Large-scale earthquakes have occurred repeatedly along the Nankai Trough, southwestern Japan, and the next one is expected to occur in the near future. These large-scale earthquakes are associated with the subduction of the Philippine Sea plate beneath the Eurasian plate. Coupling at the plate boundary is a source of large-scale earthquakes and it also causes crustal deformation in its vicinity. Consequently, an understanding of crustal deformation in the plate boundary area is important for detecting changes in plate coupling and for understanding and monitoring large-scale earthquakes. Here, we present results of an Interferometric Synthetic Aperture Radar (InSAR) time-series analysis that used data from the Advanced Land Observing Satellite (ALOS) Phased Array L-band SAR (PALSAR). Data collected during both ascending and descending orbits from 2007 to 2010 and from 2014 to 2022 were analyzed. Although widespread crustal deformation associated with plate subduction was not detected in the analysis of the former period data, it was seen in the analysis results for the latter period. The enhanced detection during the latter period may be due to the higher accuracy of the satellite orbit during the latter period compared with the former period.
Monitoring of earthquakes in the Nankai Trough requires objective and immediate detection of slow slip events (SSEs), not only under normal conditions, but also during postseismic deformation after large earthquakes that may mask SSEs. The variability of the data currently used for objective detection of long-term SSEs along the Nankai Trough has increased since the 2011 Tohoku earthquake. I identified the cause of this problem and improved the method of removing common mode errors. In addition, I extended the target area for the objective detection of SSEs to the Hyuga-nada. I furthermore proposed a framework for continuous monitoring of unsteady deformation that includes removal of postseismic deformations after large earthquakes. Postseismic deformations associated with two 1996 Hyuga-nada earthquakes were removed as an example. Unsteady displacements from 1998 to 2001, which were hidden by the postseismic deformations, were detected in the southern Hyuga-nada region. The center of the distribution of estimated slips was off the Miyazaki-Kagoshima border, and the magnitude was equivalent to Mw 6.7.