14:15 〜 14:30
[SCG74-15] 御嶽山と桜島における火山泥流レオロジーの比較
★招待講演
Most volcanoes are covered with volcanic ash fallen after various types of ash-dominated eruptions. Once the ash combines with water, it is likely to run down slopes. The flow is referred to as lahar, which is widely observed all over the world. Lahar is one of the volcanic phenomena that cause severe damage to surrounding environment, since the speed is generally far faster than that of lava and the onset time is hard to predict [E. Bélizal et al., 2013; S. Jenkins et al., 2015]. Sometimes it occurs just after an eruption [Nakayama and Kuroda, 2003] whereas a large debris flow, which broke out about 30 years after the latest eruption due to heavy rainfall, was reported [Ogiso and Yomogida, 2015]. Moreover, lahar-flow is occasionally accompanied by seismic signals [Walsh et al., 2016; Ogiso and Yomogida, 2015], so that understanding flow characteristics of lahar is important to investigate the relation between lahar-flow and seismicity leading to early detection of the onset in addition to the purpose of simulating the flow.
Based on the background, we have performed rheological measurements of mixtures of volcanic ash and water, which are major compositions of lahar. The volcanic ash used in this study was collected at Sakurajima and Ontake volcanoes in Japan. The reason why the two volcanoes are focused is that lahar had flowed there after recent eruptions although the two types of volcanic ash are apparently different. In order to reveal key features in rheology and to compare rheological characteristics, the viscosity was measured changing the particle concentration and the shear rate. An important point of our findings is that the two types of mixtures show non-linear characteristics differently. For instance, Sakurajima samples show strong shear-thinning regardless of the particle concentration whereas the viscosity fluctuates in a longer time scale than rotational period of rheometer within a certain definite range of shear rate in the case of Ontake samples. Interestingly, the range of shear rate corresponds to that at which the relation between the viscosity and the shear rate shows positive slope or shear-thickening deviating from shear-thinning. Since these non-linear characteristics are considered to be induced by variations in particles such as size and shape [C. Chang and R. Powell, 1994; D. Genovese, 2012; S. Mueller et al., 2014], we mainly discuss the rheological changes of mixtures of volcanic ash and water with consideration for the particle size distribution.
Based on the background, we have performed rheological measurements of mixtures of volcanic ash and water, which are major compositions of lahar. The volcanic ash used in this study was collected at Sakurajima and Ontake volcanoes in Japan. The reason why the two volcanoes are focused is that lahar had flowed there after recent eruptions although the two types of volcanic ash are apparently different. In order to reveal key features in rheology and to compare rheological characteristics, the viscosity was measured changing the particle concentration and the shear rate. An important point of our findings is that the two types of mixtures show non-linear characteristics differently. For instance, Sakurajima samples show strong shear-thinning regardless of the particle concentration whereas the viscosity fluctuates in a longer time scale than rotational period of rheometer within a certain definite range of shear rate in the case of Ontake samples. Interestingly, the range of shear rate corresponds to that at which the relation between the viscosity and the shear rate shows positive slope or shear-thickening deviating from shear-thinning. Since these non-linear characteristics are considered to be induced by variations in particles such as size and shape [C. Chang and R. Powell, 1994; D. Genovese, 2012; S. Mueller et al., 2014], we mainly discuss the rheological changes of mixtures of volcanic ash and water with consideration for the particle size distribution.