Collaborative Research: Earthscope integrated investigation of Cascadia subduction zone tremor, structure and process

本项目在Cascadia俯冲带进行了综合野外实验，以阐明水输运、地震滑动、幕式震颤和弧岩浆活动之间的关系。最终目标是利用地震学、大地测量学和岩石学等工具探索俯冲带的H2O过程，并将这些结果与地球动力学和地球化学的互补约束相结合。地震成像被用来照亮(i)下沉的海洋板块，在那里它发生变质；（ii）地幔楔，在那里流体可能产生含水相，如蛇纹岩，或者在火山弧下，原始岩浆。该实验旨在穿越卡斯卡迪亚系统的一部分，在那里地震延伸到近100公里的深度，从而允许调查流体释放与瓦达蒂-贝尼奥夫地震产生之间的关系。流体的运输也可能是间歇性震颤和滑动（ETS）的主要驱动因素，在卡斯卡迪亚观测到的这种现象可能比地球上任何其他地方都好，包括本实验中的震源区域。已知震源区域的震颤测量与GPS数据和现有长基线倾斜仪得出的滑动分布相结合。与地震成像一起，这些观测结果产生了一个无与伦比的数据集，用于确定震颤、滑动和成像表明下行板块变质或上覆地幔楔水化的区域之间的关系。基础实验有四个组成部分：一个集成了“大脚”的柔性阵列仪器的宽带成像阵列，三个靠近非火山震颤源的小孔径地震阵列，对PBO和PANGA GPS数据集的分析，以定义幕式滑动事件的细节，以及综合建模。宽带阵列的特点是在卡斯卡迪亚俯冲系统的一部分，包括中深度地震和Nisqually震源，在交错配置中，允许沿着走向的影响进行测试。这是两条交叉线的补充，一条穿过地壳似乎正在脱水的板块，另一条在喀斯喀特山麓取样弧的根部。地震和GPS阵列尽可能与宽带成像同时配置，以便同时定位地震和滑动并成像其震源区域。这些数据需要进行相应的分析，包括对地震波波前定向的阵列分析、远震散射波的迁移、Vp、Vs和Q的层析成像、剪切波分裂、地震重新定位、与地震板相互作用的高频相位的研究，以及为瞬态事件的检测和量化而设计的专门的GPS处理。这些结果与卡斯卡迪亚俯冲系统的详细岩石-热模型相结合进行了解释。这些结果对下行板内的脱水路径、中深度构造与地震活动性的关系、瞬态应变事件与构造的关系、地幔楔的温度、熔体和挥发物含量以及大陆地壳的生长等问题提出了新的约束。

0545441CreagerIn this project, an integrated field experiment is undertaken in the Cascadia subduction zone to elucidate the relationship between water transport, aseismic slip, episodic tremor, and arc magmatism. The ultimate goal is to explore H2O processes in subduction zones using the tools of seismology, geodesy and petrology, and to integrate these results with complementary constraints from geodynamics and geochemistry. Seismic imaging is employed to illuminate (i) the descending oceanic plate where it metamorphoses, and (ii) the mantle wedge where fluids may be producing hydrous phases such as serpentine or, beneath the volcanic arc, primary magmas. The experiment is designed to traverse the one part of the Cascadia system where earthquakes extend to nearly 100 km depth, thus permitting an investigation of the relationship between the release of fluids and the generation of Wadati-Benioff-zone earthquakes. The transport of fluids may be also a primary driver for episodic tremor and slip (ETS), a phenomenon observed in Cascadia perhaps better than anywhere else on the planet, including source regions within this experiment. Measurements of tremor from known source regions are integrated with slip distributions derived from GPS data and existing long-baseline tiltmeters. Together with the seismic imaging, these observations yield an unparalleled data set for determining the relationship between tremor, slip and the regions where imaging indicates metamorphism of the down-going plate or hydration of the overlying mantle wedge.The basic experiment has four components: a broadband imaging array of flexible-array instruments integrated with Bigfoot, three small-aperture seismic arrays near sources of non-volcanic tremor, analysis of the PBO and PANGA GPS data sets to define the details of episodic slip events, and integrative modeling. The broadband array features a dense transect across the part of the Cascadia subduction system that includes intermediate-depth earthquakes and the Nisqually earthquake hypocenter, in a staggered configuration to allow along-strike effects to be tested. This is complemented by 2 cross lines, one crossing the slab where the crust appears to be dehydrating, and one in the Cascades foothills to sample the roots of the arc. The tremor and GPS arrays are collocated with the broadband imaging as much as possible, to allow simultaneous location of tremor and slip and imaging of their source region. These data are subject to the gamut of analyses appropriate to such data, including array analysis for wave-front orientation of tremor waves, migration of teleseismic scattered waves, tomographic images of Vp, Vs and Q, shear-wave splitting, earthquake relocation, investigation of high-frequency phases interacting with the slab, and specialized GPS processing designed for the detection and quantification of transient events. The results are interpreted in conjunction with detailed petrological-thermal models of the Cascadia subduction system. These results are used to place new constraints on the dehydration pathways within the down-going plate, the relationship between structure and seismicity at intermediate depths, the relationship between transient strain events and structure, the temperature, melt and volatile content of the mantle wedge, and the growth of continental crust.