Collaborative Research: High-Resolution Study of Subduction Zone Seismicity and Structure

俯冲带是地球板块构造体系的重要组成部分。关于俯冲带的两个重大国际科学倡议的存在就反映了这一点：俯冲工厂(SubFac)和孕震区实验(SEQUE)，这两个倡议都是边缘倡议的一部分。SubFac倡议推动了在候选板岩成分(特别是水合相)的稳定性领域及其与中深度地震活动的关系方面的大量矿物学和岩石学工作。一种可行的假设是，中深度地震与俯冲板块中的变质脱水反应有关。对这一假设的检验是不完善的，主要有四个原因--板坯热结构、板坯成分、板坯地震速度结构和地震位置的不确定性。后者影响地震位置，也影响板条模型中的地震速度与候选矿物学之间的关联。该项目涉及对世界各地俯冲带的多方面地震学调查。项目的主要目标是完善下行板块内地震活动分布的知识，并改进其结构的地震模型。该项目正在应用的分析方法包括：(1)改进的波形对准技术；(2)高精度远震和局部地震定位；(3)相关震源机制确定；(4)双差(DD)地震层析成像。方法1、2和3适用于广泛的俯冲带，特别是在存在或不存在双地震带的问题上。方法1和方法4被应用于选定的一组俯冲带，在那里有足够的当地地震数据可用--智利北部、阿拉斯加、哥斯达黎加和台湾。前人的工作表明，用双谱波形对准方法验证的波形互相关(WCC)时延比用标准阈值准则选择的WCC时延具有更好的定位结果。预计这一改善有两个主要原因。首先，不可靠的WCC时延数据可以通过双谱检查来去除。其次，双谱验证过程可以为两个接近的地震提供更多的时间延迟测量，从而更好地控制它们的相对位置。根据之前对远震和本地事件所做的工作，可以预期相对位置散布的减少将接近一个数量级。将精细的波形对准技术和新的DD地震层析成像方法应用于几个俯冲带的局部数据集，以锐化地震速度结构的图像。所有拟议的目标区都是一个或多个以前地方地震层析成像研究的对象，因此有适合进行DD层析成像的数据集。根据应用DD层析成像方法对本州北部数据进行编目的初步结果，可以预期产生前所未有的清晰度的俯冲带地震图像。该项目将从根本上促进对俯冲带和相关过程的了解。俯冲带是驱动地球演化的板块构造引擎的一个基本特征。下行板的结构、应力状态和演化已经受到了大量的科学关注，但仍然相对知之甚少。该项目将提供关于下行板块内地震活动分布的改进信息，以及板块和上覆地幔楔体的地震结构的详细模型，这是地球动力学和岩石学模型的关键制约因素。特别是，关于中等深度地震的性质，有几种相互矛盾的假说。通过对几个不同俯冲带的详细研究，将获得支持或反对这些假设的地震证据。该项目将利用华盛顿大学麦迪逊分校由国家科学基金会支持的新的研究、教学和学习一体化中心(CIRTL)，根据研究结果编写基于网络的外展材料。软件和数据将与其他科学家共享，特别是通过提供使用DD断层扫描的培训。

Abstract Subduction zones are one of the most important components of the Earth's plate tectonic system. This is reflected by the existence of two major international science initiatives focused on subduction zones: the Subduction Factory (SubFac) and the Seismogenic Zone Experiment (SEIZE), both part of the MARGINS initiative. The SubFac initiative has motivated substantial mineralogical and petrologic work on stability fields of candidate slab constituents (especially hydrated phases) and their relation to intermediate-depth seismicity. One working hypothesis is that the intermediate-depth earthquakes are related to metamorphic dehydration reactions in the subducting slab. Tests of this hypothesis are imperfect for four main reasons - uncertainties in slab thermal structure, slab composition, slab seismic velocity structure, and earthquake locations. The latter impacts earthquake locations and also affects the association between seismic velocities in slab models and candidate mineralogies. This project involves a multi-faceted seismological investigation of subduction zones around the world. The principal project goals are refining knowledge of the distribution of seismic activity within down-going slabs and improving seismic models of their structure. Analysis methods that are being applied in the project include (1) improved waveform alignment techniques, (2) high-precision teleseismic and local earthquake location, (3) associated focal mechanism determination, and (4) double-difference (DD) seismic tomography. Methods 1, 2 and 3 are applied to a broad suite of subduction zones, especially where the presence or absence of a double seismic zone is in question. Methods 1 and 4 are applied to a selected set of subduction zones where adequate local earthquake data are available - Northern Chile, Alaska, Costa Rica, and Taiwan. Previous work indicates that waveform cross-correlation (WCC) time delays verified with the bispectrum waveform alignment method provide better relocation results than those selected with the standard threshold criterion. This improvement is expected for two main reasons. First, unreliable WCC time delay data can be removed using bispectrum checking. Second, the bispectrum verification process can provide more time delay measurements for two close earthquakes and thus more control over their relative positions. Based on previous work with both teleseismic and local events, close to an order of magnitude reduction in relative location scatter can be expected. Refined waveform alignment techniques and a new DD seismic tomography method are applied to local datasets from several subduction zones to sharpen images of seismic velocity structure. All of the proposed target areas have been the subject of one or more previous local earthquake tomography studies, and thus have datasets suitable for DD tomography. Based on initial results applying the DD tomography method to catalog data from northern Honshu, the production of subduction zone seismic images of unprecedented clarity can be anticipated. This project will contribute in a fundamental way to the understanding of subduction zones and related processes. Subduction zones are a fundamental feature of the plate tectonic engine that drives the evolution of the Earth. The structure, state of stress, and evolution of down-going slabs are topics that have received substantial scientific attention yet remain relatively poorly understood. This project will provide improved information on the distribution of seismic activity within down-going slabs along with detailed models of the seismic structure of the slab and overlying mantle wedge, which are critical constraints for geodynamic and petrologic models. In particular, there are several competing hypotheses about the nature of intermediate-depth earthquakes. Through a detailed study of several different subduction zones, seismic evidence for or against these hypotheses will be obtained. This project will take advantage of the new NSF-supported Center for the Integration of Research Teaching and Learning (CIRTL) at UW-Madison in preparing web-based outreach materials based on the research results. Software and data will be shared with other scientists, particularly by providing training on the use of DD tomography.