2009 年 21 巻 2 号 p. 2_15-2_24
To understand characteristics of large earthquakes occurring 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 earthquake generation, ocean bottom seismometer (OBS) is essential to observe seismic waves from earthquakes in subduction areas. Since a conventional OBS is designed for high-sensitivity observation, OBS records of large earthquake occurring near an OBS are often saturated. An accelerometer is suitable to record large amplitude seismic waves. Recently a compact accelerometer with a large dynamic range and low-power consumption is being developed. In addition, a pressure vessel of an OBS can contain much more batteries by using a large size titanium sphere. Therefore we developed new OBSs to obtain low-sensitivity (strong motion) accelerograms on the sea floor by installation of a small three-component accelerometer to a conventional long-term OBS. Three types of the OBS with accelerometer were developed. First type has a compact three-component accelerometer and the conventional 4.5 Hz velocity seismometers and used the sea floor observation of aftershocks of the 2004 Sumatra-Andaman Earthquake. Recording period of the first type is limited to two month due to capacities of batteries and a storage device. Second type has accelerometers only, however a recording period reaches one year by using a large pressure vessel. Third type records both high-sensitivity seismogram and low-sensitivity accelerograms with a recording period of one year. Both the second type and third type were used for the earthquake observation off Ibaraki. From these observations, we could obtain low-sensitivity accelerograms on sea floor with a good S/N ratio. Especially, records near the epicenter of the earthquake with magnitude of 7 off Ibaraki in 2008 were recorded without saturation. The developed OBSs with accelerometers are useful for location and studies of source process of large earthquakes.