Triggering and decay characteristics of dynamically activated seismicity in Southwest Japan

Previous research revealed a dynamically triggered earthquake front propagating along Southwest Japan, in correlation with the arrival of surface waves from the 2011 M9.0 Tohoku-oki earthquake. However, some characteristics of the dynamically initiated seismic activation have not been investigated in detail. We analyse first the dynamic triggering process, using NIED waveform data. Our results suggest that at several triggering sites in SW Japan, including volcanic areas in Kyushu, the arrival of larger amplitude Love waves may correlate better with the occurrence of first locally triggered events compared to the Rayleigh wave arrivals. We also observe that triggering takes place during relatively long-period surface waves. We search for other cases of dynamic triggering in SW Japan by remote large earthquakes and show that the distinctive phase arrivals of both Love and Rayleigh waves from the 2008 M7.9 Wenchuan earthquake triggered local events in Kyushu. These observations suggest that sharing motion on well-lubricated local faults may have started failure in these areas. Based on the analysis of an extended earthquake catalogue, we find that the stacked seismicity for the dynamically triggered regions in SW Japan after the 2011 Tohoku-oki earthquake has a significant, albeit weak increase after the megathrust event, followed by a relatively slow decay towards the background level for the next several days. The relatively slow decay may reflect the temporal pattern of stacked, swarm-like clustered seismicity, which has been mainly activated at volcanic/geothermal areas in Kyushu. The decay of seismicity within a single, activated earthquake cluster may have, however, different characteristics. The analysis of the aftershocks initiated dynamically at the Yufuin-Beppu geothermal area (Kyushu), by the 2016 M7.3 Kumamoto earthquake, shows a fast decay that may reflect quick stress recovery near higher-temperature volcanic or geothermal regions