CO2 oceanic sequestration is one of the technologies for reducing the discharge of CO2 into the atmosphere, which is considered to cause the global warming, and consists in isolating industry-made CO2 gas within the depths of the ocean. This method is expected to enable industry-made CO2 to be separated from the atmosphere for a considerably long period of time. On the other hand, it is also feared that the CO2 injected in the ocean may lower pH of seawater surrounding the sequestration site, thus may adversely affect marine organisms. For evaluating the biological influences, we have studied to precisely predict the CO2 distribution around the CO2 injection site by a numerical simulation method. In previous studies, in which a 2 degree by 2 degree mesh was employed in the simulation, CO2 concentrations tended to be evenly dispersed within the grid, giving lower concentration values. Thus, the calculation accuracy within the area several hundred kilometers from the CO2 injection site was not satisfactory for the biological effect assessment. In the present study, we improved the accuracy of concentration distribution by changing the computational mesh resolution for a 0.2 by 0.2 degree. By the renewed method we could obtain detailed CO2 distribution in waters within several hundred kilometers of the injection site, and clarified that the Moving-ship procedure may have less effects of lowered pH on marine organisms than the fixed-point release procedure of CO2 sequestration.
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.