Fig. 4

Candidate source areas for the epicenters of the 1858 earthquake swarm. a Seismicity for earthquakes with magnitude >5.0 since 1923. Circles show the hypocenters of earthquakes shallower than 100 km that were catalogued by the JMA. Candidate source areas for the 1858 swarm are shown by the red solid (Okinawa Trough area: OT), yellow dotted (Amami Island area: AM), and blue dot dashed (Ryukyu Arc area: RA) ellipses. Transparent dotted ellipse denotes the epicenters of the 1980 swarm. Red-colored contours show the distance from Naha; solid black triangle shows the location of Naha. b Magnitude–time diagram for earthquakes with magnitude >5.0 in the central OT, denoted by the solid rectangle in (a). c Magnitude–time diagram for the 1980 swarm