Possibility of tilt observation at the sea floor by using the BBOBST-NX system

Monday, 15 December 2014
Hajime Shiobara1, Aki Ito2, Hiroko Sugioka2 and Masanao Shinohara3, (1)Univ. Tokyo, Tokyo, Japan, (2)JAMSTEC Japan Agency for Marine-Earth Science and Technology, Kanagawa, Japan, (3)Univ Tokyo, Tokyo, Japan
Since 1999, we had developed the broadband ocean bottom seismometer (BBOBS) and its new generation system (BBOBS-NX), and, with them, performed several practical observations to create a new category of the ocean floor broadband seismology. Now, the BBOBS data is proved to be acceptable for broadband seismic analyses. In these studies, the period range mostly used is about 10 - 200 s, but in longer period range, i.e. geodetic range, is unknown region in the sea floor observation. High mobility of our BBOBS and BBOBS-NX can be a breakthrough to realize the geodetic observation network on the sea floor. Two kinds of attempts to expand observation range toward the geodetic one have been started since 2009, based on our BBOBS technology. One is for detecting vertical displacement by attaching an absolute pressure gauge (APG) in the BBOBS system. The highly stable frequency oscillator within the OBS recorder is adequate for precise pressure measurement of the APG. This BBOBS+APG system has been operated since 2009. In this presentation, we will report results of several test experiments for the tilt observation just beneath the sea floor by using the BBOBS-NX system, as the second one. The tilt is measured by using two horizontal mass position signals of the sensor. The first test observation was performed by using the same sensor of the BBOBS-NX at the land observatory in 2010. The result was comparable with that of the water tube tilt-meter there. After the in situ test for 2 months at the sea floor in the Shikoku Basin in 2012, we started the practical tilt observation at the sea floor off Boso peninsula (KAP3 site) as the feasibility study between April 2013 and April 2014. The deployment and recovery were performed by the ROV. In both observations, a Doppler current profiler was deployed nearby the BBOBST-NX to monitor bottom currents through the observation period. In January 2014, a slow slip event (SSE) occurred near the KAP3 site. The Mw of the SSE is 6.5, and this site seems almost above the area slipped. The tilt data of the BBOBST-NX was processed by removing for the mechanical relaxation and by using the baytap08 program. The result shows recognizable peak started from the late December 2013, but the tilt was not remained after the SSE ended. The maximum tilt is comparable to the theoretical value of a few μ radian.