TOKYO — Seismic activity in southern Japan is mystifying geologists and keeping the nation on edge.

The island of Kyushu has been struck by a series of significant earthquakes, with the epicenters moving progressively further inland. The cluster started with the deadly quakes that hit Kumamoto Prefecture last Thursday and Saturday. Temblors subsequently rocked the Mount Aso region and neighboring Oita Prefecture.

There is a known concentration of faults in the area. Still, experts say it is highly unusual to have a string of quakes measuring around magnitude 6 and stretching over such a vast area. The epicenter of the Oita jolt was about 100km away from the first Kumamoto quake.

“I don’t quite understand what is happening with the recent earthquakes, because it’s an unfamiliar phenomenon,” said Yoshihisa Iio, a professor at Kyoto University’s Research Center for Earthquake Prediction.  Source