Focal Mechanism and Seismic Activity in the Past

b. Focal Mechanism
Of the earthquakes in the Beppu-Shimabara rift zone, many show the focal mechanism (CMT solution) which has a tension axis in an N-S direction. Including the M6.5 earthquake on April 14 at 21:26 and the M7.3 earthquake on April 16 at 01:25, many of "the 2016 Kumamoto Earthquakes" have the focal mechanism which shows a strike-slip fault type with a tension axis in N-S direction.

c. Seismic activity in the past
For the seismic activity since January 1885, in the vicinity of the recent activity (Area b), there was a M6.3 earthquake on July 28, 1889, which caused damage with 19 fatalities and so on (damage: Comprehensive Bibliography of Destructive Earthquakes in Japan).

Japan Meteorological Agency
[Evaluation of "the 2016 Kumamoto Earthquakes" (May 13, 2016)]