HR: 0800h
AN: S51A-0144    [Abstracts]
TI: Free Oscillations of the Earth Observed on the Hydroseismogram Obtained in Closed Boreholes
AU: * Yanagidani, T
EM: yasan@rcep.dpri.kyoto-u.ac.jp
AF: Disaster Prevention Research Institute, Kyoto University, Gokasho, Uji, 6110011 Japan
AU: Kano, Y
EM: kano@rcep.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 Mozumi observation tunnel excavated at 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. The 3 November 2002 $M_{w}$ = 7.9 Denali earthquake (epicentral distance $\Delta$ = $51.3\deg$) provided us to investigate how a closed borehole responds to free oscillations of the Earth. We made a Fourier analysis of the hydroseismogram, or the pore pressure record, produced by the Denali earthquake. 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 record from origin time of the event, and the sampling interval was 1 second (86400 points). The spectrum peaks correspond to free oscillations were clearly observed. Identified normal modes below 5 mHz were $_{0}S_{6}$, $_{0}S_{13}$, $_{0}S_{16}$, $_{0}S_{17}$, $_{0}S_{18}$, $_{0}S_{20}$, $_{0}S_{21}$, $_{0}S_{23}$, $_{0}S_{24}$, $_{0}S_{25}$, $_{0}S_{26}$, $_{0}S_{27}$, $_{0}S_{28}$, $_{0}S_{29}$, $_{0}S_{31}$, $_{0}S_{32}$, $_{0}S_{33}$, $_{0}S_{34}$, $_{0}S_{35}$, $_{0}S_{37}$, $_{0}S_{38}$, $_{0}S_{39}$, $_{0}S_{42}$, $_{0}S_{43}$. The peaks in lowest frequency bands ($_{0}S_{2}$ to $_{0}S_{5}$) could not be identified on the hydroseismogram of the Denali earthquake. 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: 7255 Surface waves and free oscillations
DE: 7294 Instruments and techniques
DE: 5114 Permeability and porosity
DE: 1829 Groundwater hydrology
SC: Seismology [S]
MN: 2004 AGU Fall Meeting