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

The 2016 Kumamoto earthquakes in Kyushu, Japan, started with a magnitude (M) 6.5 quake on April 14 on the Hinagu fault zone (FZ), followed by active seismicity including an M6.4 quake. Eventually, an M7.3 quake occurred on April 16 on the Futagawa FZ. We investigated if any sign indicative of the M7.3 quake could be found in the space–time changes in seismicity after the M6.5 quake. As a quality control, 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 evalu‐ ated 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 Omori–Utsu 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 Gutenberg–Richter frequency–magnitude distribution to the spatiotemporal change in the seismicity revealed that the b value in the northern area increased significantly, the increase being Δb = 0.3–0.5. Significance was verified by a statistical test of Δb and a test using bootstrapping errors. Based on our findings, combined with the results obtained by a stress inversion analysis performed by the National Research Institute for Earth Science and Disaster Resilience, 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.