4:00 PM - 4:15 PM
[SCG52-03] GNSS-Acoustic seafloor geodetic observations using a Wave Glider off Tohoku and their results
Keywords:GNSS-Acoustic observation, Wave Glider, Seafloor geodetic observation, Postseismic deformation, The 2011 Tohoku-oki Earthquake
Usingthe GNSS-Acoustic (GNSS-A) observation technique, measurement results of seafloor crustal deformation associated with an earthquake cycle have been reported [e.g., Honsho et al., 2019, JGR]. In the GNSS-A observation, a sea-surface platform is required, which conducts GNSS measurement and acoustic ranging between sea-surface and seafloor. Although a research vessel has been generally employed as a sea-surface platform, it takes high financial and human resources; these matters have disturbed us to conduct frequent GNSS-A observations. Therefore, JAMSTEC and Tohoku University have developed a GNSS-A observation system that employs a Wave Glider (WG) as a sea-surface platform, which is an unmanned surface vehicle moving under wave power. After succession of the employment of a WG on a trial survey in July 2019 [Iinuma et al., 2021, Front. Earth Sci.], we have repeatedly conducted long-term campaign surveys. The technical development of GNSS-A observations using a WG will be reported in Iinuma et al. [in this meeting]. In this presentation, we report the outline of GNSS-A campaign surveys conducted May-July and October 2022; then, we present the GNSS-A positioning analysis for the WG data and interpretation of the observation results.
For the survey in May-July 2022, we deployed a WG at G22 site off Nemuro on May 11. After the campaign observations for 55 days at 19 sites mainly off Sanriku, we recovered the WG at G14 site off Miyagi. Because of the slow ship speed of WG, a fixed-point survey was mainly conducted above the center of the seafloor transponder array; however, a moving survey was also conducted along the circumference of the seafloor transponder array to detect sound speed gradients and vertical displacements.
For the survey in October 2022, we deployed a WG at G25 site off Aomori on Oct. 7. After the campaign observations for 13 days at 7 sites mainly off northern Sanriku, we recovered the WG off Miyagi on Nov. 12. The WG was scheduled to be recovered at the end of October by the training ship Shioji-maru (Tokyo University of Marine Science and Technology: TUMSAT), but due to bad sea conditions, the recovery could not be completed; then it was recovered in November using a charter vessel. To maintain the power of WG, no observations had been conducted after the recovery was abandoned in late October.
We processed the above observational data collected by WG and then estimated seafloor transponder array displacements using a method of Tomita and Kido [2022, EPS]. This method assumes a sloping sound speed structure and estimates the array displacements through a MCMC technique. Although this method requires moving survey data to estimate a shallow gradient in a sound speed structure, we occasionally failed to collect adequate moving survey data when the WG was swept away by a strong current. Thus, we improved this method to stably obtain the positioning results by providing additional prior distributions as constraints on parameters expressing the sound speed gradients. The estimated displacements are generally consistent with trends from the past observational results; thus, they are expected to be demonstrate the postseismic deformation following the 2011 Tohoku-oki earthquake. At this moment, these estimation results are preliminary. Therefore, we will carefully examine the data and the estimation results and then will report the current condition of the seafloor crustal deformation off Tohoku in the presentation.
[Acknowledgments] A part of GNSS-A observation data in this study was obtained by the JAMSTEC R/V Kaimei (the cooperation of Dr. No), R/V Shinsei-maru (Joint Usage of Research Center for Atmosphere and Ocean Science and JAMSTEC) and the TUMSAT training vessel Shioji-maru (supported by ERI JURP 2021-KOBO19 in Earthquake Research Institute, the University of Tokyo). This work was supported by JSPS KAKENHI JP19H05596.
For the survey in May-July 2022, we deployed a WG at G22 site off Nemuro on May 11. After the campaign observations for 55 days at 19 sites mainly off Sanriku, we recovered the WG at G14 site off Miyagi. Because of the slow ship speed of WG, a fixed-point survey was mainly conducted above the center of the seafloor transponder array; however, a moving survey was also conducted along the circumference of the seafloor transponder array to detect sound speed gradients and vertical displacements.
For the survey in October 2022, we deployed a WG at G25 site off Aomori on Oct. 7. After the campaign observations for 13 days at 7 sites mainly off northern Sanriku, we recovered the WG off Miyagi on Nov. 12. The WG was scheduled to be recovered at the end of October by the training ship Shioji-maru (Tokyo University of Marine Science and Technology: TUMSAT), but due to bad sea conditions, the recovery could not be completed; then it was recovered in November using a charter vessel. To maintain the power of WG, no observations had been conducted after the recovery was abandoned in late October.
We processed the above observational data collected by WG and then estimated seafloor transponder array displacements using a method of Tomita and Kido [2022, EPS]. This method assumes a sloping sound speed structure and estimates the array displacements through a MCMC technique. Although this method requires moving survey data to estimate a shallow gradient in a sound speed structure, we occasionally failed to collect adequate moving survey data when the WG was swept away by a strong current. Thus, we improved this method to stably obtain the positioning results by providing additional prior distributions as constraints on parameters expressing the sound speed gradients. The estimated displacements are generally consistent with trends from the past observational results; thus, they are expected to be demonstrate the postseismic deformation following the 2011 Tohoku-oki earthquake. At this moment, these estimation results are preliminary. Therefore, we will carefully examine the data and the estimation results and then will report the current condition of the seafloor crustal deformation off Tohoku in the presentation.
[Acknowledgments] A part of GNSS-A observation data in this study was obtained by the JAMSTEC R/V Kaimei (the cooperation of Dr. No), R/V Shinsei-maru (Joint Usage of Research Center for Atmosphere and Ocean Science and JAMSTEC) and the TUMSAT training vessel Shioji-maru (supported by ERI JURP 2021-KOBO19 in Earthquake Research Institute, the University of Tokyo). This work was supported by JSPS KAKENHI JP19H05596.