[J07-P] Poster

[J07-P-10] Possibility of tilt observation at the seafloor by a mobile ocean bottom seismometer

^○Hajime Shiobara¹, Aki Ito², Hiroko Sugioka³, Yoshio Fukao², Masanao Shinohara¹ (1.The University of Tokyo, Tokyo, Japan, 2.Japan Agency for Marine-Earth Science and Technology, Kanagawa, Japan, 3.Kobe University, Hyogo, Japan)

Since 1999, we had developed the broadband ocean bottom seismometer (BBOBS) and its new generation (BBOBS-NX), and, with them, performed many practical observations to create a new category of the ocean bottom broadband seismology. High mobility of the BBOBS and BBOBS-NX can be a breakthrough to realize the geodetic observation network on the seafloor. Two kinds of attempts to expand observation range toward the geodetic one have been started since 2009. One is for detecting vertical displacement by attaching an absolute pressure gauge with the BBOBS, and another is for tilt observation by using the BBOBS or BBOBS-NX. In this presentation, we will report results for the later. The tilt is measured from two horizontal mass position (acceleration) signals as offsets from the level. The first test was performed at the land pit in 2010. The result was comparable to that of the water tube tilt-meter there, with resolution of better than a micro radian. Next, a practical observation at the seafloor off Boso was done as the feasibility study from April 2013 for one year. In January 2014, a slow slip event (SSE) occurred near the site. The tilt data was processed by removing for the mechanical relaxation and tides. The result shows clear peak started from the late Dec. 2013, but the tilt was not remained after the SSE ended. As the data length of one-year seemed short, two tilt observations are in operation by the BBOBS-NX since 2015, one is at the same site above and another is off Tohoku area, both for two years. Moreover, we made tilt observations by the BBOBS on the seafloor 4 times at 3 sites, 2 of 4 cases were with a current profiler. Compared with the tilt data by the BBOBS-NX using a penetrated sensor, the effective resolution is more than ten micro radian, which is similar to the horizontal noise difference due to the bottom current. But, the BBOBS is still useful to detect such oceanographic events as internal bores unstably generated along seafloor slopes.