Understanding slow slip earthquake mechanisms and seismic hazard using local earthquakes: Hikurangi margin, New Zealand

俯冲带产生了一些地球上最大和最具破坏性的地震和海啸。然而，在过去的二十年里，一种全新的滑动机制被发现，在这种机制中，滑动发生得比板块运动速度快，但速度太慢，不足以产生地震波。这些事件被称为慢滑事件(SSE)，考虑到它们最近的发现，我们还不知道SSE是否抑制或增加了未来发生破坏性地面震动的大地震的可能性。实验室和数值模拟实验表明，需要流体堆积来促进这种缓慢滑移。目前缺乏的是对板块边界断裂带和缓慢滑动断层上方的岩石的物理性质的直接测量，以确定这些地区是否真的存在流体堆积(或高孔隙流体压力)。沿新西兰北岛东海岸的Hikurangi俯冲带拥有一些世界上特征最好、最浅的SSE。位于SSES区域上方的是吉斯伯恩镇，该镇经历了猛烈的泥火山喷发、气体泄漏和地表不稳定。这些地质特征表明地下存在流体和高流体压力；然而，有限的高分辨率地球物理研究证实了这一点。造成这些不稳定的流体在浅层(3公里深)积聚，流体的逃逸也可能与与慢滑事件有关的较深层流体有关。2017-2018年，在Hikurangi北缘部署了50台宽带地震仪和145台短周期仪器。这些台阵记录了这一地震活跃区发生的许多当地地震，并在2018年12月一座大型泥火山喷发前两个月捕捉到了有关地下条件的信息。我们正在寻找一名具有地球物理背景并对构造过程感兴趣的学生来分析这一大型数据集，以了解Hikurangi边缘的物理性质和各向异性，以更好地了解该地区的结构以及对泥火山和慢滑的影响。学生将i)开发本地地震目录，ii)进行层析成像以根据本地地震数据开发速度模型，iii)使用地震分裂技术来了解各向异性，以及iv)根据区域地质和构造来解释这些发现。研究成果，即恢复断裂带和覆盖板块的物理性质，可用于直接更新新西兰和其他发生缓慢滑动事件的地点的地震风险评估。学生将在新西兰与GNS的项目合作伙伴一起工作，并在吉斯伯恩地区进行实地观察。在新西兰期间，学生将被鼓励在市政厅会议上向包括地方议会、iwi、民防和普通公众在内的观众展示他们的成果。

Subduction zones produce some of the largest and most destructive earthquakes and tsunami on Earth. However, in the last two decades a completely new slip mechanism has been discovered, in which slip occurs faster than the plate motion rate but too slowly to produce seismic waves. These events are called slow slip events (SSEs) and given their recent discovery we do not yet know whether SSEs inhibit or increase the likelihood of a future large earthquake with damaging ground shaking. Laboratory and numerical modelling experiments suggest that fluid build-up is needed to facilitate this slow slip. What is currently missing are direct measurements of physical properties across the plate boundary fault zone and the rocks lying above slowly slipping faults to establish if fluid build-up (or high pore-fluid pressure) really does exist in these areas.The Hikurangi subduction zone, along the east coast of the North Island of New Zealand hosts some of the most well-characterised and shallowest SSEs in the world. Lying above the zone of SSEs is the town of Gisborne, which has experienced violent mud volcano eruptions, gas seepage and land surface instability. These geological features point to fluids and high fluid pressure in the subsurface; however, limited high-resolution geophysical studies exist to confirm whether this is the case. Fluid build-up at shallow depths (<3 km deep) causing these instabilities and fluid escapes may also be linked to deeper fluids related to the slow slip events. In 2017-2018 an array of 50 broad-band seismometers and 145 short-period instruments were deployed across the north Hikurangi margin. These arrays recorded the many local earthquakes that occur in this seismically active region and have captured information about subsurface conditions two months before a major mud volcano eruptuion in Dec 2018. We are looking for a student with a background in geophysics and interest in tectonic processes to analyse this large dataset to learn about physical properties and anisotropy along the Hikurangi margin to better understand the structure of the region and the implications for mud volcanoes and slow slip. The student will i) develop a local earthquake catalogue, ii) conduct tomography to develop a velocity model from the local earthquake data, iii) use earthquake splitting techniques to learn about anisotropy and iv) interpret these findings in light of the regional geology and tectonics. The research deliverables, namely recovery of the physical properties of the fault zone and overriding plate, can be used to directly update seismic risk assessments for New Zealand and other locations that have slow slip events. The student will spend time in New Zealand working with project partners at GNS and conduct field observations in the Gisborne area. While in New Zealand the student will be encouraged to present their results in town-hall meetings to audiences including local councils, iwi, civil defence and general public.