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The 2016 Kumamoto earthquakes started with a magnitude (M) 6.5 quake on Apr14 on the Hinagu fault zone (FZ), followed by active seismicity including anM6.4 quake. Eventually, an M7.3 quake occurred on Apr 16 on the Futagawa FZ. Weinvestigated if any sign indicative of the M7.3 quake could be found in thespace-time changes of seismicity after the M6.5 quake. We determined in advancethe 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. Significanceof this decrease was evaluated by one standard deviation of sampled changes inthe rate of occurrence. We next confirmed that seismicity with M >= 3 was wellmodeled by the OU law with p ~ 1.5 +/- 0.3, which indicates that the temporaldecay 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 inthe analysis. This significance was confirmed by a bootstrapping approach. Ourdetailed analysis shows that the large p value was caused by the rapid decay ofthe seismicity in the northern area around the Futagawa FZ. Application of theslope (the b value) of the GR frequency-magnitude distribution to thespatiotemporal change in the seismicity revealed that the b value in thenorthern area increased significantly, the increase being db = 0.3-0.5.Significance was verified by a statistical test of db and a test usingbootstrapping errors. Based on our findings, combined with the results obtainedby a stress inversion analysis performed by the NIED, we suggested that stressnear the Futagawa FZ had reduced just prior to the occurrence of the M7.3quake. We proposed, with some other observations, that a reduction in stressmight have been induced by growth of the slow slips on the Futagawa FZ.

Anomalous decrease of relatively large shocks and increase of the p and b values preceding the April 16, 2016 M7.3 earthquake in Kumamoto, Japan