The degassing fluctuation concerning sealing process before eruptions at Sakurajima volcano, Japan.

Transcription

1 The degassing fluctuation concerning sealing process before eruptions at Sakurajima volcano, Japan. Ryunosuke Kazahaya, Toshiya Mori (The University of Tokyo, Tokyo, Japan. ) Masato Iguchi (Kyoto University, Kyoto, Japan. ) 1 /24

2 Sealing of the conduit Sealing process and gas accumulation at the top of conduit before the eruption (e.g. Iguchi et al. 2008). Volcanic gas measurement is useful for examining these processes. Quantitative treatment of volcanic gas flux was limited to long-term observation (e.g. Edmonds et al. 2003). Does gas emission stop immediately before an eruption? How does the gas flux decrease? We examined how the volcanic gas flux fluctuates immediately before the eruption at Sakurajima. 2 /24

3 SO 2 visualization by using UV cameras (Mori and Burton, 2006; Bluth et al., 2007) UV Sensitive CCD UV band pass filters 200 Visualize transparent toxic SO ppm m Useful for hazard assessments Observe SO 2 flux with frequencies of the order 1Hz and plume speed Minamidake crater Showacrater Distinct the difference of the fumarolic activity of multiple vents We can detect the detailed volcanic gas fluctuation immediately before an eruption due to high temporal resolution of this system. 3 /24

4 About Sakurajima volcano. Minamidake-crater Showa-crater Two craters: Showa-crater and Minamidake-crater. There are several eruptions with strong air shocks per day now from Showa-crater. The Observation was conducted on Dec. 1 st and 2 nd, /24

5 SO 2 motion in volcanic plume Eruption plume Video camera s image SO 2 image Unfortunately, we could not obtain the SO 2 flux during and after the eruptions because the plumes of the eruptions contain volcanic ash which prevents the light penetrating through the Click plume. to play 5 /24

6 Calculation of SO 2 flux from SO 2 animation Gas mass passing through the lines per unit of time. Showa-crater s flux Minamidake -crater s flux 6 /24

7 SO 2 flux time series Sulfur dioxide flux (kg/s) Ash 5 emissions without an air shock UV cameras Traverse Focus on the two explosions! Explosions with strong air 0 10:20 10:40 11:00 11:20 11:40shock 12:00 12:20 time (mm:ss) SO 2 flux time series of UV cameras are consistent with that of conventional methods. 7 /24

8 First eruption. Stagnant gas mass was released immediately before the eruption. 2009/12/1 11:02 Click to play Air shock: 37.1 Pa(JMA; Volc. Act. Doc. Dec., 2009) 8 /24

9 Second eruption. 2009/12/1 11:15 Click to play Air shock: 97.0 Pa(JMA; Volc. Act. Doc. Dec., 2009) 9 /24

10 The model of the eruption at Sakurajima. Iguchi et al /24

11 SO 2 amount time series SO2 emission at the top of Showa crater (ppm*m^2/s) SO 2 flux gradually decreased Calibration SO 2 flux sharply decreased and Inflation started. Explosions with strong air shock SO 2 flux decreased monotonously -10 before the explosion with strong inflation :30 10:35 10:40 10:45 10:50 10:55 11:00 11:05 11:10 11:15 11: Tangential strain (nstrain) Stagnant gas mass was released. (leakage of gas) 11 /24

12 Difference between shape of two air shocks Air shock (Pa) Air shock (Pa) Air shock of first explosion (11:02) Air shock of second explosion (11:15) 12 /24

13 Characteristic of the explosions Second inflation started immediately before the first eruption. Creation of new sealing cap in a few minute is doubtful. It is possible that this phenomenon is explained by double sealing. Air shock wave forms were different between them. The difference might reflect the geometry and conduit shape around the sealing caps. It may be explained by double sealing too. 13 /24

14 Double sealing as plausible process 14 /24

15 Double sealing as plausible process Gradually sealed SO2 flux gradually decreased 15 /24

16 Double sealing as plausible process Sealing & Inflation SO2 flux sharply decreased and inflation started 16 /24

17 Double sealing as plausible process Leakage of gas Contraction Expansion at deep part Iguchi et al SO2 emission increased due to leakage 17 /24

18 Double sealing as plausible process Explosion First explosion occurred and second inflation started 18 /24

19 Double sealing as plausible process Sealing & Inflation Inflation succeeded and SO2 emission decreased monotonously 19 /24

20 Double sealing as plausible process Bigger Explosion Second bigger explosion occurred 20 /24

21 2009/12/2 Observation from KUROKAMI From this observation site, we can not measure Showa-crater s flux and Minamidake s one distinctively. SO2 flux contributed from both Showa-crater and Minamidake. Sulfur dioxide flux (kg /s) Air shock (ARIMURA) : 56 Pa Earth quake (peak height) :7.0E-5 m/s SO2 flux decreased Explosion 0 13: :40 13:45 13:50 13: Tangential Strain (nstrain) 21 /24

22 Conclusion (1) Explosions with strong air shock (>50 Pa) are associated with SO2 flux decrease and inflation at shallower part of the volcano. Stagnant gas mass was observed before the first explosion at 11:02 am Dec. 1, These observational result is consistent with the model of Iguchi et al. (2008). 22 /24

23 Conclusion (2) In Dec. 1, 2009, the inflation corresponds to second explosion started immediately after the first explosion. The shapes of these two explosion s air shock were different. It is possible that this phenomenon is explained by double sealing process. Quantitative examination for this process remains a challenge. 23 /24

24 Thank you so much. 24 /24