William with a quick quake update. According to the Japan Meteorological Agency (which oversees earthquakes, too), an earthquake with a maximum seismic intensity of M5 or stronger may occur within a week of Sunday’s quake.
They said that the earthquake on the 26th occurred on the Takano-Shirahata (高野－白旗区間) and Hinagu (日奈久区間) sections of the Hinagu fault zone (日奈久断層帯), as can be seen on the map below, and noted that there have been consecutive earthquakes in this area in the past within short periods of time.
The mainshock of the Kumamoto earthquakes in April 2016 occurred about 28 hours after the foreshock, with the epicenter around the Futagawa fault zone, which is in contact with the Hinagu fault zone. In 1931, there were three M5.5 - 5. 8 quakes in the Amakusa / Ashikita region within five days.
Also, they indicated a probability over 3% of an M7.5 quake in the Hinagu section within 30 years.
Kumanichi: https://kumanichi.com/articles/705309 The blue lines indicate fault sections which ruptured in the ‘16 quakes; red, those not yet ruptured.