Thermal and melt structure of the Juan de Fuca plate from ridge to trench to arc, inferred from seismic attenuation across the Amphibious Array

这项研究的主要目标是更好地了解海洋盆地构造板块下方的地幔。卡斯卡迪亚倡议在太平洋西北部的整个胡安德富卡板块和邻近的卡斯卡迪亚边缘部署了一个两栖地震台阵。这是地震台首次覆盖从震颤中心到海沟再到火山弧的完整板块。 海洋板块从形成、老化到最终在大陆边缘俯冲的冷却、刚性结构是众所周知的。相比之下，下面的延展性、部分熔融的地幔的性质则不太确定。熔化仅发生在大洋扩张中心下方产生岩浆的上升带下方的地幔中，还是在老化板块下方存在少量岩浆？当板块接近海沟并向下弯曲俯冲时，压裂是否会让海水过滤到板块中，被矿物质吸收并带入地幔？如果是这样，这些挥发物后来如何以及在哪里出现，影响缓慢循环的地幔的流动性？本研究进行的地震分析旨在回答这些问题。研究生将进行大部分分析，在最后一年的博士培训中扩展技能，超越早期论文研究中已经获得的知识。尽管海洋岩石圈的热结构已得到很好的了解，但有关软流圈的基本问题仍然存在。部分融化水和束缚水都被提出来解释低软流圈地震速度和粘度，但它们的分布没有得到很好的约束。目前尚不清楚软流圈的底部是否由恒定压力/同源温度定义，或者软流圈是否被夹带在俯冲带并带到深处。一旦板块俯冲，就会出现有关温度、水分布和融化的其他问题。这项研究的重点是通过体波测量的卡斯卡迪亚两栖阵列的地震衰减，因为这些物理特性比单独的地震速度研究可以更好地阐明。由于两栖阵列提供了如此全面的宽带地震数据，因此在这里可以比任何其他海洋区域更好地看到体波衰减的预期变化。与地球动力学模型和表面波衍生图像的比较将允许校准基于实验室的迟弹性理论。

The primary goal of this study is to better understand the mantle beneath tectonic plates that floor the ocean basins. The Cascadia Initiative deployed an Amphibious Array of seismic stations across the complete Juan de Fuca plate and adjacent Cascadia margin, in the Pacific Northwest. This is the first time a seismic array has covered a complete plate from spreading center to trench to volcanic arc. The cooling, rigid structure that characterizes an oceanic plate from its formation, through aging, and its eventual subduction at the continental margin is well understood. In contrast, the nature of the ductile, partially molten mantle underneath is less certain. Does melt occur in the mantle only beneath the magma-producing upwelling zone beneath the oceanic spreading center, or are small amounts of magma beneath the aging plate? As the plate approaches a trench and bends downward to subduct, does fracturing let seawater filter down into the plate, to be absorbed in minerals and carried into the mantle? If so, how and where do these volatiles later emerge to affect the fluidity of the slowly circulating mantle? The seismic analysis carried out in this study aims to answer these questions. A graduate student will carry out most of the analysis, expanding skills in a final PhD training year beyond what has already been gained in earlier thesis research.Although the thermal structure of oceanic lithosphere is well understood, basic questions regarding asthenosphere still exist. Both partial melt and bound water are proposed to explain low asthenosphere seismic velocities and viscosities but their distribution is not well constrained. It is not known if the base of the asthenosphere is defined by a constant pressure / homologous temperature, or if the asthenosphere is entrained at a subduction zone and carried to depth. Additional questions about temperature, water distribution and melt exist once plates subduct. This study focuses on seismic attenuation across the Cascadia Amphibious Array measured from body waves, since these physical properties can be better elucidated than from seismic velocity studies alone. Because the Amphibious Array provides such comprehensive broadband seismic data, the expected variations in body-wave attenuation can be seen here better than any other oceanic region. Comparisons with geodynamic models and with surface-wave derived images will allow calibration of laboratory-based theories of anelasticity.