Eruptive Activity of Mt. Usu in 1978 Detected by the Seismometrical Observation of Eruption Tremors

Abstract

During the summer of 1978, numerous small eruptions had occurred intermittently from the Gin-numa Crater at Mt.Usu, Hokkaido, Japan. From July 14 to August 14, 1978, seismic signals from SRM, the seismic station at the south rim of the summit crater, were continuously recorded by a drum-recorder at the temporary base of Usu Volcano Observatory, Hoaakido University. 374 volcanic tremors were identified on the seismograms. Since all the known eruptions which produced ash fall in the flanks are registered by a seismometer at SRM as a tremor, and no tremor which is not related to an eruption was identifed in this period, we can assume reasonably that all tremors are originating from the eruption. Hence, those tremor data give us a homogeneous data set of the eruptive activity. The amplitude and the duration of a tremor are closely related to the eruption intensity. Duration was mostly less than five minutes. Nearly equally spaced termor activity (“banded tremor”) were observed on the drum recorder seismogram during this period. Similar phemmena to banded tremor are widely observed at active volcanoes: directly corresponding to magmatic eruptions (Klyuchevskoy, Galunggung, etc.) -phreatic eruptions (Usu), or even during the pre-eruption stage (Karkar, Izu-Oshima, etc.) or during other active stages (Kilauea, Suwanosejima, etc.). About thirty percents of the continuous tremor (116 events) were accompanied with long-period isolated tremors (LT). Characteristics of LT were studied in detail using digital seismograms: LT indicates the dispersive long-period (ca,2Hz) and the very low velocity transmission (near to the sound velocity). LT always appears at the end of the eruption and never overlapped one another. The amplitude-frequency relation of LT shows a remarkable band at the large amplitude, which suggests aphysical limitation of LT generation. The waveform similarity among LT at a given station indicates that they are originating from nearly identical and repeatable source process. The time difference of the differential travel times of a pair of LT between two stations can be precisely calculated from the cross-correlation functions of the entire LT waves. The calculated time differences are consistent with the theoretical ones assuming that the LT source is exactly corresponding to each known vent (craterlet) in the Gin-numa crater. Our preferred model of LT excitation is the air-coupled Rayleigh Wave due to the successive puffiing at the shallowmost vent. When the eruptive power declines gradually towards the end of the eruption, materials temporarily close the vent. However, Puffing may occur and reopen the vent. Such process may continue until the eruptive power could not overcome the critical pressure.