5:15 PM - 6:45 PM
[SVC31-P09] Investigation into the magma intrusion pathways of Mt. Usu using three-dimensional numerical model.
Keywords:magma intrusion, numerical model, Mt. Usu, dike
Mt. Usu, located in the southwest of Hokkaido, has erupted approximately every 30 years since the 1910 eruption. Future eruptions are now a concern because 24 years have passed since the 2000 eruption. Past craters of Mt. Usu are distributed in the summit crater floor and in the northeastern half of the volcanic edifice. Recently, Mantiloni et al. (2023) developed a three-dimensional numerical simulator called SAM (Simplified Analytical Model). The aim of this study is to evaluate how topographic and tectonic stress affect the crater distribution at Mt. Usu by applying this simulator.
SAM takes into account of tectonic stress, gravitational loading due to topography, and magma buoyancy. The intruded magma is represented by a penny-shaped crack with a fixed volume and radius. Crack moves to the direction in which the stress intensity factor, calculated from magma buoyancy and the minimum compressive principal stress at the crack edge, becomes the largest.
In this study, we examined the effects of compressive tectonic stress (TEST A), topographic effects (TEST B), and the topography of Mt. Usu (TEST C). In TEST A, we set a flat topography and varied the direction and magnitude of tectonic stress. Next, in TEST B, without setting tectonic stress, we performed several simulations with various heights of mountains (TEST B-1) and depths of valleys (TEST B-2). The uniform topography in one direction provides the two-dimensional calculations to simply examine the effects of mountains and valleys. Finally, in TEST C, we calculated magma intrusion pathways using topographic data of Mt. Usu. The magnitudes of tectonic stress were varied (TEST C-1). Additionally, we conducted calculations for the widely distributed starting points in a horizontal range of 20 km * 20 km under the compressive tectonic stress in the northwest-southeast (TEST C-2).
In TEST A, the strike of the crack was aligned parallel to the compressional axis. This result is consistent with the previous studies (Odé, 1957). Therefore, we consider that SAM is applicable even in the case where tectonic stress is compressive, which was not calculated by Mantiloni et al. (2023). In TEST B-1, there were no significant changes due to height of the mountain, and all cracks intruded towards the mountain. However, cracks did not reach the mountain when starting points were located more than 12.5 km away from the mountain ridge. In addition to the mountain height, the ratio of horizontal and vertical distance from the mountain ridge affects the magma intrusion pathways. In TEST B-2, there were no significant changes due to the depth of valley. As the distance from the valley increased, the crack paths deviated less. Cracks that intruded from horizontal distances greater than 5 km from the valley axis had changed their strike from parallel to perpendicular against the valley alignment. It is necessary to examine the stress distribution in the host rock in detail. In TEST C-1, without tectonic stress, craters were distributed around the summit and the northern foot. Increasing the magnitude of the compressive tectonic stress showed a tendency for craters to align in the direction of the tectonic stress. In TEST C-2, cracks intruded from the southwestern seaside reached the coastline and near Mt. Usu, while cracks intruded from Lake Toya deviated from the lake and reached the mountainous areas.
From these calculations, magma intrusion pathways seem significantly influenced by the topographic effects, and the surface crater locations depends on the starting point of intrusion. It is necessary to estimate the direction and magnitude of tectonic stress and starting points of magma intrusion for clarifying the physical mechanism governing the crater distribution of Mt. Usu. Additionally, since we here assumed homogeneous host rock in this calculation, it is also necessary to consider the influence of heterogeneous rigidity structure.
SAM takes into account of tectonic stress, gravitational loading due to topography, and magma buoyancy. The intruded magma is represented by a penny-shaped crack with a fixed volume and radius. Crack moves to the direction in which the stress intensity factor, calculated from magma buoyancy and the minimum compressive principal stress at the crack edge, becomes the largest.
In this study, we examined the effects of compressive tectonic stress (TEST A), topographic effects (TEST B), and the topography of Mt. Usu (TEST C). In TEST A, we set a flat topography and varied the direction and magnitude of tectonic stress. Next, in TEST B, without setting tectonic stress, we performed several simulations with various heights of mountains (TEST B-1) and depths of valleys (TEST B-2). The uniform topography in one direction provides the two-dimensional calculations to simply examine the effects of mountains and valleys. Finally, in TEST C, we calculated magma intrusion pathways using topographic data of Mt. Usu. The magnitudes of tectonic stress were varied (TEST C-1). Additionally, we conducted calculations for the widely distributed starting points in a horizontal range of 20 km * 20 km under the compressive tectonic stress in the northwest-southeast (TEST C-2).
In TEST A, the strike of the crack was aligned parallel to the compressional axis. This result is consistent with the previous studies (Odé, 1957). Therefore, we consider that SAM is applicable even in the case where tectonic stress is compressive, which was not calculated by Mantiloni et al. (2023). In TEST B-1, there were no significant changes due to height of the mountain, and all cracks intruded towards the mountain. However, cracks did not reach the mountain when starting points were located more than 12.5 km away from the mountain ridge. In addition to the mountain height, the ratio of horizontal and vertical distance from the mountain ridge affects the magma intrusion pathways. In TEST B-2, there were no significant changes due to the depth of valley. As the distance from the valley increased, the crack paths deviated less. Cracks that intruded from horizontal distances greater than 5 km from the valley axis had changed their strike from parallel to perpendicular against the valley alignment. It is necessary to examine the stress distribution in the host rock in detail. In TEST C-1, without tectonic stress, craters were distributed around the summit and the northern foot. Increasing the magnitude of the compressive tectonic stress showed a tendency for craters to align in the direction of the tectonic stress. In TEST C-2, cracks intruded from the southwestern seaside reached the coastline and near Mt. Usu, while cracks intruded from Lake Toya deviated from the lake and reached the mountainous areas.
From these calculations, magma intrusion pathways seem significantly influenced by the topographic effects, and the surface crater locations depends on the starting point of intrusion. It is necessary to estimate the direction and magnitude of tectonic stress and starting points of magma intrusion for clarifying the physical mechanism governing the crater distribution of Mt. Usu. Additionally, since we here assumed homogeneous host rock in this calculation, it is also necessary to consider the influence of heterogeneous rigidity structure.