1994 Multi-site Broadband Seismic Observation at Sakurajima Volcano, Japan

Broadband observation of volcanic earthquakes has been recently conducted at many volcanoes. At the Sakurajima Volcano in Japan, one of the most active volcanoes all over the world, we conducted two broadband seismic observations successfully in last three years. However, they left some problems in terms of the number of observation sites and recording systems. This paper reports the outline of our third observation using three broadband seismometers (Streckeisen STS-II) with continuous recording at the Sakurajima Volcano from February 18 to March 28, 1994. This observation is distinguished from the previous two by the three stations operating continuously over the entire period. Although the Sakurajima Volcano had very low seismic activities during this observation period, we observed some kinds of volcanic earthquake, A-type and B-type earthquakes and volcanic tremors, particularly one series of interesting clone events

Observation of Microtremors in the Tsukuba Area, Japan, using a Portable Broadband Seismometer

Studies of microtremors have been advanced by different approaches, that is, a variety of observational studies and analyses, for both the short-period and the long-period ranges since the microtremors for each range have their own source and site characteristics in time and space. We, therefore, conducted microtremor observation around Tsukuba City, Ibaraki, Japan, in July and August, 1991, in order to clarify site characteristics of six locations around the Tsukuba Mountain, deploying a portable broadband seismometer (Streckeisen STS-2) in the field of subsurface structure and studying ground motion in both frequency ranges, simultaneously. By a comparison with an STS-1 seismometer, the STS-2 gives reliable frequency ranges higher than 0.09 Hz and 0.05 Hz in horizontal and vertical components, respectively. The correlation of the reference site and the other sites implies that the source of microtremors shares common characteristics for the lower range ( 1 Hz), particularly 1.2 ～ 2.5 Hz, in this area. Three types of dominant peak frequencies for the range of 0.1 ～ 1 Hz are revealed: (1) the peak frequencies ranging from 0.2 to 0.3 Hz observed near Mt.Tsukuba can be explained by the topographic high model of Bard (1982). Two frequency peaks ranging (2) from 0.2 to 0.4 Hz and (3) from 0.5 to 0.8 Hz, observed at the stations on alluvials, are related to any vertical resonance in soft layers, consistent with other geological information. Amplitude ratios at sedimentary sites with respect to TSK (Mt.Tsukuba), a rock site, are greater than unity over 5 Hz where the ratios are reported to be smaller than unity in many areas, which implies relatively hard sedimentary layers in the Tsukuba area

Muography of the active Sakurajima volcano: recent results and future perspectives of hazard assessment

Sakurajima volcano is one of the world’s most active volcanoes with over 3,000 of explosive eruptions during the last five years. A muography observatory is under construction in international collaboration since 2017 at a distance of approx. 2,800 m in south-west direction from the active craters. Currently, the Sakurajima Muography Observatory is operating with 11 Multi-Wire-Proportional-Chamber-based Muography Observation Systems that are covering a sensitive surface area of 8.25 square meters. This work is focusing on the volcanological implications of muographic monitoring of Sakurajima: (i) tephra deposition, and erosion of the surface region exist due to heavy rains and post-eruptive lahars; (ii) magmatic plug formation was observed beneath the active craters after the deactivation of Showa crater in 2018 and after a dormant period of Central craters in 2020; (iii) machine-learning-based processing of daily muographic images achieved a fair area under the receiver operating characteristic curve score of 0.76

Seismic exploration at Fuji volcano with active sources : The outline of the experiment and the arrival time data

Fuji volcano (altitude 3,776m) is the largest basaltic stratovolcano in Japan. In late August and early September 2003, seismic exploration was conducted around Fuji volcano by the detonation of 500 kg charges of dynamite to investigate the seismic structure of that area. Seismographs with an eigenfrequency of 2 Hz were used for observation, positioned along a WSW-ENE line passing through the summit of the mountain. A total of 469 seismic stations were installed at intervals of 250-500 m. The data were stored in memory on-site using data loggers. The sampling interval was 4 ms. Charges were detonated at 5 points, one at each end of the observation line and 3 along its length. The first arrival times and the later-phase arrival times at each station for each detonation were recorded as data. P-wave velocities in the surface layer were estimated from the travel time curves near the explosion points, with results of 2.5 km/s obtained for the vicinity of Fuji volcano and 4.0 km5/s elsewhere

Characteristics and source modeling of broadband seismic signals associated with the hydrothermal system at Satsuma-Iwojima Volcano, Japan

The broadband seismic data observed at Satsuma-Iwojima volcanic island in southwestern Japan show a wide variety of interesting features. One of them is the periodic and frequency-dependent amplitude modulation of volcanic tremors. Another interesting feature is the occurrence of very long period (VLP) seismic pulses synchronized with the tremor amplitude modulation. Both the tremor amplitude modulation and the VLP pulses have fairly regular intervals of 46-50 min, suggesting a spatially fixed, non-destructive repetitive source. We performed waveform inversions to reveal the source mechanism of the VLP pulses, assuming a point source embedded in a medium that realistically represented the topography of the island. Numerical tests showed that the Green's functions used in the inversion were significantly affected by the topography. The observed VLP pulses were well explained by the rapid expansion of an inclined crack 100 m beneath the floor of the summit crater, which had a seismic moment of 3 × 10^10 Nm. Both the characteristic amplitude modulation and the VLP pulses are explained systematically by a model consisting of a water-filled crack and a network of gas paths. The water-filled crack was heated above the boiling temperature of the water in it. A sudden break of the unstable superheated state caused sudden vaporization and a pressure increase, which was observed as the VLP pulse. The network of gas paths consisted of swollen portions and constricted portions, which worked as tremor generators when vapor from the water-filled crack passed through

Source mechanisms of vulcanian eruptions at Mt. Asama, Japan inferred from volcano seismic signals.

GSL Special PublicationsPart of "Fluid Motions in Volcanic Conduits : A source of seismic and acoustic signals