[SVC38-P28] Characteristics of shallow volcanic earthquakes beneath the summit of Unzen Volcano
Keywords:Unzen, volcanic earthquake, particle motion, spectral analysis
First, we relocated the hypocenters of these earthquakes from Jan. 1 to Sep. 7 2018. The relocated hypocenters are distributed beneath the lava dome and Mt. Fugen, and they are classified into 3 groups. Focal depths of Group 2 and 3 are located 1km and 2km below sea level under Mt. Fugen, while Group 1 (three earthquakes in this period) locates above sea level beneath the lava dome. The first motions of P-waves for Group 1 are all “down” in this study, and the duration times are longer than those of Group 2 and 3.
Next, we calculated particle motions of these earthquakes using relocated hypocenters. The result indicates that the earthquakes of Group 2 and 3 have clear P and S phases, which are similar to tectonic earthquakes. On the other hand, no obvious S phase is recognized in the seismograms of Group 1.
Spectral analysis shows that the seismograms of Group 1 are lacking in high frequency components and the low frequency ( 2 - 5 Hz) is dominant compared with Group 2 and 3.
We will discuss candidates for the source mechanism of these earthquakes based on the results of wave form analyses.