Collaborative Research: Unlocking the secrets of slow slip by drilling at the northern Hikurangi subduction margin, New Zealand: CORK observatory development and installation

新西兰北岛东部近海的希库朗伊俯冲带的特征为了解海洋板块在大陆地壳下逆冲（或“俯冲”）时应力如何产生和释放的基本方面提供了全球独特的窗口。最近，地震学家已经认识到俯冲巨型逆冲断层在偶发的“慢滑事件”中经历滑动。在过去十年发现慢滑移事件之前，我们的断层滑移范式主要集中在大地震中的地震滑移。自从发现慢滑移以来，大多数研究都集中在深层（20-40公里深度）慢滑移事件，但我们现在意识到浅层慢滑移事件（5-15公里深度）的存在更容易进行近源调查。这些浅层慢断层滑动事件集中在海洋下方，因此陆上仪器无法测量发生了多少表面变形。这项研究利用计划中的国际海洋发现计划钻探，在两个钻孔中安装传感器，测量表明间歇性断层活动引起的浅地壳变化的特性。 钻孔仪器的高级研究人员将指导职业生涯中期的科学家，传授此类复杂实施和研究的广泛知识。日本、新西兰和德国的合作者将与美国调查人员一起参与这项国际研究，当地社区将受益于与新西兰附近游轮相关的科学家外展活动。这项研究是作为“社区提案”进行的，所有数据将立即公开。地球上最浅、有据可查的慢滑移事件发生在新西兰吉斯伯恩近海的希库朗吉北部边缘，深度为 5-15 公里。它们大约每 1-2 年复发一次，板块边界界面上的滑移可达 15-20 厘米。吉斯伯恩慢滑移事件异常接近海底、大的慢滑移事件震级（相当于地震震级 Mw ~6.3-6.8）和短的复发间隔，使其成为评估近海俯冲边缘对大型、频繁、有据可查的变形瞬变的近源水文、地球化学和热响应的理想环境。钻孔观测数据将阐明流体压力、流体流动和温度在慢滑移事件发生中所起的作用，以及慢滑移事件变形对弧前流体和地球化学循环的影响。该项目将建立钻孔设施并建立开放数据访问。对 ROV 恢复的前两年数据进行分析，将评估慢滑移源附近的变形、水文和地球化学响应，并记录浅层慢滑移的热状态。其中一个钻孔装置将瞄准巨型逆冲断层浅部周围的区域，而第二个钻孔中的传感器将瞄准慢滑移事件源上方约 5 公里的覆盖板块。

The Hikurangi subduction zone offshore the eastern North Island of New Zealand has characteristics that provide a globally unique window into a fundamental aspect of how stress builds up and is relieved as an oceanic plate thrusts (or ?subducts?) beneath continental crust. Recently, seismologists have recognized that subduction megathrust faults undergo slip in episodic ?slow slip events?. Prior to the discovery of slow slip events in the last decade, our paradigms of fault slip were focused on seismic slip in major earthquakes. Since the discovery of slow slip, most studies have focused on deep (20-40 km depth) slow slip events, but we now realize that shallow slow slip events (5-15 km depth) exist that are much more accessible to near-source investigations. These shallow slow fault slip events are centered beneath the ocean so onshore instruments have not been able to measure how much surface deformation occurs. This study takes advantage of planned International Ocean Discovery Program drilling to install sensors in two boreholes that will measure properties indicative of change in the shallow crust that the episodic fault activity causes. Senior researchers in borehole instrumentation will mentor mid-early career scientists, passing along extensive knowledge of such complex implementation and research. Japanese, New Zealand, and German collaborators will participate with the US investigators in this international study, and local communities will benefit from scientist outreach associated with cruises off New Zealand. This study is being undertaken as a ?community proposal?, whereby all data will be immediately publicly available. The shallowest, well-documented slow slip events on Earth occur at the northern Hikurangi margin, offshore Gisborne, New Zealand, at 5-15 km depth. They recur approximately every 1-2 years, with slip on the plate boundary interface of up to 15-20 cm. The unusually close proximity of the Gisborne slow slip events to the seafloor, the large slow slip event magnitudes (equivalent to an earthquake magnitude Mw ~6.3-6.8), and short recurrence intervals makes this the ideal setting for evaluating the near-source hydrological, geochemical, and thermal response of the offshore subduction margin to large, frequent, well-documented deformation transients. The borehole observatory data will clarify the role that fluid pressures, fluid flow, and temperature play in the occurrence of slow slip events, as well as the influence of slow slip event deformation on fluid and geochemical cycling within the forearc. This project will establish the borehole facility and set-up open data access. Analysis of the first two years of data, recovered by ROV, will evaluate the deformation, hydrological, and geochemical responses near the source of slow slip, as well as document the thermal regime of shallow slow slip. One of the borehole installations will target the region surrounding the shallow portion of the megathrust fault , while sensors in a second borehole will target the over-riding plate ~5 km above the slow slip event source.