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

俯冲带产生了地球上一些最大最具破坏性的地震和海啸。然而，在过去的二十年里，人们发现了一种全新的滑动机制，其中滑动发生的速度比板块运动速度快，但速度太慢，无法产生地震波。这些事件被称为慢滑事件（sse），鉴于它们最近的发现，我们还不知道sse是否会抑制或增加未来具有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，需要流体积聚来促进这种缓慢滑动。目前缺少的是对板块边界断裂带和缓慢滑动断裂带上的岩石的物理性质的直接测量，以确定这些地区是否确实存在流体积聚（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最具特色和最浅的sse。位于sse区域上方的吉斯本镇经历了猛烈的泥火山喷发、气体渗漏和地表不稳定。这些地质特征表明地下存在流体和高流体压力；然而，现有的高分辨率地球物理研究有限，无法证实这是否属实。造成这些不稳定性和流体泄漏的浅深度流体积聚（<3公里深）也可能与与慢滑事件相关的深层流体有关。2017-2018年，在北Hikurangi边缘部署了50个宽带地震仪和145个短周期仪器。这些阵列记录了发生在这个地震活跃地区的许多局部地震，并在2018年12月一次大型泥火山爆发前两个月捕获了有关地下状况的信息。我们正在寻找一名具有地球物理学背景和对构造过程感兴趣的学生来分析这个大型数据集，以了解Hikurangi边缘的物理特性和各向异性，从而更好地了解该地区的结构以及泥火山和慢滑的影响。学生将i)编制当地地震目录，ii)从当地地震数据进行断层扫描以开发速度模型，iii)使用地震分裂技术来了解各向异性，iv)根据区域地质和构造解释这些发现。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有慢滑事件的地区的地震风险评估。该学生将在新西兰与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，将鼓励学生在市政厅会议上向包括地方议会、iwi、民防和公众在内的听众介绍他们的研究结果。

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.