ALBERT

Description: Geospatial Information Authority of Japan published a framework to support the transformation for positioning applications, POS2JGD, to provide gridded models for transform between observation and the reference epochs of Japanese Geodetic Datum 2011. While the present gridded model is created from national GNSS CORS with an average spacing of 20 km, higher spatial resolution will inevitably be required with the development of positioning techniques in the near future. Thus Yamashita et al. (2022) proposed a modeling approach to combine GNSS CORS and InSAR Time Series Analysis (InSAR TSA) using ALOS-2 data, and implied that it could contribute to improving model spatial resolution. However, ALOS-2 data is available from 2015, and does not cover the reference epoch, so we evaluate performance of the approach proposed by Yamashita et al. (2022) in the case that InSAR TSA result is temporally extrapolated to transform into the reference epoch.This study focuses on Chiba region in Japan, where rapid subsidence is locally detected by geodetic leveling. Considering displacement derived from geodetic leveling (2012~2022) as true value, we estimate transformation error of gridded model derived from GNSS CORS (same as POS2JGD), and gridded model derived from GNSS CORS and InSAR TSA. For temporal extrapolation, the velocity obtained by InSAR TSA is extrapolated linearly. As a result, applying simple linear extrapolation of InSAR velocity still contributes to improvement of model spatial resolution in subsidence area with large velocity. In this presentation, we report difference in the results obtained from the two methods.

Description: Geodetic observations can be often powerful tools for natural disaster monitoring. To proactively monitor the potential of disaster risk, spatiotemporally comprehensive observations are required. Synthetic aperture radar (SAR) is one of promising techniques for the purpose. An interferometric SAR (InSAR) method has been utilized to broadly and locally map ground surface changes. However, the standard InSAR is not always effective because it does not have sufficient measurement accuracy due to various errors. Against the background, InSAR time series analysis is an effective technique to improve the detectability by statistically processing a large number of InSAR images. We processed ALOS-2 satellite data observed over 7 years from 2014 to 2021 for InSAR time series analysis. In general, it is difficult for InSAR to distinguish spatially long-wavelength tectonic signals from long-wavelength noises which are comparable. To overcome the shortcoming, we derived a complete nationwide surface change map that has both tectonic wide-ranging deformation and locally distributed deformation, by incorporating displacements which are measured at nationwide deployed GNSS sites. Our deformation map successfully detects various types of land deformations such as inflation/deflation of volcanoes, ground subsidence, landslides, post-seismic deformation which have slowly proceeded with a few cm/mm per year. In this presentation, we will show the analysis results and the effectivity of InSAR-based nationwide land monitoring. Acknowledgments: ALOS-2 data were provided based on the joint research agreement with JAXA and under a cooperative research contract between GSI and JAXA. The ownership of ALOS-2 data belongs to JAXA.