5:15 PM - 7:15 PM
[SVC32-P05] Crustal Deformation of Mt. Usu Derived from Two Periods of Airborne Laser Survey Data
Keywords:Airborne Laser Survey, Mt. Usu, Showa Shinzan, Crustal Deformation, Differential Analysis
Mount Usu is known as an active volcano, and it has erupted multiple times throughout recorded history. Significant crustal deformations have been observed in the foothill areas during eruptions in the 20th century, including those in 1910, 1943, 1977, and 2000 (Soya et al., 2007, etc.). This study uses elevation difference analysis from aerial laser survey data taken in two periods to clarify the crustal deformations around Mount Usu since 2009.
The data used in the analysis includes aerial laser survey data conducted by the Hokkaido Development Bureau Muroran Development Construction Department in 2009 (historical data), and data from a 2024 aerial laser survey conducted by the same bureau (new data). The mesh data for the new data has a 0.5-meter interval, while the historical data's mesh interval was 1 meter. Therefore, the original data from the historical survey was reprocessed to create 0.5-meter interval mesh data, from which the elevation differences were calculated.
The elevation differences for both periods are shown in Figure 1. Overall, areas with significant terrain modifications due to sand and gravel extraction or construction activities near the foothills show larger elevation differences. However, notable terrain changes are also observed near the summit and the surrounding foothills. First, within the summit crater (Figure 2), erosional landforms caused by surface collapses on the southwestern slopes of the potential dome features "Usu Shinzan" and "Ogariyama," which constitute the summit of Mount Usu, are identified (negative elevation differences). Additionally, beneath these surface collapses, regions with positive elevation differences are formed, indicating that sediments have accumulated. The surface collapses continue in the collapsed areas on the southern side of the lava dome "Dai Usu," where collapse had already occurred during the 2009 survey, and sediment accumulation is observed downstream. Moreover, in the Gin-numa crater formed during the 1977–78 eruption, sediment accumulation at the crater floor is observed due to the collapse of the crater wall.
On the other hand, characteristic terrain changes are observed on the western side of the summit crater, including Showa Shinzan and the Nagayagawa river to the west. "Showa Shinzan" is a lava dome formed during the 1943–45 eruption. The process of lava dome formation (uplift) was recorded in the Mimatsu diagram (Mimatsu, 1962), and after volcanic activity ceased, subsidence (elevation decrease) continued, as reported (subsidence rates near Showa Shinzan were 20–35 cm over 11 years from 1982 to 1993, Hokkaido University Usu Volcano Observatory, 1994). The elevation difference diagram in this study also confirms similar subsidence in the lava dome near Showa Shinzan and surrounding potential domes. Although the rate of subsidence has slowed compared to previous speeds, it was found to be approximately 20–30 cm over 15 years from 2009 to 2024. Additionally, this subsidence is consistent with the deformation observed in a more recent analysis of interferometric SAR data from the ALOS-2 satellite by the Geospatial Information Authority of Japan (GIAJ) (GIAJ website, Figure 4). At the outer edge of the lava dome forming the summit of Showa Shinzan, small-scale collapses are occurring, and erosion is evident near the source area with sediment accumulation on the lower slopes. Moreover, in the Nagayagawa river flowing to the east of Showa Shinzan, erosion is observed on the outer side of the river meander, which is the attack slope, and sediment accumulation is observed on the inner side of the meander.
In the past, subsidence around Mount Usu and Showa Shinzan was confirmed through interferometric SAR, but it was difficult to capture finer details of local sediment movement, subtle uplifts, and subsidence. By conducting differential analysis of the two periods of aerial laser survey data, this study was able to capture not only the broad-scale crustal deformation of the volcanic body but also more detailed topographical changes such as collapses, erosion, and sediment accumulation.
The data used in the analysis includes aerial laser survey data conducted by the Hokkaido Development Bureau Muroran Development Construction Department in 2009 (historical data), and data from a 2024 aerial laser survey conducted by the same bureau (new data). The mesh data for the new data has a 0.5-meter interval, while the historical data's mesh interval was 1 meter. Therefore, the original data from the historical survey was reprocessed to create 0.5-meter interval mesh data, from which the elevation differences were calculated.
The elevation differences for both periods are shown in Figure 1. Overall, areas with significant terrain modifications due to sand and gravel extraction or construction activities near the foothills show larger elevation differences. However, notable terrain changes are also observed near the summit and the surrounding foothills. First, within the summit crater (Figure 2), erosional landforms caused by surface collapses on the southwestern slopes of the potential dome features "Usu Shinzan" and "Ogariyama," which constitute the summit of Mount Usu, are identified (negative elevation differences). Additionally, beneath these surface collapses, regions with positive elevation differences are formed, indicating that sediments have accumulated. The surface collapses continue in the collapsed areas on the southern side of the lava dome "Dai Usu," where collapse had already occurred during the 2009 survey, and sediment accumulation is observed downstream. Moreover, in the Gin-numa crater formed during the 1977–78 eruption, sediment accumulation at the crater floor is observed due to the collapse of the crater wall.
On the other hand, characteristic terrain changes are observed on the western side of the summit crater, including Showa Shinzan and the Nagayagawa river to the west. "Showa Shinzan" is a lava dome formed during the 1943–45 eruption. The process of lava dome formation (uplift) was recorded in the Mimatsu diagram (Mimatsu, 1962), and after volcanic activity ceased, subsidence (elevation decrease) continued, as reported (subsidence rates near Showa Shinzan were 20–35 cm over 11 years from 1982 to 1993, Hokkaido University Usu Volcano Observatory, 1994). The elevation difference diagram in this study also confirms similar subsidence in the lava dome near Showa Shinzan and surrounding potential domes. Although the rate of subsidence has slowed compared to previous speeds, it was found to be approximately 20–30 cm over 15 years from 2009 to 2024. Additionally, this subsidence is consistent with the deformation observed in a more recent analysis of interferometric SAR data from the ALOS-2 satellite by the Geospatial Information Authority of Japan (GIAJ) (GIAJ website, Figure 4). At the outer edge of the lava dome forming the summit of Showa Shinzan, small-scale collapses are occurring, and erosion is evident near the source area with sediment accumulation on the lower slopes. Moreover, in the Nagayagawa river flowing to the east of Showa Shinzan, erosion is observed on the outer side of the river meander, which is the attack slope, and sediment accumulation is observed on the inner side of the meander.
In the past, subsidence around Mount Usu and Showa Shinzan was confirmed through interferometric SAR, but it was difficult to capture finer details of local sediment movement, subtle uplifts, and subsidence. By conducting differential analysis of the two periods of aerial laser survey data, this study was able to capture not only the broad-scale crustal deformation of the volcanic body but also more detailed topographical changes such as collapses, erosion, and sediment accumulation.