Collaborative Research: Unraveling the habitat and dynamics of slow slip events through integrated borehole observations in the northern Hikurangi subduction margin

俯冲带板块边界，一个构造板块俯冲或俯冲到另一个构造板块之下，就会产生世界上最大的地震和海啸，2004年印度洋9.2级地震和2011年日本北部近海9.0级东北冲绳地震就是鲜明的例证。最近，科学家们已经认识到，俯冲巨型逆冲断层在幕式慢动作地震中发生滑动，或持续数天至数月的缓慢滑动事件。在某些情况下，慢滑事件被观察到是在大俯冲地震之前(并可能触发)的，这增加了理解它们的必要性。发生在靠近地球表面(5-15公里深)的慢滑事件存在于新西兰近海的Hikurangi俯冲带，是近源调查唯一可以接触到的地方。2018年，在Hikurangi俯冲区的国际海洋发现计划科学钻探考察期间安装了两个观测站(穿透海底400米以下)。天文台的传感器现在正在连续测量慢滑事件源头附近地壳性质的时间变化，这将有助于揭示它们的原因、后果以及与破坏性地震的关系。该项目提供资金用于分析从观测站收集的数据，并支持另一个前往观测站的任务，以取回和更换观测站的仪器弦，使其能够继续运行五年。该项目支持研究生和本科生的培训。缓滑事件(SSE)涉及断层上的瞬时无地震滑动(持续数周至数月)，滑动速度介于板块边界位移速率和产生地震波所需的滑动速度之间。导致SSE的物理机制，它们在板块边界应变积累和释放中的作用，以及它们与俯冲推覆的破坏性地震滑移的关系，人们知之甚少，部分原因是大多数研究充分的俯冲带SSE发生在太深的地方，不适合近场研究。一个明显的例外是新西兰北部的Hikurangi俯冲带，在那里，特征良好的SSE发生在海底以下5-15公里内，并可能传播到海沟。2018年初，作为IODP探险375的一部分，在Hikurangi SSE北部的近场安装了两个钻孔观测站，以监测通过多个SSE循环的形变、热、水文和地球化学响应。该项目将：(1)恢复从观测站回收的数据和样本并对其进行分析；(2)在2023年初拟议的研究巡航期间，更换其中一个钻孔中的化探取样和温度传感仪器串，该钻孔跨越变形前沿附近的活动逆冲。这些数据将与海底传感器网络(绝对压力计、地震仪和流体流量计)集成在一起，这些传感器网络也记录了2019年大型慢滑事件期间沿钻井横断面的记录，提供了前所未有的机会来整合大型慢滑事件近场的多个海底和海底下观测。该奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

Subduction zone plate boundaries, where one tectonic plate dives or subducts beneath another tectonic plate produce the world’s largest earthquakes and tsunami, as starkly demonstrated by the M9.2 Indian Ocean earthquake in 2004, and the 2011 M9.0 Tohoku-Oki earthquake offshore northern Japan. Recently, scientists have recognized that subduction megathrust faults undergo slip in episodic slow-motion earthquakes, or slow slip events lasting days to months. In some cases, slow slip events have been observed to precede (and possibly trigger) major subduction earthquakes, increasing the need to understand them. Slow slip events occurring close to Earth's surface (5-15 km depth) exist at the Hikurangi subduction zone offshore New Zealand and are uniquely accessible to near-source investigations. In 2018, two observatories (penetrating up to 400 m below the seafloor) were installed during an International Ocean Discovery Program scientific drilling expedition at the Hikurangi subduction zone. Sensors in the observatories are now continuously measuring temporal changes in properties of Earth's crust near the source of slow slip events and will help reveal their causes, consequences, and relationship to destructive earthquakes. This project provides funding to analyze the data collected from the observatories, and to support another mission to the observatories to retrieve and replace the instrument strings at the observatories to enable their continued operation for another five years. The project supports the training of graduate and undergraduate students.Slow slip events (SSEs) involve transient aseismic slip on a fault (lasting weeks to months) at sliding velocities intermediate between plate boundary displacement rates and those required to generate seismic waves. The physical mechanisms leading to SSEs, their role in plate boundary strain accumulation and release, and their relationship to destructive seismic slip on subduction thrusts are poorly known, due in part to the fact that most well-studied subduction zone SSEs occur far too deep for near-field investigations. A notable exception to this is the northern Hikurangi subduction zone, New Zealand, where well-characterized SSEs occur within 5-15 km below the seafloor, and possibly propagate to the trench. In early 2018, as part of IODP Expedition 375, two borehole observatories were installed in the near field of northern Hikurangi SSEs to monitor deformation, thermal, hydrological, and geochemical responses through multiple SSE cycles. This project will: (1) recover and conduct analyses of data and samples recovered from the observatories; and (2) replace the geochemical sampling and temperature sensing instrument string in one of these boreholes, spanning an active thrust near the deformation front, during a proposed research cruise in early 2023. These data will be integrated with a network of seafloor sensors (absolute pressure gauges, seismometers, and fluid flowmeters) that were also recording along the drilling transect during a large slow slip event in 2019, offering an unprecedented opportunity to integrate multiple seafloor and subseafloor observations in the near-field of a large slow slip event.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.