HR: 0800h
AN: S41A-0237    [Abstracts]
TI: Free Oscillations of the Earth Observed by Closed Borehole Wells
AU: * Yanagidani, T
EM: yanagidani@rcep.dpri.kyoto-u.ac.jp
AF: Disaster Prevention Research Institute, Kyoto university, Gokasho, Uji, 6110011, Japan
AU: Kano, Y
EM: kano@eqh.dpri.kyoto-u.ac.jp
AF: Disaster Prevention Research Institute, Kyoto university, Gokasho, Uji, 6110011, Japan
AB: We have made observations of pore pressure under undrained condition by an airtight borehole penetrating an artesian, or a confined aquifer in the Atotsu tunnel excavated in the Kamioka Mine, central Japan. We confirmed that the relation between pore pressure change and stress change is a zero-order system for a wide range of frequency and that stress change, strictly speaking strain change, induced within the rock mass shared by the skeletal framework of rock and pore fluid. Examining the pore pressure measured using closed borehole wells, we detected free oscillations of the Earth excited by earthquakes such as the 26 December 2004 Mw = 9.1 Sumatra-Andaman Islands earthquake (epicentral distance Δ= 51.1°) and other M7 to 8 events. We made a Fourier analysis of the pore pressure record produced by the earthquakes. We examined (1) whether the closed borehole has sufficient sensitivity to identify free oscillations, and (2) how the closed borehole responds to spheroidal modes and troidal modes. The poroelastic theory predicts that pore pressure should respond only to spheroidal modes since pore pressure change is proportional to volumetric strain change. No pore pressure response is expected from shear strain that is produced by troidal modes. However, it is controversial whether pore pressure responds to shear strain, since phases corresponding S- and Love waves have been usually detected on hydroseismograms. We calculated the spectrum of the 24 hours time windows (86400 points) with shifting the time window by 1 hour from 24 hours before the origin time of the event to 24 hours after that. The spectrum peaks correspond to entire fundamental spheroidal modes were clearly observed. The Q of each mode is calculated by fitting the decay of the amplitude of each peak. The peaks whose eigenfrequencies are less than 1 mHz (0S0, 0S2, 0S3, 0S4, and 0S5) clearly appear 5 hours after the event. On the other hand, no spectrum peak corresponding troidal modes was observed. These results confirm that the poroelastic theory correctly predicts the pore pressure response.
DE: 1223 Ocean/Earth/atmosphere/hydrosphere/cryosphere interactions (0762, 1218, 3319, 4550)
DE: 1294 Instruments and techniques
DE: 1835 Hydrogeophysics
DE: 7255 Surface waves and free oscillations
DE: 7294 Seismic instruments and networks (0935, 3025)
SC: Seismology [S]
MN: 2007 Fall Meeting