Seismicity and Structure of Subducting Slabs

俯冲带承载了地球大部分的地震矩释放。虽然大部分地震是板块之间沿着板块界面运动的结果，但由于俯冲板块内部变形，也会发生显著的地震活动。虽然它们很少达到与俯冲巨型逆冲断层相关的地震的规模或频率，但这些地震通常位于主要俯冲带向陆地方向的城市深处，具有破坏性的潜力，如1970年7.9级安卡什地震。（秘鲁--南美历史上最致命的地震）、2001年7. 7级的萨尔瓦多地震，以及最近，2017年7.1级普埃布拉/墨西哥城地震（例如，Melgar等人，2018年）。在许多情况下，此类地震的发生仍然令人惊讶，许多地区下行板块承载此类大规模地震的能力尚不确定。这些地震是由大规模板块驱动力产生的应力、板块几何形状变化产生的局部应力（例如，板块倾角、板块撕裂等的变化），以及当俯冲板块下降到地球内部时与俯冲板块的结构和演化有关的流变学因素（Hacker等人，2003; Abers等人，2013年）。了解这种地震的分布的关键是我们能够准确地绘制出它们在板块内的位置，特别是相对于彼此，以及俯冲板块的表面（例如，Abers等人，2013年）。这个项目最初将着重于改进若干区域个案研究的地震目录（从南美洲开始），准确地绘制俯冲洋板块的发震结构、板块内活动断层的位置和破坏方式以及板块内应力和应变分布的变化。这项工作将主要通过对全球地震数据进行详细分析和建模来完成，并在可获得和有用的情况下纳入当地获得的数据。所需的许多技术以前已经开发，但学生将调整现有的方法，以满足研究和数据集的要求，并将参与开发新的方法来评估地震数据的要求。该项目还将通过结合地震观测和地球动力学建模，回答这些地震与板块的地球动力学背景及其结构和流变学演变的关系问题（Hacker等人，2003年）。改进控制板地震活动的位置将使我们能够发展我们的理解，这些地震如何与板的热结构，因为它开始升温，矿物相变，板在俯冲过程中经历，这些过程可能在本地化或触发地震活动中发挥的作用。最后一个目标是了解在这种情况下发生的最大地震（M7-8）与其附近的背景地震活动之间的关系。特别是，解决这些地震如何激活发震板块的如此大的部分，以及它们的空间发生是否可以预测，并纳入地震灾害模型的问题。了解这些地震的破裂程度与板块的孕震结构的比较，将使我们能够解决这样一个问题，即这些地震可以告诉我们什么关于板块的地球动力学（这样的地震是否破裂了孕震板块的全部范围？它们是否会在两个双重地震带中破裂？），并且通过与较小的地震活动性（例如，克雷格等人，2014）。该项目的这一方面可能涉及对这些地震进行更复杂的地震学建模，并考虑其余震序列。

Subduction zones host the majority of the Earth's seismic moment release. Whilst much of this is the result of motion between plates along the plate interface, significant seismicity also occurs within subducting plates, as they deform internally. Although they rarely reach the size or frequency of earthquakes associated with the subduction megathrust, these earthquakes, often located deep beneath cities landwards of major subduction zones, have the potential to be devastating, as seen in the magnitude 7.9 1970 Ancash (Peru - the deadliest earthquake in South American history), the 2001 magnitude 7.7 El Salvador earthquake, and, more recently, the magnitude 7.1 2017 Puebla/Mexico City earthquake (e.g., Melgar et al., 2018). In many cases, the occurrence of such earthquakes remains a surprise, with the capacity of the downgoing plate to host such large-magnitude earthquakes uncertain in many regions. These earthquakes arise from the combination of the stresses derived from large-scale plate-driving forces, localised stresses arising from changes in slab geometry (e.g., changes in slab dip, slab tears, etc), and rheological factors relating to the structure and evolution of the subducting plate as it descends into the Earth's interior (Hacker et al., 2003; Abers et al., 2013). Critical to understanding the distribution of such earthquakes is our ability to accurately map out their location within the plate, in particular with respect to each other, and to the surface of the subducting plate (e.g., Abers et al., 2013). This project will initially focus on improving earthquake catalogues for a number of regional case studies (starting with South America), accurately mapping out the seismogenic structure of the subducting oceanic plate, the location and mode of failure of active faults within the plate, and how the distribution of stress and strain vary within the plate. This will principally be done through the detailed analysis and modelling of global seismic data, incorporating locally-acquired data where available and useful. Many of the techniques required have been previously developed, but the student will tailor existing approaches to the requirements of the study and datasets available, and will be involved in the development of new approaches to the assessment of the seismic data as required. The project will also aim to answer the question of how these earthquakes relate to the geodynamic setting of the slab, and its structure and rheological evolution, through the combination of seismological observations and geodynamic modelling (Hacker et al., 2003). Improved controls on the location of slab seismicity will allow us to develop our understanding of how these earthquakes relate to the thermal structure of the slab as it starts to heat up, and the mineralogical phase transitions that the slab undergoes during subduction, and the role that these processes may play in localising or triggering seismicity. A final aim will be to understand the relationship between the largest earthquakes occurring in such settings (M7-8), and the background seismicity in their vicinity. In particular, addressing the issue of how these earthquakes activate such large sections of the seismogenic slab, and whether their spatial occurrence can be predicted, and incorporated into seismic hazard models. Understanding the rupture extent of these earthquakes in comparison to the seismogenic structure of the slab, will allow us to address the question of what these earthquakes can tell us about both the geodynamics of the slab (Do such earthquake rupture the full extent of the seismogenic slab? Do they rupture through both double seismic zones?), and may also throw light on their causative mechanism, through comparison to smaller seismicity (e.g., Craig et al., 2014). This aspect of the project may involve more complex seismological modelling of these earthquakes, and consideration of their aftershock sequences.