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

在这个项目中，在卡斯卡迪亚俯冲带进行了一项综合的野外实验，以阐明水的输送、无地震滑动、幕式地震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的工具探索俯冲带中的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)下降的大洋板块在那里变质，以及(Ii)地幔楔形，在那里流体可能产生水合相，如蛇纹岩，或在火山弧下产生原生岩浆。该实验旨在穿越卡斯卡迪亚系统的一部分，那里的地震延伸到近100公里深，从而能够调查流体释放与瓦达蒂-贝尼奥夫带地震发生之间的关系。流体的运输也可能是阵发性震颤和滑动(ETS)的主要驱动因素，卡斯卡迪亚观测到的这一现象可能比地球上任何其他地方都要好，包括本实验中的震源区。从已知震源区测量的震颤与由GPS数据和现有长基线倾斜仪得出的滑移分布相结合。与地震成像一起，这些观测产生了一个无与伦比的数据集，用于确定地震、滑动和成像指示下行板块变质或上覆地幔楔体水化的区域之间的关系。基本实验包括四个组成部分：与BigFoot集成的柔性阵列仪器的宽带成像阵列，非火山地震震源附近的三个小口径地震阵列，用于确定幕式滑动事件细节的PBO和Panga GPS数据集的分析，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有一个密集的横断面，其中包括中深地震和尼斯奎利地震震源，呈交错配置，以便测试沿走向的影响。另外还有两条交叉线，一条穿过地壳似乎脱水的地方，另一条在喀斯喀兹山脚下，采样弧形的根部。地震和GPS阵列尽可能与宽带成像并置，以便同时定位地震和滑动，并对其震源区进行成像。这些数据受到适用于这些数据的各种分析的影响，包括对震动波的波前取向的阵列分析、远震散射波的偏移、VP、VS和Q的层析成像、剪切波分裂、地震重新定位、与板块相互作用的高频相的调查，以及为检测和量化瞬变事件而设计的专门的GPS处理。结合卡斯卡迪亚俯冲系统的详细岩石学-热学模型对结果进行了解释。这些结果被用来对下行板块内的脱水路径、中深部构造与地震活动的关系、瞬变应变事件与构造的关系、地幔楔体的温度、熔融和挥发分含量以及陆壳的生长施加新的约束。

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.