Seismicity and Structure of Subducting Slabs

俯冲带承载了地球上大部分的地震力矩释放。虽然这很大程度上是板块之间沿板块界面运动的结果，但当俯冲板块内部变形时，也会发生重大的地震活动。尽管这些地震很少达到与俯冲大逆冲带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陆地的城市深处，具有破坏性的潜力，如1970年安卡什7.9级地震（秘鲁-南美洲历史上最致命的地震），2001年萨尔瓦多7.7级地震，以及最近的2017年普埃布拉/墨西哥城7.1级地震（例如，Melgar等人，2018）。在许多情况下，这种地震的发生仍然是一个意外，因为在许多地区，下行板块承载如此大震级地震的能力是不确定的。这些地震是由大尺度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、板块撕裂等变化）以及与俯冲板块下降到地球内部时的结构和演化相关的流变因素共同作用而产生的（Hacker et al., 2003; Abers et al., 2013）。了解此类地震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表面的位置（例如，Abers等人，2013年）。该项目最初将侧重于改进一些区域案例研究的地震目录（从南美洲开始），准确绘制出大洋俯冲板块的发震结构，板块内活动断层的位置和失效模式，以及板块内应力和应变的分布如何变化。这将主要通过对全球地震数据的详细分析和建模来完成，并在可用和有用的情况下结合当地获得的数据。所需的许多技术以前已经开发出来了，但学生将根据研究和可用数据集的要求定制现有方法，并将根据需要参与开发评估地震数据的新方法。该项目还旨在通过地震学观测和地球动力学建模的结合，回答这些地震与板块的地球动力学背景及其结构和流变演化之间的关系（Hacker et al., 2003）。对板块地震活动位置的改进控制将使我们能够进一步了解这些地震与板块开始升温时的热结构之间的关系，以及板块在俯冲过程中所经历的矿物相变，以及这些过程在定位或触发地震活动方面可能发挥的作用。最后的目标是了解发生在这些地区的最大地震（7-8级）与附近地区的背景地震活动性之间的关系。特别是，解决这些地震如何激活如此大的发震板块的问题，以及它们的空间发生是否可以预测，并纳入地震危险模型。了解这些地震的破裂程度与板块的发震结构的比较，将使我们能够解决这些地震可以告诉我们关于板块的地球动力学的问题(这样的地震会使发震板块的全部范围破裂吗？它们是否穿过两个地震带破裂？)，并且通过与较小地震活动的比较（例如，Craig et al., 2014），也可能阐明它们的致病机制。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模拟，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.