11:00 〜 13:00
[SCG57-P02] Tidal analysis of pore pressure in NanTroSEIZE boreholes at the Nankai margin, SW Japan.
キーワード：International Ocean Discovery Program (IODP), Long Term Borehole Monitoring System (LTBMS), Pore pressure, Subduction zone, Earthquake, Fault
The Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE) is a multi-expedition drilling project aiming to investigate the key role of hydromechanical properties in fault mechanics and processes at the high-risk Nankai margin, southwestern Japan. There, the northwestward subduction of the Philippine Sea plate beneath the Eurasian plate results in large magnitude (>8) earthquakes, tsunamis, and slow earthquakes. As part of this initiative, Long Term Borehole Monitoring Systems(LTBMSs) were deployed at three International Ocean Discovery Program (IODP) boreholes to collect real-time data providing information on the in-situ hydrologic processes and crustal deformation. This data includes pore pressure which can be used as a proxy for volumetric strain.
The monitored pore pressure exhibits oscillations with diurnal (~24h) and semi-diurnal (~12h) periods resulting from Earth tides forcing. Because Earth tides are predictable, tidal analysis programs can be used to extract the phase and amplitude of the tidal signal, which depend on the in-situ hydraulic and poroelastic properties. We use BAYTAP-08 (Tamura and Agnew, 2008), a modified version of the Bayesian Tidal Analysis Program - Grouping Model program of Tamura et al. (1991), to decompose the tidal response from the pore pressure recorded above the updip limit of the locked seismogenic zone (Site C0002), at the megasplay fault zone and its footwall (Site C0010), and at the frontal thrust of the accretionary prism (Site C0006). Overall, the response of the main diurnal and semi-diurnal tidal components is constant over the 8-year period analyzed, with a minor to zero phase shift and amplitude ratio ~0.6-0.7. This suggests constant hydraulic and poroelastic responses to tectonic loading, which we attempt to determine from the amplitude and phase of the main tidal components M2 and O1, and to compare with measurements on core samples and drilling data.
The removal of the tidal signal computed with BAYTAP-08 provides a clearer residual (i.e. detided) pore pressure signal. The detided pore pressure exhibits hydraulic anomalies and seems to have a long-term variation, which may either be instrumental or arise from subseafloor strain modulations related to plate convergence and seismic activities. In particular, high-frequency transients can be correlated with local earthquakes or slow earthquakes, and large magnitude remote earthquakes, which provides an opportunity to investigate the hydromechanical response for these events.
The monitored pore pressure exhibits oscillations with diurnal (~24h) and semi-diurnal (~12h) periods resulting from Earth tides forcing. Because Earth tides are predictable, tidal analysis programs can be used to extract the phase and amplitude of the tidal signal, which depend on the in-situ hydraulic and poroelastic properties. We use BAYTAP-08 (Tamura and Agnew, 2008), a modified version of the Bayesian Tidal Analysis Program - Grouping Model program of Tamura et al. (1991), to decompose the tidal response from the pore pressure recorded above the updip limit of the locked seismogenic zone (Site C0002), at the megasplay fault zone and its footwall (Site C0010), and at the frontal thrust of the accretionary prism (Site C0006). Overall, the response of the main diurnal and semi-diurnal tidal components is constant over the 8-year period analyzed, with a minor to zero phase shift and amplitude ratio ~0.6-0.7. This suggests constant hydraulic and poroelastic responses to tectonic loading, which we attempt to determine from the amplitude and phase of the main tidal components M2 and O1, and to compare with measurements on core samples and drilling data.
The removal of the tidal signal computed with BAYTAP-08 provides a clearer residual (i.e. detided) pore pressure signal. The detided pore pressure exhibits hydraulic anomalies and seems to have a long-term variation, which may either be instrumental or arise from subseafloor strain modulations related to plate convergence and seismic activities. In particular, high-frequency transients can be correlated with local earthquakes or slow earthquakes, and large magnitude remote earthquakes, which provides an opportunity to investigate the hydromechanical response for these events.