Measuring seismicity diversity and anomalies using point process models: case studies before and after the 2016 Kumamoto earthquakes in Kyushu, Japan

Abstract

Abstract This paper reviews seismic activity in and around the Kumamoto region before and after the April 16, 2016, Kumamoto earthquake of M7.3 using statistical models such as stationary, two-stage, and non-stationary epidemic-type aftershock sequence (ETAS) models to examine seismicity anomalies. Our findings are summarized as follows. First, most of the earthquake clusters before April 2016 are explained by the stationary ETAS model, except for a few clusters of swarm activity, one of which was remotely induced by the 2011 Tohoku-Oki earthquake (M9). The non-stationary ETAS model describes changes in the rate of background seismicity of swarm activity. Second, we revealed seismic quiescence relative to the stationary ETAS model in the foreshock sequence from the M6.5 earthquake on April 14, 2016, and further in the aftershock activity of the 2000 M5.0 earthquake that occurred in the shallower extension of the M6.5 foreshock zone. Thirdly, the main-fault and two off-fault aftershock clusters of the M7.3 mainshock show different features, caused by static triggering effects of the mainshock and/or effects induced by fault weakening. Finally, the b-value increased stepwise over time during the entire period of foreshocks and aftershocks, the reason of which is explained.