Fig. 2

a Flowchart of our analysis. We use plate boundary configurations, the GCMT catalog, and the ISC-GEM catalog as input data. Grid points are set on the plate boundary at 10-km intervals. We analyze interplate earthquakes from the GCMT catalog. On the plate boundary, we define stick–slip patches according to the magnitude of the interplate earthquakes using Eqs. 1, 3, and 4. The seismic slip of the patches is calculated at the grid points based on the magnitudes of the GCMT and the ISC-GEM catalog earthquakes. The cumulative seismic slip on the stick–slip grid points is divided by the period of investigation, yielding the slip rate. The plate convergence rate minus the slip rate is defined as the slip-deficit rate on each stick–slip grid point. If the slip-deficit rate is larger than half of the relative plate motion, the grid point is regarded as a “slip-deficient grid”; otherwise, it is a “sufficiently slipping grid”. The largest elliptical area that contains no sufficiently slipping grid points is regarded as the area of hidden potential earthquakes. b Schematic illustration of the estimation of hidden potential earthquakes. The red and black solid circles show “slip-deficient” and “sufficiently slipping” grid points, respectively. The black open ellipse centered at the grid point shown by a cross is an example of source area estimation for hidden potential earthquakes. This ellipse contains four “slip-deficient” grid points. The gray open ellipse is an example of an unreliable ellipse as it does not contain any “slip-deficient” grid points and is terminated by the edge of the plate boundary. Each grid point has an ellipse drawn in the same way