ALBERT

Description: This presentation covers the 10 year history of GGOS Japan. As many as six Japanese national institutes have been operating large-size geodetic facilities with VLBI and/or SLR in Japan and also in Antarctica, and they had been involved in each geodetic service rather independently. In 2013, we initiated "GGOS Working Group" in Japan to enhance the communications among the Japanese institutes and global geodesists and also to encourage collaboration beyond each technique. GGOS Japan is basically a loose network, but it has a chair, a secretary, an outreach representative, working groups (currently active: Data DOI working group) and representatives of each geodetic technique (VLBI, SLR/LLR, GNSS, DORIS and gravimetry). We regularly host annual meetings and sometimes small meetings with focused issues. We have also involved in organising or supporting scientific sessions in domestic and international conferences. During the 10 years history, it was positioned as a GGOS Affiliate in 2017 and we renamed it as "GGOS Japan" in 2019. Basara Miyahara was elected as the GGOS President in 2019. GGOS Japan's webpages are being updated on the GGOS Website, and we have contributed to GGOS YouTube channels. We often set focused activities. We are currently working on the review of ITRF2020 from an operating point of view, and we are also working on a possible best location for a future GGOS Cores site in Japan. In the future we hope to collaborate with international communities, in particular with other GGOS Affiliates.

Description: In the central to western Nankai Trough, seismic reflection surveys were conducted in 2018-2020. Compiled seismic dataset was used for picking BSRs (bottom-simulating reflectors). BSR-derived heat flow is calculated from the depth and the temperature at BSR, and the average thermal conductivity between seafloor and BSR. Heat flow is highest near the trough axis off Muroto (near the central Nankai Trough). In the forearc region, heat flow varies between 50-70 mW/m^2, but it is lowest in the forearc off Hyuga-Nada, westernmost portion of the Nankai Trough. On the forearc area off Muroto, the topography is characterized by a large landward embayment including the trough axis and deformation front. Within 20 km landward from the deformation front, heat flow is ~80 mW/m^2 in this embayed area, whereas it is 40-60 mW/m^2 on either side of embayment. Further landward, we found a low heat flow (~30 mW/m2) region above the subducted seamount. We propose that the heat flow is affected by the subduction of seamount. In the Hyuga-nada forearc region off eastern Kyushu, the Kyushu-Palau Ridge (KPR) is obliquely subducting toward N30W since several Ma B.P. Across KPR we observed a‘bowl-shape’negative heat flow anomaly with its width ~50 km. The heat flow outside is ~45 mW/m^2, whereas it is ~25 mW/m^2 above the subducted KPR. We discuss possible mechanisms of variable heat flow around the subducted seamount with implication for the earthquake likelihood in the shallow portion of seismogenic zone.

Description: As one of the seafloor geodetic techniques, precise seafloor positioning by the GNSS—Acoustic ranging combination technique (GNSS-A) is applied for the observations of the crustal deformation in the plate subduction zones (e.g., Spiess et al., 1998; Fujita et al., 2006). For the precise positioning with the GNSS-A, it is required to appropriately cancel or correct the effects of sound speed variation on acoustic travel time. We have developed static GNSS-A analysis methods where the sound speed effects were simultaneously corrected with well-distributed acoustic data, by introducing the perturbation field model (Watanabe et al., 2020). Based on the empirical Bayes approach, it was implemented in an open-source software GARPOS (the latest version is v1.0.1, https://doi.org/10.5281/zenodo.6414642), in which hyperparameters are selected to minimize the Akaike Bayesian Information Criterion (ABIC; Akaike, 1980). Watanabe et al. (under review, preprint https://doi.org/10.21203/rs.3.rs-1881756/v1) developed the upgraded version of GARPOS, i.e., GARPOS-MCMC (the latest version is v1.0.0, https://doi.org/10.5281/zenodo.6825238), with a full-Bayes GNSS-A analysis scheme, where the hyperparameters are also expressed as probability density functions. The parameters are estimated with the Markov chain Monte Carlo method, which enabled us to directly sample from the joint posterior of parameters including any hyperparameters and evaluate the correlations between those parameters. However, it requires computational resources as the number of acoustic data becomes large. To overcome the disadvantage, we introduced the widely applicable Bayesian information criterion (WBIC; Watanabe, 2013) for model selection for some hyperparameters, to partly take an empirical Bayes approach, and implemented it on GARPOS-MCMC.

Description: Although GNSS-A (Global Navigation Satellite System–Acoustic ranging combination technique) observation is a technology that measures steady or sudden seafloor crustal deformations at the centimeter level, the technical capabilities are inferior to those of terrestrial GNSS observation in terms of accuracy and frequency. Therefore, many technological developments are currently underway. We are conducting error factor analysis through simulation and experimental research in order to improve the observation accuracy and frequency of SGO-A, which is operated by the Japan Coast Guard. By investigating the effects of high-rate GNSS on GNSS-A, development of representation and modeling methods of underwater sound speed fields, and equipment and angle-dependent characteristics due to sonar characteristics, we are progressing in developing quantitative evaluation and correction methods for errors. The accuracy research of GNSS-A is closely related to the accuracy research of GNSS. In the near future, we would like to construct a unified error correction method for instruments and observation envirnments, similar to GNSS. Regarding the observation frequency, we have reached the limit of observation frequency using ships around 2020, and the development of new sea surface platforms is necessary. For example, research on autonomous buoys (wave glider), moored buoys, and flying-boat type UAVs is underway. Various marine engineering applications other than GNSS-A are underway for a research field of sea surface platforms, and this field may be further updated in the future. In this presentation, we will also discuss new sea surface platforms for GNSS-A.

Description: Around the Japanese Islands, major subduction zones along the plate boundaries of the Pacific Plate and the Philippine Sea Plate have repeatedly caused megathrust earthquakes. GNSS-Acoustic ranging combination technique (GNSS-A) is an effective tool to measure the absolute position on the seafloor, from which we can visualize the plate boundary conditions at these subduction zones. The Japan Coast Guard has been conducting GNSS-A observations at the sites deployed along the Japan Trench and the Nankai Trough, named the Seafloor Geodetic Observation Array (SGO-A). At the SGO-A sites, we have been periodically conducting campaign observations for approximately 20 years. In these two decades, technological advancements in our observation and analysis techniques have enabled us to detect shallow slow slip events lasting for a year (Yokota and Ishikawa 2020). Our decadal observations have revealed the processes related to the 2011 Tohoku-oki Earthquake (Watanabe et al. 2021). We have also been developing a csv-based data format for GNSS-A observation data, which we have been discussing in a working group of the Inter-commission Committee on Marine Geodesy (ICCM) of the International Association of Geodesy (IAG).