Assessing the causes of slab seismicity through dehydration, temperature and strain: A global approach

地球的大部分地震活动集中在俯冲带，要么是浅冲断带，要么是中等深度(约50~300公里深)或深(~600公里深)的板条地震活动。中等深度的地震活动通常被认为是由脱水脆化引起的，即俯冲地壳或地幔中的含水矿物的脱水增加了低渗透率岩石中的孔隙流体压力，使岩石表现得脆弱；尽管最近的研究因缺乏实验室观察而对这一假说提出了质疑。双地震带在一些俯冲板块(如本州)中很明显，但在其他板块中不太明显，这是因为震中位置相对于两层地震活动之间的间距存在较大的误差，这在不同的板块之间是不同的。由于地震活动的下层被认为是由板片地幔中蛇纹石的脱水引起的，因此对全球双地震带盛行的认识有限，限制了对板片地幔蛇纹化和脱水的制约。在这个项目中，我们将重新定位54个小弧形截面内的远震地震，并将这些地震活动分布与先前存在的2D热和矿物学模型进行比较，重点放在板条矿物脱水最严重的位置。双差重定位技术teletomoDD将与嵌套的区域-全球三维速度模型一起使用，与现有的全球目录相比，这将显著减少分散并改善地震震中的绝对位置。通过更好地了解中深部地震活动及其与热学和岩石学模型的关系，我们可以深入了解板块的孕震过程，并有助于理解与俯冲带有关的六个关键问题：1)中深部地震活动的主要原因是什么现象？2)在哪里可以观察到双地震带？3)在双地震带中，什么是下层地震活动的原因？4)是什么控制了板块地震活动的最大深度？5)板块地震活动的分布是否与火山作用的变化有关？6)板块地震活动与深部板块耦合的变化是否有关系？地球的大部分地震活动集中在俯冲带，那里寒冷、致密的洋壳和地幔下降到一个更浮力的板块之下，具有产生大地震和火山活动的能力。地震活动要么发生在浅层逆冲带，要么表现为中深度(~50~300公里)或深(~600公里)的板片地震活动。引起浅层地震的机制相对较好，而中深度地震活动的原因则不太确定。这在一定程度上是由于地球材料在高压和高温下预期的延展性行为，而这些条件在实验室很难再现。然而，通过将俯冲地壳和地幔内的温度和矿物学反应的数值模型与地震活动观测相结合，我们可以更好地了解哪些板块条件与观测到的地震活动最密切相关。在这个项目中，我们将使用更准确地再现地震波传播路径的技术来重新定位中深度地震，并减少现有全球地震活动目录中地震位置的误差。然后，我们将全面比较板片地震活动的分布与现有的温度和矿物学模型，以确定哪些特定的反应对潜在的中深度大地震的产生做出了最大贡献。

The majority of Earth's seismicity is focused at subduction zones, either in the shallow thrust zone or as intermediate-depth (~50 to 300 km depth) or deep (to ~600 km depth) slab seismicity. Intermediate-depth seismicity is commonly thought to be caused by dehydration embrittlement, in which the dehydration of hydrous minerals in the subducted crust or mantle increases the pore fluid pressure in low-permeability rocks, allowing rocks to behave brittlely; although recent studies have called this hypothesis into question due to a lack up supporting laboratory observations. Double seismic zones are obvious in some subducting slabs (e.g., Honshu), but their presence in other slabs is less clear due to the relatively large errors in hypocentral locations relative to the spacing between the two layers of seismicity, which varies among different slabs. As the lower layer of seismicity is thought to be caused by the dehydration of serpentine in the slab mantle, having a limited view of the global prevalence of double seismic zones limits the constraints on slab mantle serpentinization and dehydration. In this project, we will relocate teleseismic earthquakes within 54 small arc sections and compare these seismicity distributions with pre-existing 2D thermal and mineralogical models, with a focus on the locations of greatest dehydration of slab minerals. The double-difference relocation technique teletomoDD will be used with nested regional-global 3D velocity models, which will significantly reduce scatter and improve the absolute locations of earthquake hypocenters in comparison to existing global catalogs. By better understanding intermediate-depth seismicity and its relation to thermal and petrological models, we can gain insight to the seismogenic processes in slabs and contribute to the understanding of six key questions pertaining to subduction zones:1) What phenomenon is responsible for the majority of intermediate-depth seismicity?2) Where are double seismic zones observable?3) In double seismic zones, what is responsible for the lower layer of seismicity?4) What controls the maximum depth of slab seismicity?5) Is the distribution of slab seismicity correlated with changes in volcanism?6) Is there a relationship between slab seismicity and changes in deep plate coupling?The majority of Earth's seismicity is focused at subduction zones, where cold, dense oceanic crust and mantle descend beneath a more buoyant plate, with the ability of producing large earthquakes and volcanism. Seismicity occurs either in the shallow thrust zone, or as intermediate-depth (~50 to 300 km depth) or deep (to ~600 km depth) slab seismicity. The mechanisms that cause shallow earthquakes are relatively well understood, whereas the cause of intermediate-depth seismicity is less certain. This is due in part to the expected ductile behavior of Earth materials at high pressures and temperatures, conditions that are difficult to reproduce in the laboratory. However, by combining numerical models of temperature and mineralogical reactions within subducting crust and mantle to observations of seismicity, we can better understand what slab conditions are most closely associated with observed seismicity. In this project, we will relocate intermediate-depth earthquakes using techniques that more accurately reproduce the paths of seismic wave propagation and reduce errors in earthquake locations from existing global seismicity catalogs. We will then comprehensively compare the distributions of slab seismicity with existing temperature and mineralogical models to identify which particular reactions contribute most to the creation of potentially large intermediate-depth earthquakes.