Seismicity and structure changes following the 2016 Kumamoto earthquake

The 2016 Kumamoto earthquake sequence has a complex character including three earthquakes of magnitude greater than 6 that occurred within 30 hours, that ruptured two major active faults (Hinagu and Futagawa), and for which seismicity was remotely triggered at the Aso volcano and Beppu areas, which are both to the east of the mainshock epicenter. We analyze continuous seismic records observed by NIED, JMA, and universities in Kyushu to understand the increase in seismicity and estimate changes in the seismic structure caused by the Kumamoto earthquakes. For the seismicity, we find about 200,000 new events based on the template matching technique. Among these earthquakes, we find small events that appear to be triggered by the foreshock. For the change in the structure, we use ambient seismic field correlations of all 9 components and find some areas have a significant decrease in seismic velocity after the largest foreshock (April 14), and other areas only change after the mainshock (April 16). We also observe a larger velocity change at the Aso area than vicinities.