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底部构造板块下的地幔。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站阵列。这是地震台阵首次覆盖从扩张中心到海沟再到火山弧的完整板块。 大洋板块从形成到老化，再到最终在大陆边缘俯冲，其特征是冷却、坚硬的结构，这一点已为人们所熟知。相比之下，下面部分熔融的韧性地幔的性质就不那么确定了。熔融是否只发生在地幔中，在大洋扩张中心之下的产生岩浆的上涌带之下，或者少量的岩浆在老化板块之下？当板块接近海沟并向下弯曲以俯冲时，断裂是否会让海水向下渗透到板块中，被矿物质吸收并带入地幔？如果是这样的话，这些挥发物后来是如何以及在哪里出现的，从而影响了缓慢循环的地幔的流动性？本研究所进行的抗震分析旨在回答这些问题。研究生将进行大部分的分析，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。虽然海洋岩石圈的热结构已经很好地理解，但关于软流圈的基本问题仍然存在。部分熔融和束缚水都被提出来解释低的软流圈地震速度和粘度，但它们的分布没有得到很好的限制。目前尚不清楚软流圈的底部是否由恒定的压力/同源温度所定义，或者软流圈是否在俯冲带被携带并被带到深处。一旦板块俯冲，关于温度、水分布和融化的其他问题也存在。这项研究的重点是地震衰减整个卡斯卡迪亚两栖阵列测量体波，因为这些物理特性可以更好地阐明比单独从地震速度研究。由于两栖台阵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面波派生图像的比较将允许校准基于实验室的滞弹性理论。

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.