Large earthquakes that hit Kumamoto Prefecture's Aso and Kumamoto areas as well as Oita Prefecture in Kyushu on the morning of April 16 were separate events, a spokesman for the Japan Meteorological Agency (JMA) said at a press conference later that day.
While likely influenced by the "main shock," a 7.3-magnitude quake that struck at 1:25 a.m. on April 16, JMA earthquake and tsunami monitoring division head Gen Aoki stated that the three temblors are considered separate due to the distance between the three regions. He added, "I cannot think of any other cases in recent years where a sequence of earthquakes was observed across such a wide area." He admitted, "We're a little unsure about the coming (seismic) activity."
The 7.3-magnitude earthquake occurred a little north of an area where the Hinagu and Futagawa faults cross. These faults are also thought to have caused the large earthquake on April 14.
After the main shock, increased earthquake activity was seen to northeast of where the faults meet. At 3:55 a.m., a magnitude-5.8 earthquake, registering as high as an upper-6 on the Japanese 7-point intensity scale, hit the Aso area of Kumamoto Prefecture. At 7:11 a.m., a magnitude-5.3 earthquake registering up to a lower 5 on the Japanese scale hit central Oita Prefecture.
Regarding the JMA's initial inability to recognize the April 14 quake -- which measured up to a 7 on the Japanese scale -- as a foreshock, Aoki said, "When an earthquake occurs, it is generally very difficult to predict whether a larger earthquake is going to happen."