Japan Geoscience Union Meeting 2014

Presentation information

Oral

Symbol S (Solid Earth Sciences) » S-SS Seismology

[S-SS30_28AM2] New perspective of great earthquakes along subduction zones

Mon. Apr 28, 2014 11:00 AM - 12:42 PM Main Hall (1F)

Convener:*Kyuichi Kanagawa(Graduate School of Science, Chiba University), Takashi Furumura(Center for Integrated Disaster Information Research (CIDIR) Interfaculty Initiative in Information Studies, The University of Tokyo), Shuichi Kodaira(Institute for Research on Earth Evolution Japan Agency for Marine-Earth Science and Technology), Masanobu Shishikura(Active Fault and Earthquake Research Center, GSJ/AIST), Chair:Saneatsu Saito(Japan Agency for Marine-Earth Science and Technology)

12:15 PM - 12:30 PM

[SSS30-10] Enigmatic phase lead of pore pressure: 11+ years of ACORK monitoring at the frontal decollement of Nankai Trough

*Masataka KINOSHITA1, Yasuyuki KANO2 (1.JAMSTEC, 2.Kyoto Univ. DPRI)

Keywords:Nankai Trough, ACORK, ODP

For more than 11 years we have been conducting a continuous monitoring of downhole pore pressures at multiple sub-bottom intervals in ODP Holes 808I and 1173B situated landward and seaward of the deformation front in the Nankai Trough off Cape Muroto. We found that the pressure response to the semi-diurnal ocean tide (M2), both amplitude and phase, gradually change during the observed period. The M2 amplitudes at most depths in Hole 808I decay as their phase delay (up to 45 degrees), ONLY IF the amplitude is larger than ~0.2 of that for the seafloor. On the other hand, we observe an anomalous phase LEAD (up to -40 degrees) if the relative amplitude is lea than ~0.2. We hypothetize that the recorded pore pressure is a combination of two components; one with larger amplitudes and phase-delay and the other with small amplitude and phase-lead. The former can be interpreted as the decrease in formation compliance relative to that of the system, or as the decrease in hydraulic diffusivity around the sensors. The mechanism of the latter variation remain still enigmatic. Existence of gas-rich layer next to the sensor, as suggested by wang and Davis (1996), is difficult to generate such a large phase lead. The predicted earth tide at this site does not coincide with the observed phase. Thermal expansion/contraction caused by the flow within the casing, induced by tidal loading, may cause this phase shift, but a simple thermal/hydrological diffusion cannto explain both the amplitude and phase simultaneously. A complex process including some unknown mechanism may be in operating at Hole 808I.