桜島・昭和火口の火山活動に伴うハーモニック微動のピーク周波数

抄録

We investigated temporal changes in the peak frequency of harmonic tremors accompanied by eruptions at the Showa crater of the Sakurajima volcano from January 1 to September 30, 2015. The peak frequency characteristics of harmonic tremors at the summit crater of Minamidake from 1982 to 2002 were also compared. We calculated the running spectrum of the harmonic tremors of eruptions at the Showa crater and detected their peak frequency based on the findings of Maryanto et al. (2008) by modifying the parameters. Based on the value of peak frequency and associated temporal changes, we classified harmonic tremors at the Showa crater into the following three types: nearly constant (NC), positive gliding (PG), negative gliding (NG), and others. A sharp transition, with abrupt peak frequency of the harmonic tremor changes over a brief period, was also observed at the Showa crater. Assuming that the fundamental frequency corresponds to changes in the length of the gas pocket, the length of the gas pocket was estimated to change from 110 m to 220 m at the Showa crater. Compared with the features of peak frequencies of harmonic tremors at the summit crater of Minamidake (Maryanto et al., 2008), those at the Showa crater present three differences: 1) Compared with HTB of the summit crater of Minamidake (Maryanto et al., 2008), Depending on the event, NC values had different frequency bands at the Showa crater. HTBs occur several hours after swarms of B-type earthquakes and their peak frequencies remained within a certain range (Maryanto et al., 2008). 2) The increase in f₀ of PG at the Showa crater was smaller than that of HTE at the summit crater of Minamidake. HTEs occur several minutes after an eruption, as well as accompanying remarkably strong eruptions and their peak frequencies showed a gradual increase (Maryanto et al., 2008). 3) NG and sharp transitions were only recognized in harmonic tremors during volcanic activity at the Showa crater.