A Seismic Investigation of the Magmatic and Hydrothermal Systems of Nabro Volcano and other East African Rift Volcanoes

火山地震学的基本目标之一是发展对活动岩浆系统动力学的了解。通过研究火山的地震活动，有可能绘制出火山地下的地图：确定是什么驱动和供给了岩浆系统，并追踪其随时间的演变。这对评估火山危害和预测喷发行为具有重要意义。了解火山流体如何烧结对于评估火山的地热能生产潜力也是至关重要的。地震层析成像是一种强大的地球物理工具，用于在各种尺度上对地球内部进行成像。该技术对地震资料进行反演，以约束二维或三维P波和S波的速度结构及其波速比(Vp/Vs)。为了更好地理解导致火山动荡的地下活动岩浆过程(例如Greenfield等人，2016年；Wilks等人，2020年)，它经常在地方范围内应用于使火山变形的火山。对火山下方地震速度结构的了解有助于识别深部含熔体区域和流体，限制岩浆和热液系统之间的相互作用。横波分裂是另一种越来越多地用于研究火山活动，特别是地下应力变化的地震方法(例如，Nowacki等人，2018年)。当地震波传播通过各向异性材料时，就会发生横波分裂；通过测量地震波传播速度和方向的变化，可以表征地下的各向异性。测量地震各向异性提供了动态过程的指示，例如形变，这对于其他技术来说是不可见的(Wookey，2012)。纳布罗火山是一座远离裂谷的火山，位于埃塞俄比亚-厄立特里亚边界附近达纳基尔微板块的中央部分(Hamlyn等人，2014年)。尽管纳布罗火山在2011年喷发，但其岩浆管道系统的结构仍然是一个谜。总的来说，仍不清楚岩浆是如何供应给远离扩张轴线的火山的；此外，它们在适应伸展方面的作用也不清楚(Maccaferri等人，2014年)。2011年8月至2012年10月在纳布罗周围建立的地震网络产生了14个月的地震数据。这些数据以前曾被Hamlyn等人部分分析过。(2014年)；然而，由于人工挑选P波和S波到达量的耗时性质，大部分地震资料尚未编目。最近，一种新的基于卷积神经网络的深度学习模型被应用于来自纳布罗的地震数据(Lapins等人，2020)。这产生了33,950个事件目录(图1)，远远超过以前人工识别的事件目录，表明NaBro是地震层析成像反演和横波分裂各向异性分析的主要条件。除了纳布罗，东非裂谷系统中还有其他火山，它们将成为地震调查其地下过程的有趣目标。例如，Corbetti火山和Aluto火山都进行了几个月的地震监测，因此有大量未分析的地震数据可供利用。Aluto目前是一种可行且多产的地热资源(Hochstein等人，2017年)，并计划在Corbetti建立一座地热发电厂；对这些火山的热液和岩浆系统的分析将有助于优化地热生产。

One of the fundamental objectives of volcano seismology is to develop an understanding of the dynamics of active magmatic systems. By studying the seismicity of a volcano, it is possible to map the volcanic subsurface:determining what drives and feeds the magmatic system and tracing its evolution through time. This has important implications for assessing volcanic hazard and predicting eruptive behaviour. Understanding how volcanic fluidsinteract is also vital for evaluating a volcano's geothermal energy production potential. Seismic tomography is a powerful geophysical tool used to image the Earth's interior at a variety of scales. Thetechnique inverts seismic data to constrain the 2D or 3D P- and S-wave velocity structures and the ratio between them (Vp/Vs). It has been frequently applied on a local scale to deforming volcanoes, in order to better understandthe subsurface active magmatic processes responsible for volcanic unrest (e.g. Greenfield et al., 2016; Wilks et al., 2020). Knowledge of the seismic velocity structure beneath a volcano helps to identify melt-bearing regions andfluids at depth, constraining interactions between magmatic and hydrothermal systems. Shear wave splitting is another seismic method that is being increasingly applied to the investigation of volcanologicalprocesses, particularly stress variations in the subsurface (e.g. Nowacki et al., 2018). Shear wave splitting occurs when seismic waves propagate through an anisotropic material; by measuring the resulting variation inseismic wave speed and direction of propagation, the anisotropy of the subsurface can be characterised. Measuring seismic anisotropy provides an indication of dynamic processes, such as deformation, which are invisible to othertechniques (Wookey, 2012). Nabro is an off-rift volcano located on the central part of the Danakil microplate near the Ethiopia-Eritrea border (Hamlyn et al., 2014). Despite the fact that Nabro erupted in 2011, the structure of its magma plumbing system remains a mystery. In general, it is still unclear how magma is supplied to such volcanoes offset from the axis of spreading; further, their role in accommodating extension is not well understood (Maccaferri et al., 2014).A seismic network established around Nabro from August 2011 - October 2012 yielded 14 months of seismic data. These data have been partially analysed previously by Hamlyn et al. (2014); however, most of the seismicityhas not been catalogued, due to the time-consuming nature of manually picking P- and S-wave arrivals. A new deep learning model for automating phase arrival detection, based on convolutional neural networks, has recentlybeen applied to the seismic data from Nabro (Lapins et al., 2020). This has produced a catalogue of 33,950 events (Figure 1), far more than the previous catalogues of manually-identified events, indicating that Nabro is a primecandidate for both a seismic tomography inversion and a shear wave splitting anisotropy analysis. As well as Nabro, there are other volcanoes in the East African Rift System that would be interesting targets fora seismic investigation into their subsurface processes. For example, both Corbetti and Aluto volcanoes have been seismically monitored over several months and hence there is a wealth of unanalysed seismic data to exploit. Alutois currently a viable and productive geothermal resource (Hochstein et al., 2017), and there are plans to establish a geothermal power plant at Corbetti; an analysis of the hydrothermal and magmatic systems of these volcanoes willhelp optimise geothermal production.