Takeshi Tameguri

Evaluation of recent activity at Satsuma-Iwojima —Felt earthquake on June 8, 1996—

Emission of large amount of volcanic gas occurs at the summit crater of Satsuma-Iwojima volcano located south of Kyushu, Japan. Sakurajima Volcanological Observatory (SVO), DPRI, Kyoto University has conducted seismic observation at a permanent station 1.5 km WNW of the summit crater since June 1995. Volcanic earthquakes at the volcano are classified into A-type (high frequency), B-type (low frequency), C-type (monochromatic) and tremor. A-type earthquake generated by shear fracture and B-type earthquake with volumetric source are dominant at the volcano. Volcanic activity of Satsuma-Iwojima was evaluated from seismicity of the volcanic earthquakes. During the period of 1995–1998, the seismicity has remained at a constant level with seismic energy release rate of 7×1014 erg/year. The seismicity was as high as that during 1975–1977. The energy release rate of volcanic earthquakes at the volcano has been stable at least for recent 20 years. Within the stable activity, a minor increase in activity occurred in mid-1990’s, as shown by a felt earthquake (M2.9) on June 8, 1996. The earthquake on June 8, 1996 may be related to topographic changes and ground deformation around the summit crater, because a crack with length of 100 m was found at southeast rim of the summit crater in October 1996 and the benchmark near the crack moved eastward by 6 cm during the period from June 1995 to April 1997. The stable activity for 20 years and minor inflation of activity estimated from seismic observation coincides with those from SO2 flux, highest fumarolic temperatures and gas equilibrium temperatures. Although daily number of B-type earthquakes with smaller amplitude increased almost 10 times in September 1998, this increase in the seismicity did not accompany any change of eruptive activity. Unstable seismicity of B-type earthquake may reflect fluctuations of condition in the gas conduit.

Reexamination of moment tensors for initial motion of explosion earthquakes using borehole seismograms at Sakurajima volcano, Japan

We estimated the seismic moment tensor for the initial phase of explosion earthquakes associated with Vulcanian explosions at the andesitic volcano Sakurajima. We applied a waveform inversion method to 24 events observed with borehole seismometers at 6 stations assuming a single triangular source time function. The three diagonal components representing volumetric change were dominant in the estimated moment tensors. The three diagonal components were positive and had almost the same values with the difference among them less than 10% for most of the explosion earthquakes, while non-diagonal components were negligibly small compared with the diagonal ones. Durations of source time functions had a range of 0.2–0.5 s and seismic moments were 1011 ∼ 1012 Nm. We conclude that the explosion earthquakes are initiated by an isotropic expansion.