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，会产生世界上最大的地震和海啸，2004年的9.2级印度洋地震和2011年日本北部沿海的9.0级东北大地震就是明证。最近，科学家们已经认识到，俯冲巨型逆冲断层在间歇性的慢动作地震或持续数天至数月的慢滑动事件中发生滑动。在某些情况下，已经观察到慢滑事件先于（并可能触发）主要的俯冲地震，这增加了对它们的了解的需要。在新西兰近海的Hikurangi俯冲带，发生在接近地球表面（5-15公里深度）的慢滑事件，是近源调查的唯一途径。2018年，在Hikurangi俯冲带的国际海洋发现计划科学钻探探险期间，安装了两个观测站（深入海底以下400米）。现在，观测站里的传感器正在不断地测量慢滑事件发生地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，用于分析从各天文台收集的数据，并支持另一项前往各天文台的任务，以检索和更换各天文台的仪器串，使它们能够再继续运行五年。该项目支持研究生和本科生的培养。慢滑事件涉及断层上的瞬态地震滑动（持续数周至数月），其滑动速度介于板块边界位移率和产生地震波所需的速度之间。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，以及它们与俯冲冲断上破坏性地震滑动的关系都知之甚少，部分原因是大多数研究得很好的俯冲带sse发生得太深，无法进行近场调查。新西兰北部的Hikurangi俯冲带是一个明显的例外，在那里，在海底以下5-15公里范围内出现了特征明显的sse，并可能传播到海沟。2018年初，作为IODP远征375的一部分，在Hikurangi SSE北部近场安装了两个钻孔观测站，通过多个SSE循环监测变形、热、水文和地球化学响应。本项目将：(1)对从天文台回收的数据和样本进行恢复和分析；(2)在2023年初的研究巡航期间，更换其中一个井眼的地球化学采样和温度传感仪器串，该井眼跨越了变形前沿附近的活动冲断。这些数据将与海底传感器网络（绝对压力表、地震仪和流体流量计）相结合，这些传感器网络也在2019年的一次大型慢滑事件期间沿着钻井样带进行记录，为在大型慢滑事件的近场整合多个海底和海底观测提供了前所未有的机会。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值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.