日本地球惑星科学連合2018年大会

講演情報

[JJ] Eveningポスター発表

セッション記号 S (固体地球科学) » S-VC 火山学

[S-VC44] 島弧の火成活動と火山ダイナミクス

2018年5月24日(木) 17:15 〜 18:30 ポスター会場 (幕張メッセ国際展示場 7ホール)

コンビーナ:鈴木 雄治郎(東京大学地震研究所)、中村 仁美(海洋研究開発機構・地球内部物質循環研究分野)、入山 宙(国立研究開発法人 防災科学技術研究所)

[SVC44-P06] プリニー式噴火噴出物のソース一定2次元モデルにおける堆積物GSD構造の距離変化

*入山 宙1寅丸 敦志2山本 哲生3 (1.国立研究開発法人 防災科学技術研究所、2.九州大学 大学院理学研究院 地球惑星科学部門、3.北海道大学 低温研究所)

キーワード:2次元ソース一定モデル、粒径分布、降下火砕堆積物、GSD構造、Critical level、距離変化

In a constant supply from a point source at a certain height in the one-dimensional sedimentation column, our 1DCS model (one-dimensional constant source model) can construct the overall vertical structure of grain-size distribution (GSD) in the fall deposits controlled by the supply duration and the source GSD (Iriyama et al., in press). Especially, the upper limit of the existence of the largest particle (critical level) characterizes the sedimentary feature. In the ash cloud of plinian eruptions, GSDs are modified by the segregation during the transportation to the 1D source position. So, in order to establish the model to realistic plinian eruptions, in this study, we develop a 2D model in which the lateral transport in the ash cloud and the segregation effect from the ash cloud are taken into account.

In the two-dimensional constant source (2DCS) model, we call the GSD above the vent as the 2D source GSD. We assume: 1) the 2D source GSD is constant during the supply duration, 2) the velocity field in the ash cloud is constant with time, 3) the fall velocity only depends on the particle size and is constant with time, 4) no lateral transport after segregation from the bottom of the ash cloud occurs (no wind condition).

We obtain the lateral change of the GSD in the ash cloud which decreases exponentially with distance from the 2D source due to the segregation effect. The GSD in the sediment (sediment GSD) corresponds to the GSD in the ash cloud satisfying the time relation for each particle. We analyze the lateral change of the critical level with a use of given velocity field in the ash cloud. The critical level decreases with the distance depending on degree of the particle segregation. For example, rapid expansion of ash cloud reduces the degree of particle size segregation, then the lateral change of the critical level becomes gradual. The decreasing rate of critical revel with distance become high with decreasing the supply duration because the total amount of the largest particles decreases relatively. Using the 2DCS model and observed values (e.g., the 2D source GSD obtained by geological survey and the velocity field expected by the satellite image), the lateral changes of the GSD structures in the pyroclastic fall deposits can be predicted and can be compared to the observed ones.