[S-CG45] Ocean Floor Geoscience

[SCG45-13] Preliminary results of ocean bottom pressure observation around Kikai submarine caldera, SW Japan

Yuko Asai¹, *Nobukazu Seama^1,2, Tomoko Hanyu², Tetsuo Matsuno^2,1, Hitoshi Hirose^3,1, Takuto Minami¹, Hiroko Sugioka^2,1 (1.Department of Planetology, Graduate School of Science, Kobe University, 2.Kobe Ocean Bottom Exploration Center, Kobe University, 3.Research Center for Urban Safety and Security, Kobe University)

Keywords:Ocean bottom pressure variation, Ocean tide, Loading effect on crust

We perform tidal analysis of ocean bottom pressure variation, which were measured by 4 ‘OBEMP (Ocean Bottom Electro-Magnetometer with absolute Pressure gauge)’ instruments at observation sites around Kikai submarine caldera, SW Japan. These measurements were made during periods of April/2018-Oct./2018, Nov./2018-Sept./2019, Nov./2018-March/2019, and Nov./2019-March/2020, with their water depths of 392m, 312m, 206m, and 196m, respectively. Tidal analysis of these data is performed using BAYTAP-G program (Tamura et al., 1991) to derive tidal amplitudes, factors, and phases of 14 diurnal and semidiurnal tidal groups, and these data are decomposed into three parts: 1) a tidal part, 2) a part that is correlated with the ocean bottom temperature measurement data, and 3) a nontidal part. The tidal factors are estimated at ~2 and ~ 3.5 for diurnal and semidiurnal tidal groups, respectively. Some tidal factors and phases that are well estimated with small errors indicate slight difference among the observation sites. Comparison of tidal factors and phases between our results and a global ocean tide model, NAO.99b (Matsumoto et al., 2000), also shows slight differences. These differences could be due to local loading effect on crust with relation to the existence of Kikai submarine caldera. We will perform further analysis with respect to site location difference, period dependence, and time variation to investigate possible local loading effect on crust. The result from the nontidal part indicates two characteristic features. One is a sudden increase of pressure values (~30 hPa) with in 6 hours, which occurred on August 22, 2018 at one observation site. We will investigate possible causes of the sudden pressure increase. The second one is pressure variation of 20 hPa with its period longer than a week. This variation was observed at two observation sites during Nov./2018-March/2019, which was the only duration that we made simultaneous observation at two observation sites. The similar pressure variations at two sites that are separated by 45 km in distance suggest that these variations are due to a same cause. Since pressure variation at ocean bottom corresponds to the sum of air pressure and pressure due to total mass of ocean water above observation site, we compare our pressure variation with air pressure variations observed at three Japan Meteorological Agency observation sites (Makurasaki, Tanegashima, and Yakushima) far around Kikai submarine caldera, whose data were provided by Japan Meteorological Agency. The overall air pressure variations at three sites are the same with a very small difference within ~2 hPa, suggesting air pressure variations at our ocean bottom observation sites would be the same. The comparison indicates no correlation for the period longer than a week, suggesting two things. The air pressure variations are well compensated by change in total mass of ocean water (change in ocean height level), and leads no effective variation on pressure at ocean bottom. Thus, the pressure variation of 20 hPa with its period longer than a week should be due to pressure due to total mass of ocean water above two observation sites that is probably caused by ocean current variation. We will analyze period dependence of correlation between ocean bottom pressure variations and air pressure variations. We will present all these results and possible implications from them.