HR: 1340h
AN: OS43B-0548    [Abstracts]
TI: T-phase observed at deep seafloor boreholes
AU: * Araki, E
EM: araki@jamstec.go.jp
AF: JAMSTEC, 2-15 Natsushimacho, Yokosuka, KN 237-0061 Japan
AU: Stephen, R A
EM: rstephen@whoi.edu
AF: WHOI, Woods Hole Oceanographic Institution, Woods Hole, MA 02543 United States
AU: Shinohara, M
EM: mshino@eri.u-tokyo.ac.jp
AF: ERI, Univ of Tokyo, 1-1-1 Yayoi, Bunkyo-ku, TK 113-0032 Japan
AU: Kanazawa, T
EM: kanazawa@eri.u-tokyo.ac.jp
AF: ERI, Univ of Tokyo, 1-1-1 Yayoi, Bunkyo-ku, TK 113-0032 Japan
AU: Suyehiro, K
EM: suyehiro@jamstec.go.jp
AF: JAMSTEC, 2-15 Natsushimacho, Yokosuka, KN 237-0061 Japan
AB: T-phases are seismic phases that have an acoustic propagation path in the ocean. They are mostly observed on hydrophones in the ocean sound channel or on shore based observatories. Although the acoustic energy of the T-phase is considered to be trapped in the SOFAR channel in order to travel long distances without attenuation, we have observed T-phases in the deep seafloor far below the SOFAR channel. It is not clear why T-phases should be observed in on the deep seafloor . To characterize the propagation of the T-phase in the deep ocean, we inspected long-term seismic records from two deep seafloor borehole seismometers in the Philippine Sea and the NW Pacific. These borehole seismometers (WP1 in the Philippine Sea, WP2 in the NW Pacific) are among the four ocean borehole seismic stations (JT1, JT2 near Japan Trench, WP1 and WP2) deployed during the Ocean Drilling Program legs 186 (JT1 and JT2), 191 (WP2), and 195 (WP1). The WP1 and WP2 stations employ broadband borehole seismometers cemented in the oceanic basement. The depths of the seafloor and the seismometer below the seafloor are 5.7km and 561m for WP1 and 5.5km and 460m for WP2 respectively. Despite their sensor locations at deep seafloor much below the SOFAR channel, we could see many of the seismic events accompanied the T-phases below 10Hz. Long-term observation from the seafloor boreholes enabled us to cover the whole regional seismicity. The distribution of seismic events observed with the T-phases and their T-phase amplitude compared with those of P/S phases, are useful to infer characteristics of acoustic wave propagation in the sea. The WP1 and the WP2 stations have been operational since March 2002 and October 2000 respectively. Over one year's worth of seismic data at each station were recovered by the ROV "Kaiko" (10km depth capable) until the loss of the vehicle in April 2003. Both stations are expected to record additional data for more than a year. We plan to recover them by a replacement vehicle "Kaiko 7K" in the near future.
DE: 9355 Pacific Ocean
DE: 7294 Instruments and techniques
DE: 7203 Body wave propagation
DE: 4259 Ocean acoustics
DE: 1223 Ocean/Earth/atmosphere interactions (3339)
SC: Ocean Sciences [OS]
MN: 2004 AGU Fall Meeting