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

合作研究：北美俯冲地体的地震和大地测量成像

• 批准号: 0409950
• 负责人: Douglas Christensen
• 金额: $ 53.18万
• 依托单位: University of Alaska Fairbanks Campus
• 依托单位国家: 美国
• 项目类别: Continuing Grant
• 财政年份: 2004
• 资助国家: 美国
• 起止时间: 2004-07-01 至 2010-06-30
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0409950&HistoricalAwards=false
• 关键词: Collaborative; Research; Seismic; Geodetic; Imaging

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.

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，并驱动了大部分与俯冲有关的构造运动。 地体碰撞也可能对大型板内地震的规模、耦合和破裂特征产生深远影响。目前的增生与阿拉斯加俯冲系统的异国情调的珊瑚礁是目前活跃的这一过程的少数几个例子之一。 在这个地区，一个地壳增厚的地区--雅库塔特--的碰撞发生在已知的最大的破裂粗糙体上，是1964年阿拉斯加Mw9.2地震的一部分。 碰撞产生的山脉沿着阿拉斯加海岸，也许远内陆，并可能推动向西挤压阿拉斯加的远端部分。 最近，利用BEAAR PASSCAL实验的接收器功能，在阿拉斯加中部70至150公里深处的俯冲板块顶部对一个不寻常的层（可能是加厚的地壳）进行了成像。 如果与浅部结构相连，这将是迄今观测到的最大的地壳增厚的深俯冲碎片。 如此厚的地壳的俯冲可能有助于解释1964年粗糙体的大小。 然而，由于深层和浅层结构之间缺乏连续性，因此很难判断;这些信号是否成像了地球上最大的厚俯冲地壳，或者其他东西？无论如何，俯冲带对逆冲带的力学有什么影响？该项目对阿拉斯加海岸线和BEAAR之间区域的俯冲板块、上板块和介入变形进行了成像。这里俯冲穿过并经过1964年破裂带。 宽频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，提供了最长的连续断面的俯冲带，但可用的，超过700公里的走向，以下板从海沟到海岸，最后看到在150公里的深度。 与此同时，大地测量学和地震活动性的结合被用来对目前与板块界面相关的变形进行成像，板块界面在地球上第二大已知地震中破裂。 变形的建模，结合成像，阐明了锁定区的性质，最大的粗糙的起源，和板间逆冲过程的结构控制。 这些结果被用来测试中深地震起源的想法，通过在地震活动性、应变和结构的逆冲带下倾端的过渡处以高分辨率采样。实验包括以密集的间隔部署30个宽带地震仪，在高分辨率地震活动性将提供大多数信息的地方补充短周期地震仪，和GPS测量的地表变形 稀疏的常设地震和大地测量站提供区域控制。许多地震活动和全球定位系统站点都在同一地点，因此可以节省同时进行大地测量和地震实地工作的费用。 这些数据，当整合，将提供一个完整的图片的岩石圈增生及其对大地震的产生的影响。