Estimated interplate aseismic slip. Green arrows show movements of the continental plate against the Philippine Sea plate. Ellipses centeredon the tips of green arrows indicate 1σ errors. Red circles represent the epicenters of low-frequency earthquakes during the corresponding period. Solid lines indicate isodepth contours with 20-km intervals. Areas enclosed by solid lines show the adopted fault patch. (a)–(e) correspond to the 2010 event, (f)–(i) correspond to the 2003 event, and (j)–(m) correspond to the 1997 event. These periods correspond to those in Fig. 3. The slip area first appeared beneath southwest Shikoku and expanded into the Bungo Channel, over time, during the 2003 and 1997 events. We used a variable scale of vectors to show characteristic features in the time evolution of each event.