17:15 〜 19:15
[SVC32-P06] 岩手山における長周期地震とその火山活動推移における位置付け
キーワード：火山性地震、長周期地震、火山活動推移
Long-period (LP) and very long-period (VLP) events are thought to reflect fluid movement within hydrothermal systems in the shallow parts of volcanoes. Understanding their generation mechanism and activity is crucial for understanding volcanic activity. In this presentation, we report on the characteristics of LP events observed at Mt. Iwate and discuss their role in the context of volcanic activity.
Mt. Iwate is a composite volcano composed of Nishi-Iwate, which exhibits geothermal and fumarolic activity, and Higashi-Iwate located on its eastern side. Throughout recorded history, the volcano has experienced both magmatic eruptions at Higashi-Iwate and phreatic eruptions at Nishi-Iwate. Although no eruption occurred during the 1998 unrest, seismic activity and crustal deformation indicated magma ascent from the deep eastern side, followed by westward progression of activity. Geological studies also suggest the presence of a east-west trending fault zone, which may facilitate westward migration of volcanic fluid. After the 1998 unrest, Mt. Iwate remained relatively quiet. However, a slight increase in volcanic earthquakes beneath the summit area was observed since 2020, and seismic activity as well as strain and tilt changes around the volcano show noticeable change since early 2024. The early-stage crustal deformation is attributed to the intrusion of an east-west oriented dike at depths of 3-4 km beneath Higashi-Iwate. In contrast, the deformation observed after June 2024 was primarily concentrated near the western end of Nishi-Iwate, specifically around Mt. Omatsukurayama and Mt. Mitsuishiyama. Coinciding with the occurrence of deformation in Nishi-Iwate, LP events with a dominant period of about 10 seconds also began to be observed intermittently. The waveforms of LP events typically consisted of 4–5 cycles of 10-second period oscillations, exhibiting harmonic overtones. Notably, dilatational volumetric strain was observed at nearby stations about 100 seconds before the onset of LP events. Additionally, a quasistatic strain offset, indicating a volume change before and after the event, was also observed.
To investigate the generation mechanism of LP events, we performed moment tensor analysis. In the inversion, virtual point sources aligned with 500 m spacing both for horizontal and vertical were used to estimate the optimal source location and time functions of six moment tensor component and three single forces. The Green function was calculated using the finite difference method to account for the topography represented by a 50 m DEM. Despite relatively low signal-to-noise ratio of observed LP events, the optimal mechanism of almost all the analyzed LP events corresponded to the volumetric oscillation of a prolate ellipsoid with an east-west major axis at approximately 1.5 km below sea level near Omatsukurayama. This result suggests that LP events are caused by pressure perturbation in a fluid-filled resonator. However, no clear relationship was observed between the magnitude of LP events and the strain changes before and after the events. The estimated source location of LP events lies between the earthquake cluster in Nishi-Iwate caldera and the deformation source on the west side of Mt. Iwate, an area where the seismicity of ordinary volcanic earthquakes is relatively low. Considering that the inflation of the deformation source on the west side became dominant after the onset of LP activity in June 2024, these LP events and the associated short-term strain change are likely passive responses and pressure relaxations within the fluid system connecting eastern and western regions of Mt. Iwate. To understand the short- and mid-term activity of Mt. Iwate, it is thus important to understand the properties of these shallow fluid pathways.
Mt. Iwate is a composite volcano composed of Nishi-Iwate, which exhibits geothermal and fumarolic activity, and Higashi-Iwate located on its eastern side. Throughout recorded history, the volcano has experienced both magmatic eruptions at Higashi-Iwate and phreatic eruptions at Nishi-Iwate. Although no eruption occurred during the 1998 unrest, seismic activity and crustal deformation indicated magma ascent from the deep eastern side, followed by westward progression of activity. Geological studies also suggest the presence of a east-west trending fault zone, which may facilitate westward migration of volcanic fluid. After the 1998 unrest, Mt. Iwate remained relatively quiet. However, a slight increase in volcanic earthquakes beneath the summit area was observed since 2020, and seismic activity as well as strain and tilt changes around the volcano show noticeable change since early 2024. The early-stage crustal deformation is attributed to the intrusion of an east-west oriented dike at depths of 3-4 km beneath Higashi-Iwate. In contrast, the deformation observed after June 2024 was primarily concentrated near the western end of Nishi-Iwate, specifically around Mt. Omatsukurayama and Mt. Mitsuishiyama. Coinciding with the occurrence of deformation in Nishi-Iwate, LP events with a dominant period of about 10 seconds also began to be observed intermittently. The waveforms of LP events typically consisted of 4–5 cycles of 10-second period oscillations, exhibiting harmonic overtones. Notably, dilatational volumetric strain was observed at nearby stations about 100 seconds before the onset of LP events. Additionally, a quasistatic strain offset, indicating a volume change before and after the event, was also observed.
To investigate the generation mechanism of LP events, we performed moment tensor analysis. In the inversion, virtual point sources aligned with 500 m spacing both for horizontal and vertical were used to estimate the optimal source location and time functions of six moment tensor component and three single forces. The Green function was calculated using the finite difference method to account for the topography represented by a 50 m DEM. Despite relatively low signal-to-noise ratio of observed LP events, the optimal mechanism of almost all the analyzed LP events corresponded to the volumetric oscillation of a prolate ellipsoid with an east-west major axis at approximately 1.5 km below sea level near Omatsukurayama. This result suggests that LP events are caused by pressure perturbation in a fluid-filled resonator. However, no clear relationship was observed between the magnitude of LP events and the strain changes before and after the events. The estimated source location of LP events lies between the earthquake cluster in Nishi-Iwate caldera and the deformation source on the west side of Mt. Iwate, an area where the seismicity of ordinary volcanic earthquakes is relatively low. Considering that the inflation of the deformation source on the west side became dominant after the onset of LP activity in June 2024, these LP events and the associated short-term strain change are likely passive responses and pressure relaxations within the fluid system connecting eastern and western regions of Mt. Iwate. To understand the short- and mid-term activity of Mt. Iwate, it is thus important to understand the properties of these shallow fluid pathways.