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

通过脱水、温度和应变评估板片地震活动的原因：全球方法

• 批准号: 1246955
• 负责人: Clifford Thurber
• 金额: $ 10.66万
• 依托单位: University of Wisconsin-Madison
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2013
• 资助国家: 美国
• 起止时间: 2013-01-01 至 2014-12-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1246955&HistoricalAwards=false
• 关键词: Assessing; causes; slab; seismicity; through

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.

地球的大部分地震活动集中在俯冲带，无论是在浅冲断带或中深度（~50至300公里深度）或深（~600公里深度）板状地震活动。中深度地震活动通常被认为是由脱水脆化引起的，其中俯冲地壳或地幔中含水矿物的脱水增加了低渗透性岩石中的孔隙流体压力，使岩石表现出脆性;尽管最近的研究由于缺乏支持的实验室观察而对这一假设提出了质疑。在某些俯冲板块中，双地震带是明显的（例如，但它们在其他板块中的存在不太清楚，这是由于震源位置相对于两层地震活动性之间的间距的相对较大的误差，这在不同的板块之间是不同的。由于低层地震活动被认为是由板幔中的蛇纹岩脱水引起的，对全球双地震带普遍存在的有限认识限制了对板幔蛇纹岩化和脱水的约束。在这个项目中，我们将重新定位54个小弧段内的地震活动性，并将这些地震活动性分布与现有的二维热模型和矿物模型进行比较，重点关注板状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，这将大大减少分散并改善地震震源的绝对位置。通过更好地理解中深地震活动及其与热模型和岩石学模型的关系，我们可以深入了解板块中的孕震过程，并有助于理解与俯冲带有关的六个关键问题：1）什么现象是大多数中深地震活动的原因？2)哪里可以观测到双重地震带？3)在双地震带中，是什么导致了低层地震活动？4)是什么控制着板块地震活动的最大深度？5)板块地震活动的分布与火山活动的变化有关吗？6)板状地震活动与深部板块耦合变化之间是否有关系？地球的大部分地震活动集中在俯冲带，在那里，寒冷，致密的海洋地壳和地幔下降到一个更有浮力的板块之下，有能力产生大地震和火山活动。地震活动发生在浅冲断带，或作为中深度（~50至300 km深度）或深（~600 km深度）板状地震活动。引起浅层地震的机制相对较好地理解，而中深层地震活动的原因则不太确定。这部分是由于地球材料在高压和高温下的预期韧性行为，这些条件在实验室中难以重现。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性的观测相结合，我们可以更好地理解什么样的板块条件与观测到的地震活动性最密切相关。在本项目中，我们将使用更准确地再现地震波传播路径并减少现有全球地震活动目录中地震位置错误的技术重新定位中深度地震。然后，我们将全面比较板地震活动性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，最有助于创造潜在的大型中深地震。