4:15 PM - 4:30 PM
[SCG55-33] Quantitative effect evaluation and detection of internal tides on GNSS-A seafloor crustal deformation observations
Keywords:GNSS-A, internal tides
Among various ocean dynamics, internal tides are one of the error factors in various underwater acoustic measurements because they are difficult to observe directly. GNSS-A (Global Navigation Satellite System – Acoustics combination) observation, which is important for observing earthquakes and plate tectonics, is a technique for observing seafloor crustal movements using underwater acoustics. The relative position of an acoustic station installed on the seafloor in advance is determined by acoustic ranging between a ship or the like on the sea surface and an acoustic station installed on the seafloor in advance. Furthermore, by determining the sea surface position using GNSS, the position of the seafloor can be accurately grasped. Thus, even in GNSS-A, where underwater acoustic ranging plays an important role, the influence of internal tides is thought to exist.
In the field of GNSS-A, Kido et al. (2008) first pointed out the influence of the tidal cycle, but since then, it has not been sufficiently studied. In this study, to quantitatively clarify the influence of internal tides, past data from the SGO-A observation network measured by the Japan Coast Guard on the east coast of Japan was analyzed. In this study, we use the mean sound speed change and sound speed spatial deviation during GNSS-A observations extracted by GNSS-A analysis software called GARPOS (Watanabe et al., 2020) and the XBT and XCTD observation data during GNSS-A observations to discuss the effect of the numerically reproduced internal tide on the field during GNSS-A observations. The figure shows the results of XBT and XCTD observations during GNSS-A observations measured in the Kumano Nada Sea in January 2014. The effect of the mode 1 internal tide on the sound speed field at this time matched the actual measured XBT and XCTD observation results. In addition, the mean sound speed change and sound speed spatial deviation from GNSS-A were quantitatively consistent. The horizontal spatial wavelength of the internal tide can be estimated from the relationship between the mean sound speed and the spatial deviation in GNSS-A observations. On the Nankai Trough side (south of western Japan), there is a clear difference in the distribution of the estimated wavelengths during observations considered to be mode 1 from the XBT and XCTD observation data and during observations considered to be other modes, suggesting that the wavelengths of each mode actually affect the GNSS-A observation data. In other words, it can be said that the horizontal spatial wavelength of the internal tide can be estimated from the GNSS-A observation data. The direction of the internal tide inflow can also be determined from the relationship between the mean sound speed and the spatial deviation, and a synchronized state can be confirmed at some nearby observation points. However, there are cases where the directions are very complex, and the GNSS-A observation data shows that the actual internal tide changes due to various factors.
This study provided a concrete understanding of the impact of the internal tide on GNSS-A and its mechanism for the first time, and at the same time, it was found that GNSS-A detects the internal tide secondarily. A detailed elucidation of the effect of internal tides on GNSS-A may contribute to improving the sophistication of these observations and to our understanding of internal tides.
Achnowledgement: This study was supported by ERI JURP 2024-Y-KOBO12 in Earthquake Research Institute, the University of Tokyo, by SECOM science and technology foundation, and by JSPS KAKENHI Grant Number JP21H05200 in Grant-in-Aid for Transformative Research Areas (A) “Science of Slow-to-Fast Earthquakes.”
In the field of GNSS-A, Kido et al. (2008) first pointed out the influence of the tidal cycle, but since then, it has not been sufficiently studied. In this study, to quantitatively clarify the influence of internal tides, past data from the SGO-A observation network measured by the Japan Coast Guard on the east coast of Japan was analyzed. In this study, we use the mean sound speed change and sound speed spatial deviation during GNSS-A observations extracted by GNSS-A analysis software called GARPOS (Watanabe et al., 2020) and the XBT and XCTD observation data during GNSS-A observations to discuss the effect of the numerically reproduced internal tide on the field during GNSS-A observations. The figure shows the results of XBT and XCTD observations during GNSS-A observations measured in the Kumano Nada Sea in January 2014. The effect of the mode 1 internal tide on the sound speed field at this time matched the actual measured XBT and XCTD observation results. In addition, the mean sound speed change and sound speed spatial deviation from GNSS-A were quantitatively consistent. The horizontal spatial wavelength of the internal tide can be estimated from the relationship between the mean sound speed and the spatial deviation in GNSS-A observations. On the Nankai Trough side (south of western Japan), there is a clear difference in the distribution of the estimated wavelengths during observations considered to be mode 1 from the XBT and XCTD observation data and during observations considered to be other modes, suggesting that the wavelengths of each mode actually affect the GNSS-A observation data. In other words, it can be said that the horizontal spatial wavelength of the internal tide can be estimated from the GNSS-A observation data. The direction of the internal tide inflow can also be determined from the relationship between the mean sound speed and the spatial deviation, and a synchronized state can be confirmed at some nearby observation points. However, there are cases where the directions are very complex, and the GNSS-A observation data shows that the actual internal tide changes due to various factors.
This study provided a concrete understanding of the impact of the internal tide on GNSS-A and its mechanism for the first time, and at the same time, it was found that GNSS-A detects the internal tide secondarily. A detailed elucidation of the effect of internal tides on GNSS-A may contribute to improving the sophistication of these observations and to our understanding of internal tides.
Achnowledgement: This study was supported by ERI JURP 2024-Y-KOBO12 in Earthquake Research Institute, the University of Tokyo, by SECOM science and technology foundation, and by JSPS KAKENHI Grant Number JP21H05200 in Grant-in-Aid for Transformative Research Areas (A) “Science of Slow-to-Fast Earthquakes.”