久地震和大地测量站(PBO)提供区域控制。许多地震活动和GPS站点是并置的，因此同时进行大地测量和地震现场工作可以节省成本。这些数据综合起来后，将提供关于地体增长及其对大地震产生的影响的全面图景。

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.