マグマ供給システムの自己制御機構 : 観測量と力学的モデルとの比較  [in Japanese] Self-control system of magma plumbing : Comparisons between observations and mechanical models  [in Japanese]

Krafla火山やKilauea火山では, 噴火割れ目位置(岩脈貫入の距離)の時系列ダイアグラムや, 地殻変動, 積算噴出量, 噴出量/供給量比, 地震活動による応力緩和などの時系列が互いに関連して変化している。これをもとに, マグマ供給系の自己制御機構を提案し, 貫入順序やマグマ供給系の基本パターンをモデル化した。自己制御機構のバリエーションは, マグマの生産率, 地殻の拡大率, 応力緩和過程で決まる。自己制御機構が働く場所は, 深さ方向の応力勾配を含む見かけの浮力が中立となる深さ付近である。クラックの相互作用をもとにしたゼラチンを用いたアナログ実験は自己制御機構による貫入順序を説明できる。自己制御機構による応力変動の方程式により, 側噴火の時代と中心噴火を繰り返す火山の成長を, 火山体の応力変動をパラメータとして定量的に評価できる。

The self-control system of magma plumbing is proposed based on the following field observations of volcanie activity on Krafla and Kilauea volcanoes: the time series diagram of fissure eruption sites (the distance from the center of a volcano vs. time) (Takada, 1995), the stress variation from the surface deformation around the volcanic center the diagrams of cumulative discharge volume (mass), and the eruptive volume-supplied volume ratio, and the timing of stress relaxation including faulting. Examples of Mauna Loa, Kliuchevskoi, Etna, and Piton de la Fournaise volcanoes are also discussed. Then, the intrusion sequence and the funda-mental pattern of magma plumbing are modeled. These models are applicable especially for volcanoes with a cyclic activity of the flank eruption phase and the central eruption phase (Takada, 1995). Convergence times as the period of the flank eruption phase, and recurrence times as the period of the central eruption phase on various volcanoes are plotted on the output-stress diagram proposed by Takada (1994c). A variation of the time series diagram is controlled by the magma production rate, the crustal extension rate and the relaxation process. Volcanoes with a short convergence time (1-10 years) and along recurrence time (more than 100 years) are plotted near the rift trend, that is, the region where regional extension and magma production are balanced. The magma supply is easy to be suppressed under this condition. In cases of volcanoes plotted on the region away from the rift trend, the convergence time decreases whereas the recurrence time increases. The stress relaxation plays an important role under this condition. The self-control system works well at the level where cracks arrest, that is, at the level of apparent neutral buoyancy including the stress gradient with depth (Takada, 1989; 1994c). For example, the continental crust can provide more than two levels satisfying the above condition: the shallow crust accomodating dike intrusions with fissure eruptions and the middle crust. Seismicities of several volcanoes also support this hypothesis. The analogue experiments using gelatin were carried out based on the crack interaction theory. The results supports the model of intrusion sequence by the self-control system. Differential equations on the stress variation by the self-control system are introduced. Solving the equations, the volcano growth with a cyclic activity of flank eruption phase and the central eruption phase is parameterized as the stress change.