A notable WNW-ESE alignment of crustal earthquakes, which have been active since the latter half of 1989, was observed along a narrow zone from Amakusa-nada through Izumi City to Kakuto area; it nearly corresponds to a discontinuity presumed in the subducting plate. Focal mechanisms of some events suggested left lateral movements with T-axes of north or northwest trends. Though not comparable in length with the above, alignments of similar trends are also inferred in Central Kyushu, where seismic activities are mainly characterized by NE-SW tectonic lines such as Beppu-Shimabara and Amakusa-nada grabens and Futagawa-Hinagu active fault system.
In a rather small area west off Satsuma peninsula, South Kyushu, crustal earthquakes also swarmed. The seismic zone was composed of two narrow planes steeply dipping and intersecting with each other. The events along the WNW-ESE alignment mentioned above also swarmed especially at the places where the alignment crosses Amakusa-nada graben, NE-SW fault systems in the northwest of Kagoshima prefecture and a graben-like structure trending NE-SW at Kakuto area. Generally speaking, high seismic activity is observed in an area where tectonic lines or alignments of different trends intersect.
Sporadic shallow earthquakes were noticed along “Nobeoka” tectonic line geologically inactive.