This article from the Asahi Shimbun provides a bit more information on last-night’s quake. Here’s a quote:
“The 10:08 p.m. earthquake is the first since April 19 to register an intensity of lower 5 in the prefecture. It is believed to be an aftershock of the series of earthquakes that started in mid-April in and around Kumamoto Prefecture, the agency said.”
The article also says that operation of the nuclear reactors just south of Kumamoto was not affected.