The April 16 "main shock" earthquake that hit Kumamoto Prefecture and nearby areas was on par with the deadly 1995 Great Hanshin Earthquake and, according to a University of Tokyo professor, was around 16 times more energetic than a powerful foreshock that struck the area on April 14.
The main shock, which struck in the early hours of April 16, was a magnitude-7.3 temblor, while another large quake that hit on April 14 was magnitude-6.5. Other earthquakes, possibly triggered by the 7.3-magnitude temblor, have also occurred in the Aso region of Kumamoto Prefecture and in Oita Prefecture.
"The magnitude-7.3 earthquake in the early morning of April 16 had about 16 times the energy of the (magnitude-6.5) earthquake that occurred on the night of April 14, and it affected a wide area with strong tremors," says professor of seismology Naoshi Hirata, head of the Earthquake Research Institute at the University of Tokyo. "For an earthquake caused by an inland fault line it was quite large. Such large earthquakes influence the areas around them, so the Oita earthquake may have been triggered by (the April 16 main shock)."
According to the Japan Meteorological Agency (JMA), in between the magnitude-6.5 earthquake on the night of April 14 and 11 a.m. on April 16, there were a total of 252 earthquakes in the Kumamoto and Aso regions of Kumamoto Prefecture and in Oita Prefecture registering a 1 or higher on the Japanese 7-point seismic intensity scale.
The JMA considers the earthquakes in these three areas as coming from separate sources, but Gen Aoki, head of the JMA's earthquake and tsunami monitoring division, says, "Since the main shock in the Kumamoto region, there has been increased seismic activity in Kumamoto and Oita prefectures. I think the main shock has exerted some influence."
All of the areas experiencing earthquakes -- the Kumamoto and Aso regions of Kumamoto Prefecture and central Oita Prefecture -- are located in or near the Beppu-Shimabara rift zone, where changes in the Earth's crust can easily cause warps in the ground structure. There are no major fault lines known in the Aso region, but near the source of the Oita Prefecture quakes is a part of the Beppu-Haneyama fault line zone, which extends to the border with Kumamoto Prefecture.
Takashi Furumura, another professor of seismology at the University of Tokyo's Earthquake Research Institute, says, "When a single large fault line cracks open, the first earthquake that occurs is the main shock. But this time the quake occurred in an area with a complex structure of multiple fault lines running next to each other, which may have caused the larger main shock to come later. There is a possibility for a larger aftershock than what we have seen so far, and vigilance is needed."