Seismicity and Structure of Subducting Slabs

俯冲带是地球地震时刻释放的主要场所。虽然这在很大程度上是板块之间沿着板块界面运动的结果，但在俯冲板块内部变形时，也会发生重大的地震活动。虽然它们很少达到与俯冲巨型逆冲有关的地震的规模或频率，但这些地震往往位于主要俯冲带的城市下方深处，具有毁灭性的潜力，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级地震以及更近的2017年普埃布拉/墨西哥城7.1级地震(如梅尔加等人，2018年)。在许多情况下，这种地震的发生仍然令人惊讶，在许多地区，下行板块容纳这种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(例如板块倾角变化、板块撕裂等)产生的局部应力以及与俯冲板块下降到地球内部时的结构和演化有关的流变因素共同引起的(Hacker等人，2003年；Abers等人，2013年)。对于理解这种地震的分布，关键是我们有能力准确地绘制出它们在板块内的位置，特别是相对于彼此的位置，以及到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)，准确地绘制俯冲大洋板块的孕震结构、板块内活动断层的位置和破裂方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进行详细分析和建模，在现有和有用的情况下纳入当地获得的数据。许多所需的技术都是以前开发的，但学生将根据研究和可用的数据集的要求定制现有的方法，并将根据需要参与开发新的地震数据评估方法。该项目还将结合地震学观测和地球动力学模型，回答这些地震如何与板块的地球动力学背景及其结构和流变演化有关的问题(Hacker等人，2003年)。对板块地震活动位置的改进控制将使我们能够加深对这些地震如何与板块开始升温时的热结构有关的理解，以及板块在俯冲过程中经历的矿物学相变，以及这些过程在定位或触发地震活动中可能发挥的作用。最终目的将是了解在这种情况下(7-8级)发生的最大地震与其附近背景地震活动之间的关系。特别是，处理这些地震如何激活孕震板块的如此大的部分的问题，以及是否可以预测其空间发生情况，并将其纳入地震危险模型。与板块的孕震结构相比，了解这些地震的破裂程度将使我们能够解决这样一个问题，即这些地震能告诉我们关于板块的地球动力学的什么信息(这样的地震是否完全破裂了孕震板块？它们是否穿过了两个双重地震带？)，并可能通过与较小的地震活动进行比较，揭示它们的成因机制(例如，Craig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模拟，并考虑它们的余震序列。

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.