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

该项目是美国在日本本州岛近海熊野盆地钻井现场安装钻孔观测站的组成部分。综合大洋钻探计划（IODP）在这一地区的钻探是“NantroSEIZE”（南开海槽地震带实验）的一部分，这是一个多阶段的国际合作，旨在了解俯冲巨型逆冲断层的地震带。整个钻探计划的一个基本目标是建立一个跨越孕震区上倾极限的分布式长期观测网络。在这个网站上，孔隙压力，应变和温度将监测弧前盆地沉积物和下面的增生楔。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，以限制地层压缩性和水力扩散率，以及（3）地层孔隙压力，以确定环境值并记录水文瞬变。 此外，从操作的角度来看，观测站作为一个先导孔和试验台，对于在该位置进行更深隔水管钻井的规划操作至关重要。需要对地震活动性、孔隙压力和应变进行长期的同位监测，以测试应变局部化和沿沿着断层锁定的假设，以及在围岩内和沿沿着断层引发超流体静力压力的板块边界弱点的假设。对于前一种应用，孔隙压力提供了体积应变的灵敏指示，补充了应变仪的观测。 选定的地点对于监测特别重要，因为它是计划网络中最向陆地的地点，并且位于最近一次大地震（1944年Tonankai M8.2事件）期间的显著滑动区上方，并且还位于2004年板块边界系统上的一组甚低频（VLF）逆冲地震上方。

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.