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[MIS34-P14] Change in b-values following the three M6 or 7-class earthquakes in the 2016 Kumamoto earthquake sequence
Keywords:b-value, stress, detectability of earthquake, Bayesian estimation, aftershock sequence, Gutenberg-Richter law
Data analyzed in this study was taken from the JMA catalog (last accessed on 11 May 2016). A study area was chosen to cover the aftershock area just before the M7.3 event and the epicenter of the M7.3 event, and four periods were considered: from 1 August 2015 to the occurrence of the M6.5 event (Period 0), between the occurrences of the M6.5 and M6.4 events (Period 1), between the occurrences of the M6.4 and M7.3 events (Period 2), and after the M7.3 event (Period 3). For each of the four periods, the b-value and temporal change in detection capability (i.e, the parameter of the magnitude-frequency distribution proposed by Ogata & Katsura ) were estimated simultaneously. Cases with the constraint of a common b-value over successive periods (e.g., b-values of Periods 1, 2, and 3 are the same while the one in Period 0 is different.) were also considered. In total, there were eight cases (models), depending on which periods have a common b-value, and the best model was selected from the eight with Akaike's Bayesian Information Criterion (ABIC).
The best model is the case where Periods 0, 1, and 2 have a common b-value of 0.736 and only Period 3 has a different value of 0.941; the b-value varies at the timing of the M7.3 event whereas it does not change with the M6.5 and M6.4 events. The difference of ABIC values between this model and the one where the b-values over the four periods are the same is 7.9, which suggests the statistical significance of the best model. Remarkable increase in the b-value following a major/megathrust earthquake has been found [e.g., Tormann et al., 2015, Nature Geoscience]. This increase will correspond to decrease of stress due to a large slip, because it has been suggested that the b-value is correlated inversely with stress [e.g., Scholz, 2015, GRL]. From this viewpoint, the result of this study suggests that the high stress in the focal area of the Kumamoto sequence was not decreased by the M6.5 and M6.4 events but the M7.3 event has released it.