Collaborative Research: Development of a Long-term Hydrologic Observatory Aabove the Seismogenic Zone Offshore the Kii Peninsula, Japan

该项目是将安装在日本本州岛近海熊野盆地钻探现场的钻孔观测站的美国组成部分。该地区的综合大洋钻探计划(IODP)钻探是南开海槽地震孕育区实验(NantroSEIZE)的一部分，该实验是一个多阶段的国际合作，旨在了解俯冲巨型逆冲的地震孕育区。建立一个跨越孕震带上倾界限的分布式长期观测网是整个钻探计划的一个基本目标。在这个地点，将监测弧前盆地沉积物和下伏的沉积楔形物中的孔压、应变和温度。监测的具体目标是记录(1)应变的时间模式，(2)地层压力对已知潮汐载荷的响应，以限制地层可压缩性和水力扩散率，以及(3)地层孔压，以确定环境值并记录水文瞬变。此外，从操作的角度来看，观测站是规划该地区更深的立管钻井作业的试验孔和试验台。需要对地震活动、孔隙压力和应变进行长期监测，以检验应变局部化和沿断层锁定的假设，以及在围岩和沿断层引发超静水压流体压力的板块边界弱点假说。对于前一种应用，孔压提供了体积应变的灵敏指示，与应变仪观测结果相辅相成。选定的地点对监测特别重要，因为它是规划的网络中最靠近陆地的地点，紧挨着上一次大地震(1944年托南凯8.2级地震)的显著滑动带上方，也位于2004年板块边界系统上的一系列甚低频(VLF)推覆地震的上方。

This project is the U.S. component of a borehole observatory to be installed at a drillsite in the Kumano Basin, offshore Honshu Island, Japan. Integrated Ocean Drilling Program (IODP) drilling in this area is part of "NanTroSEIZE" (the Nankai Trough Seismogenic Zone Experiment), a multi-stage, international collaboration to understand the seismogenic zone of subduction megathrusts. Establishment of a distributed long-term observatory network spanning the up-dip limit of the seismogenic zone is one fundamental goal of the overall drilling program. At this site, pore pressure, strain, and temperature will be monitored in forearc basin sediments and the underlying accretionary wedge. The specific objectives of monitoring are to document (1) temporal patterns of strain, (2) formation pressure response to known tidal loading to constrain formation compressibility and hydraulic diffusivity, and (3) formation pore pressure, both to determine ambient values and to record hydrologic transients. Additionally, from an operational standpoint, the observatory is critical as a pilot hole and testbed for planning operations for deeper riser drilling at this location.Collocated long-term monitoring of seismicity, pore pressure, and strain are needed to test hypotheses for strain localization and locking along faults, and for plate boundary weakness that invoke suprahydrostatic fluid pressures within wall rocks and along faults. For the former application, pore pressure provides a sensitive indication of volumetric strain that complements strainmeter observations. The selected site is especially critical for monitoring, because it is the landward-most site in the planned network, and lies immediately above a zone of significant slip during the last great earthquake (1944 Tonankai M 8.2 event), and also above a cluster of very low frequency (VLF) thrust earthquakes on the plate boundary system in 2004.