  Crustal deformation due to the 2016 earthquake sequence in Kumamoto, Japan,
that culminated in a preceding earthquake of magnitude M6.5 and a subsequence
M7.3 earthquake 28 hours later, caused stress perturbation on and around the
causative Futagawa-Hinagu fault zone. Monitoring changes in seismicity pattern
along this zone plays a role in understanding the process before and after
major earthquakes. For this purpose, we used stress-dependent laws in
statistical seismology: the Gutenberg-Richter frequency-size law and the
Omori-Utsu aftershock-decay law. Results obtained by using these laws show a
zone of high stress near the eventual epicenters of the M6.5 and M7.3
earthquakes before the start of the Kumamoto sequence, and after it, a
decreasing trend in stress along the Futagawa-Hinagu fault zone. Our detailed
analysis suggests an aseismic pre- and after-slip along the causative fault
zone, which is consistent with other geophysical observations. The pre-slip
locally reduced stress just prior to the M7.3 earthquake near its epicenter.
Recently, a system was proposed by Gulia and Wiemer (2019) that utilizes the
Gutenberg-Richter frequency-size law to judge, immediately after a large
earthquake, whether it was the mainshock or a foreshock to a future event. A
comparison of results suggests that further research is needed to improve this
system to take into consideration the spatial variation of frequency-size
distribution that allows the exploration of the possibility of a local pre-slip
of a future nearby earthquake.
