Recent progress in ocean bottom seismic observation and new results of marine seismology

For past ten years, the technology for ocean bottom seismic observation has been greatly developed. As a result, various seismic observations have been carried out around Japan and new geophysical findings are obtained. For sea floor seismic observation, Long-Term Ocean Bottom Seismometer (LT-OBS) and Broad Band OBS (BB-OBS) were developed. As a result of long term observations, precise seismicity in marine area, especially under the landward slope of the Japan and Kuril trenches, can be obtained. Ocean Bottom Cabled Seismometers (OBCS), where the sensors are equipped in a hermetically-sealed case and these cases are connected with cables, is the best solution for real-time observation in marine area. A new OBCS system to make a high density observation in the marine areas was developed recently. The first OBCS system with a total length of 25 km and 4 observation nodes was produced. The installation of the first OBCS system carried out in end of August, 2010. At the landing station, the data are stored to hard disks, and the data are transmitted to Ea data center. It is becoming clear that emplacement of seismometers into the crust can provide low noise environments. Four borehole geophysical observatories in the western Pacific area were constructed from 1999 to 2001 and the data from the borehole sensors were obtained for more than two years. The tiltmeters in the sea floor borehole recorded the earth tides clearly and structure of the oceanic plate is obtained using data from the borehole seismometers.