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.
Highlights
The 2016 Kumamoto earthquakes, including the mainshock (M7.3), were a series of shallow, strong earthquakes that occurred at and around 01:25 JST on April 16, 2016 (16:25 UTC on April 15), near Kumamoto City in Kyushu, Japan; the first foreshock (M6.5) occurred at 21:26 JST (12:26 UTC) on April 14, 2016.Several major earthquakes occurred inland in Kyushu at shallow depths (≤ 30 km) within a 100–200-km radius preceding the 2016 Kumamoto mainshock
Prior to the Kumamoto earthquakes We first analyzed the seismicity around the Kumamoto region (Fig. 1a) from 2010 until the 2016 Kumamoto earthquake, where the data of earthquakes of M1.0 and larger were completely detected in the Hypocenter Catalog of the Japan Meteorological Agency (JMA)
We applied the single epidemic-type aftershock sequence (ETAS) model and the two-stage ETAS model to search for a change-point in the entire period using the X-windows-based ETAS applications (XETAS) program
Summary
Several major earthquakes occurred inland in Kyushu at shallow depths (≤ 30 km) within a 100–200-km radius preceding the 2016 Kumamoto mainshock. In January and April 1975, two events of M6.5 and M6.3 (the Northwest Kagoshima-ken earthquakes) occurred successively at distances of 40 and 65 km, respectively, south of the 2016 Kumamoto mainshock. Fukuoka-ken Seiho-oki earthquake) off the northern coast of Kyushu and 110 km north of the Kumamoto event. In November 2015, a shallow M7.0 earthquake (the Satsuma-hanto Seiho-oki earthquake) occurred off the west coast of Kagoshima Prefecture, Kyushu, approximately 200 km southwest of the Kumamoto M7.3 earthquake. The March 11, 2011, M9.0 TohokuOki mega-earthquake occurred approximately 1200 km northeast of the Kumamoto event
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