IAVCEI 2013 Scientific Assembly - July 20 - 24, Kagoshima, Japan
Forecasting Volcanic Activity - Reading and translating the messages of nature for society

4W_2I-P3

Date/Time: July 24 Poster

Volcano deformation caused by magma recession during a vulcanian explosion atShowa-crater of Sakurajima, JapanTakashi Minowa, Ryohei Kawaguchi, Takeshi NishimuraGeophysics, Science, Tohoku University, Japan
E-mail: minowa@zisin.gp.tohoku.ac.jp

It is necessary to investigate spatio-temporal changes of pressure source during the eruptions to understand thedynamics of magma in volcanic conduit. In this study, we present a simple moving pressure source during avulcanian eruption and calculate volcano deformation. We compare the results with the observed data at Showa
crater of Sakurajima to understand the spatio-temporal change of magma recession.
We simplify the vulcanian eruption as follows. The explosion is triggered by a removal of a "cap" that pressurizes
magma in the conduit beforehand. The magma head propagates downward as volcanic ash ejects. During themagma head falls down, the normal stress that works on the conduit wall weaken. At the same time, the drag force
generated by magma flow is applied on the conduit wall, and its reaction force works on the bottom of conduit.
We calculate volcano deformation by using a 3-D boundary element method.

We duplicate topography of

Sakurajima by 10 m meshed DEM (GSI) and make a cylindrical conduit with a radius of 15 m under Showa crater.
Radial tilt and strain changes caused by the magma depression and drag force at the ground surfaces are calculated
at Arimura and Harutayama stations which are located at distances of 2.1 km and 3.2 km, respectively, from theShowa crater. The initial position of the magma head is set at 650 m altitude. We firstly explain deformation at
Arimura caused by the normal stress. The tilt does not change remarkably during the magma head is dropped from650 m to 0 m. The tilt starts to subside toward the crater at about 0 m and turns to uplift at about -1300 m. The
radial strain shows extension at the beginning and turns to contract when the magma head reaches at about -950
m. The tilt caused by the drag force and its reaction force shows uplift toward the crater, and then turns to subsidewhen the magma head reaches at about -1300 m. The strain shows contraction at the beginning, and then turns
to show extension at about -950 m. Similar changes are shown at Harutayama station. These calculations indicatethat recession process of magma in the conduit can be quantified by the tilt and the strain data.
We compare the calculation results with observation records of a Showa crater eruption on Feb 6, 2008, which isreported in Iguchi (2008). The observed radial strain shows a change from extension to contraction about 10 minafter the start of eruption. The calculation indicates that the magma head downs to -950 to -1150 m at that time.
When the magma head becomes deeper than -2000 m, the tilt caused by normal stress shows uplift toward thecrater, while such a change is not observed. This fact suggests that the eruption stopped at the depth shallower
than -2000 m or that another deep deflation source exist.
The observed uplift toward the recorded just after the start of the eruption may be explained by upward drag force
due to magma ascent at shallower part of the conduit.

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