Deep low-frequency earthquake activity associated with the 2018 eruptions in the Kirishima volcano group, Japan

Abstract

Deep low-frequency (DLF) earthquakes have occurred at depths of 10–30 km in the Kirishima volcano Group, Japan. Here we investigate the DLF earthquake activity that was associated with the 2018 eruptions, compare these DLF earthquakes with those associated with the 2011 eruptions, and provide inferences on magmatic fluid ascension during these two eruptions. We apply a new matched-filter method to the continuous waveform data from the 2017–2018 period to comprehensively detect the DLF earthquake activity surrounding the 2018 eruptions. This new method can detect microearthquakes using a single seismic station based on an index that is computed as the product of mutual information and the correlation coefficient to measure the similarity between the template and target waveforms. We perform the same analysis using the 2010–2011 waveform data for comparison with the DLF earthquake activity associated with the 2011 eruptions. We detect 75 DLF earthquakes at approximately 25 km depth during the 2017–2018 period, whereas we detect 1302 DLF earthquakes at similar depths during the 2010–2011 period. Although the number of detected 2017–2018 events is small, we identify two swarms of DLF earthquake activity in March and July 2017. The March 2017 swarm coincides with the appearance of mud pots and jet fumaroles at the surface, and the July 2017 swarm coincides with the initiation of crustal deformation, which indicates the inflation of a deep magma reservoir. Furthermore, the occurrence rate of DLF earthquakes increased slightly after the March 2018 eruptions. Although the occurrence rate of DLF earthquakes associated with the 2018 eruptions was much lower than that associated with the 2011 eruptions, the slight increase in DLF earthquakes during the 2018 eruptions implies a connection between the deep magmatic fluid ascension and shallow volcanic unrest in 2018, which is similar to that observed during the 2011 eruptions. Such a close temporal relationship between the DLF earthquakes and surface volcanic activity suggests that the pressure disturbance within volcanic conduits propagates rapidly from depth.