十勝岳で観測される火山性地震のスペクトル解析 : 特に単純なスペクトル構造をもつ地震について

抄録

Mt. Tokachi is an active volcano situated at the central part of Hokkaido, Japan. Its recent major eruptions occurred in 1926 and 1962. There are several craters in summit area. Among them the 62-IICrater is the most active one and shows continuous plume activities since the 1962 eruption. In the neighbouring 62-ICrater, geothermal activity renewed at the end of 1983, and then gradually intensified and enlarged its area. Various phenomena such as ejection of hot mud, small eruptions with column height of less than 100m, and natural ignition of sulfer were observed at the 62-ICrater in 1985. A vault, 30m long, was newly established in 1985 for volcano observation by Usu Volcano Observatory(UVO), Hokkaido University as a part of the National Project for Prediction of Volcanic Eruptions. The facility named Tokachidake-Kansoku-Koudou (TKC) is located at about 2.0 km northeast from the active craters and operated by the solar battery and radio telemetry successfully in the mountaneous environment under heavy snow and severe cold temperature in long winter. Seismic and geodetic data from TKC are telemetered to UVO together with the data from other staions at Mt.Tokachi. Those new data provide us a good homogeneous seismic data with especially improved signal-to-noise ratio. In this study, classfication of volcanic earthquakes is discussed based on the data observed at TKC by examining their frequency characteristics for the period from December, 1985 to March, 1987 (1178 events). Spcctral analysis of the classified earthquake groups and seismicity of each group will give us better knowledge about source mechanism of those volcanic earthquakes and hence the processes taking place under Tokachi volcano. Seismicity in this period is characterized by relatively steady one, though minor earthquake swarms sometimes occurred. In the last part of the period from December 20. 1986 to the present (as of March 1987), volcanic tremors appeared on the seismograms 8 times. For classifying earthquake groups, firrst, we selected two groups according to their frequency characteristics on the monitor seismograms (4 cm/sec), and labelled HF (high-frequency earthquakes, 418 events) or LF (low-frequency earthquakes, 268 events), respectively. Most of other earthquakes were labelled MF (medium-frequency earthquakes, 415 events) unless their frequency characteristics are unique and not regarded those between HF and LF. Spectra of HF, MF, and LF are generally characterized by broad spectral peak located at about 10 Hz or more, several Hz and 3 Hz, respectively.