  The 2016 Kumamoto earthquakes started with a magnitude (M) 6.5 quake on Apr
14 on the Hinagu fault zone (FZ), followed by active seismicity including an
M6.4 quake. Eventually, an M7.3 quake occurred on Apr 16 on the Futagawa FZ. We
investigated if any sign indicative of the M7.3 quake could be found in the
space-time changes of seismicity after the M6.5 quake. We determined in advance
the threshold magnitude, above which all earthquakes are completely recorded.
We then showed that the occurrence rate of relatively large (M >= 3)
earthquakes significantly decreased 1 day before the M7.3 quake. Significance
of this decrease was evaluated by one standard deviation of sampled changes in
the rate of occurrence. We next confirmed that seismicity with M >= 3 was well
modeled by the OU law with p ~ 1.5 +/- 0.3, which indicates that the temporal
decay of seismicity was significantly faster than a typical decay with p = 1.
The larger p value was obtained when we used data of the longer time period in
the analysis. This significance was confirmed by a bootstrapping approach. Our
detailed analysis shows that the large p value was caused by the rapid decay of
the seismicity in the northern area around the Futagawa FZ. Application of the
slope (the b value) of the GR frequency-magnitude distribution to the
spatiotemporal change in the seismicity revealed that the b value in the
northern area increased significantly, the increase being db = 0.3-0.5.
Significance was verified by a statistical test of db and a test using
bootstrapping errors. Based on our findings, combined with the results obtained
by a stress inversion analysis performed by the NIED, we suggested that stress
near the Futagawa FZ had reduced just prior to the occurrence of the M7.3
quake. We proposed, with some other observations, that a reduction in stress
might have been induced by growth of the slow slips on the Futagawa FZ.
