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The ocean broadband downhole seismometer (OBDS) was successfully emplaced in Hole 794D in the Japan Sea during ODP Leg 128, and OBDS data were obtained continuously aboard JOIDES Resolution during a 60-hr period of real-time observation. During the real-time observation, air-gun profiling was done around Hole 794D using nine ocean-bottom seismometers (OBSs). The OBDS successfully observed air-gun signals from the seismic profiles, five local earthquakes, and an earthquake in Burma (mb 5.4). Signal-to-noise ratios of air-gun signals and local earthquakes observed by the OBDS are higher by over 20 dB than those observed by OBS. The epicentral distance of Burma earthquake was 38.99° to the OBDS. The OBDS could record P, S, and some surface waves from this teleseismic event with good signal-to-noise ratio for both the vertical and horizontal components. Broadband seismic noise spectra (5 m-30 Hz) in the ocean region were revealed for the first time by the OBDS observation. The noise level falls between the two levels of noisy and quiet conditions on land above 10 s. The noise level increases toward longer period. Signals from earthquakes larger than ms = 6 at 30° distance would rise above noise at an 100-s period. Following the real-time observation, an OBDS seafloor recorder was installed and operated for 30 days in event detection mode and time window mode. There was no triggered event. Very long-period data of 1-hr sampling showed only gradual variations without abrupt steplike changes. This indicates that the clamping we used for the OBDS sensors in the hole was sufficiently stable for such a long-term broadband observation. The results obtained in this paper strongly suggest that a broadband downhole seismometer performs well and is dispensable for constructing broadband digital seismograph networks on the ocean floor. INTRODUCTION Observational seismology has entered a new era with the use of high-performance broadband digital seismographs. Land seismic networks enjoy this technical advance and have been improving their instrumentation toward wide dynamic range and frequency band, with easy data access observation (e.g., IRIS, GEOSCOPE). The new set of data from these observations advances high-resolution seismological studies of the Earth's interior. It has been noted, however, that this new observational window is still limited spatially in the sense that these networks include little of Earth's surface that is covered by water. There are, however, many difficulties in setting up permanent seismic stations under the sea. These difficulties include the following: (1) Station environment, whether seafloor or borehole observation is required. Station design is also important, whatever the environment. Even if island observations prove to be adequate, vast areas of open ocean will remain (e.g., in the northwestern Pacific). (2) Sustaining stable long-period observation. (3) Retrieving data. (4) Supplying power. Resolution of these difficulties varies depending on the objectives. More importantly, we lack observational long-period data on which to base our discussion, except for the pioneering experiment of the Columbia-Point Arena Ocean-Bottom Seismic Station (Sutton et al., 1965, 1989). About 7 yr after the last attempted ocean sub-bottom seismic observation (Duennebier, Stephen, et al. (1987c), a new-generation seismic experiment to emplace an ocean broadband downhole seismometer (OBDS) was successfully conducted on Ocean Drilling 1 Tamaki, K., Suyehiro, K., Allan, J., McWilliams, M., et al., 1992. Proc. ODP, Sci. ‰swto, 127/128, Pt. 2: College Station, TX (Ocean Drilling Program). 2 Laboratory for Earthquake Chemistry, Faculty of Science, the University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113, Japan. 3 Ocean Research Institute, the University of Tokyo, 1-15-1 Minamidai, Nakano-ku, Tokyo 164, Japan. 4 Department of Earth Sciences, Faculty of Science, Chiba University, 1 -33 Yayoi-cho, Chiba 260, Japan. 5 Present address: Ocean Research Institute, the University of Tokyo, 1-15-1 Minamidai, Nakano-ku, Tokyo 164, Japan. Program (ODP) Leg 128 in the Japan Sea (Ingle, Suyehiro, et al., 1990). The borehole instrument was installed at 714.5 m below seafloor (mbsf) within a basalt section near the bottom of Hole 794D. The water depth was 2807 m. Briefly, the experiment was intended to initiate a seismic broadband observation in the backarc basin of the northern Japan Plate subduction zone to obtain seismic information of the hitherto unprobed crust and upper mantle there. Then, a long-term observation was necessary to capture natural earthquakes larger than moderate size. The seismic sensor is a modified version of a Guralp CMG-3 three-axis feedback-type accelerometer with six-channel outputs of DC-to-30 Hz low-gain and 10 m-to-30 Hz high-gain acceleration signals. The analog signals are digitized by a 16-bit A/D converter at 80 Hz/ch sampling rate and telemetered uphole through the logging cable. Our experiment during Leg 128 consisted of two phases; first, to receive the signals aboard JOIDES Resolution to conduct real-time observation and, second, to connect the logging cable to the seafloor recording system specially constructed for this experiment and deploy it for a long-term observation. During the first phase, an air-gun firing experiment was conducted to detail the crustal structure, including anisotropy together with observation using an ocean-bottom seismometer (OBS) array (Hirata et al., this volume; Shinohara et al., this volume). The seafloor recorder for the second phase was designed to record 60 Mb of selected data triggered by earthquakes and for predetermined time windows after appropriate digital filtering. This data set was retrieved after 8 months in May 1990. We shall describe in this paper the characteristics of the seismic signal and noise obtained from this experiment with our focus on long-period data. DATA ACQUISITION Real-Time Experiment A digital broadband (DC-30 Hz) seismometer was emplaced in Hole 794D in the Japan Sea on 26 September 1989. The seismometer housing was locked within the basaltic rock section at 714.5 mbsf at a water depth of 2807 m. Following the installation of the OBDS in Hole 794D, we conducted real-time observation (Fig. 1). The purposes of this observa-

Performance of the Ocean Broadband Downhole Seismometer at Site 794