2023 Volume 71 Pages 13-23
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.
