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洋盆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两栖地震台站阵列。这是第一次地震台阵覆盖了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，到最终在大陆边缘俯冲，其冷却、坚硬的结构是众所周知的。相比之下，地下部分熔融的韧性地幔的性质就不那么确定了。熔融是只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，还是少量的岩浆存在于老化板块下方？当板块接近海沟并向下弯曲以俯冲时，破裂是否让海水向下渗入板块，被矿物吸收并带入地幔？如果是这样的话，这些挥发物后来是如何以及在哪里出现，以影响缓慢循环的地幔的流动性？本研究进行的抗震分析就是为了回答这些问题。一名研究生将进行大部分分析，在博士最后一年的培训中，在早期论文研究的基础上扩展技能。尽管对大洋岩石圈的热结构有了很好的了解，但关于软流层的基本问题仍然存在。部分熔融和束缚水被认为可以解释低的软流层地震速度和粘性，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由恒定的压力/相应的温度定义的，还是软流圈被带到俯冲带并带到深处。板材减去后，温度、水分分布和熔体还会有其他问题。这项研究的重点是从体波测量的卡斯卡迪亚两栖阵列的地震衰减，因为这些物理特性可以比仅从地震速度研究更好地阐明。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。通过与地球动力学模型和面波图像的比较，可以对基于实验室的滞弹性理论进行校准。

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.