Development of ocean bottom strong-motion seismograph with multi-seismic sensors

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To understand characteristics of large earthquakes occurred in a subduction zone, it is necessary to study an asperity where large earthquakes occur repeatedly. Because observation near an asperity is needed for studies of asperities, ocean bottom seismometer (OBS) is essential to observe seismic waves from earthquakes in subduction area. Since a conventional OBS is designed for high-sensitivity observation, OBS records of large earthquake occurred near OBS are often saturated. To record large amplitude seismic waves, a servo-type accelerometer is suitable. However it was difficult for OBS to use an accelerometer due to large electric power consumption. Recently a servo-type accelerometer with a large dynamic range and low-power consumption is being developed. In addition, a pressure vessel of OBS can contain much more batteries by using a large size titanium sphere. Although it becomes possible that a small three-component accelerometer was only installed to a conventional long-term OBS, it is important to enlarge a dynamic range of records for various analyses. In this study, we developed an OBS with both accelerometer and ordinary seismometer, and obtained both high-sensitivity seismogram and low-sensitivity (strong motion) accelerograms on the sea floor. In September 2006, we deployed 5OBSs with both accelerometer and ordinary seismometer off lbaragi, and successfully recovered in October, 2007. From five OBSs, we obtained many records from large earthquakes to small earthquake without saturation of the records. It can be concluded that fundamental technology of multi-seismometers in one OBS package is established.