5:15 PM - 7:15 PM
[SVC32-P04] Geodetic Observation around Mt. Usu by Leveling Survey
Keywords:Mt. Usu, Leveling Survey, Pre-eruptive Magma Process
Mt. Usu is an active volcano located on the southern rim of the Toya Caldera in southern Hokkaido. Eruptions at Mt. Usu occur cyclically at intervals of 30 to 50 years, with four eruptions recorded since 1900. It is inferred that magma reservoirs exist at depths of approximately 4-6 km and 8-10 km beneath Mt. Usu (e.g., Tomiya et al., 2010), but geophysical observations have made it difficult to precisely determined their location and size.
Leveling survey is a highly precise method that measures elevation differences between benchmarks along a survey route. By conducting repeated surveys, vertical displacement at each benchmark over the given period can be determined. Although this method requires considerable time and effort, making its temporal resolution inferior to satellite geodetic methods such as GNSS and InSAR, it offers superior accuracy in measuring vertical deformation (Aoki, 2016).
Around Mt. Usu, there are several leveling routes: one passing through the western foot from Toyako Town (WF Route), another running along the coast of Uchiura Bay at the southern foot (UB Route) and a third connecting these two routes, encircling the volcano from northwestern to southeastern foot through Sobetsu Town. Repeated leveling surveys have been conducted along these routes by the Geospatial Information Authority of Japan (GSI). Prior to the 2000 eruption, leveling surveys captured signals of preparatory processes of the eruption in the deep magma reservoir (Murakami, 2003).
The surveys were conducted by Hokkaido University along both Encircling Route around Mt. Usu and the western shore of Lake Toya in 2006. In this study, surveys were conducted in 2023 and 2024 along both the 2006 route and UB Route to the first-order benchmark 7188. This presentation examines the spatial patterns of vertical deformation around Mt. Usu during both eruptive and quiescent periods from both the latest and past leveling survey results. The datasets used in this study include past survey results from GSI and Hokkaido University.
During the 1977–1978 and 2000 eruptions, deformation patterns exhibited similar trends. In these active periods, significant uplift of approximately 10 cm was observed on the northwestern and northeastern flanks, while large subsidence of several tens of centimeters was recorded on the northern flank.
During quiescent periods, only minor deformation of a few millimeters per year were observed. On the western foot, a distinct displacement pattern was noted: subsidence was observed near the uplift zone of the 2000 eruption, while uplift was observed slightly away from this zone to the northwest and southwest. On the northwestern foot, subsidence rates increased significantly after the 2000 eruption, suggesting a combination of steady long-term subsidence and relaxation of the uplift zone. Along the UB Route, between 1985–1986 and 1991–1992, subsidence was observed eastern benchmarks from benchmark J6. While uplift was recorded western up to the first-order benchmark 7189, approximately 5 km northwest. This displacement pattern is suggested to indicate magma accumulation in the deep magma reservoir before the 2000 eruption. Between 2004 and 2009, a similar pattern of eastern subsidence and western uplift is observed, but the uplift beyond benchmark 7189 showed little change. This could be attributed to post-seismic deformation after major earthquakes like Tokachi-Oki Earthquake in 2008. To account for this effect, a second-degree polynomial fitting method (Munekane, 2021) is applied to remove the crustal deformation due to tectonic stresses with the GEONET coordinate time-series. After this correction, the uplift west of benchmark 7189 decreased, but compared to the elevation change from 1985-1986 to 1991-1992, the area showing uplift relative to benchmark J6 expanded.
Leveling survey is a highly precise method that measures elevation differences between benchmarks along a survey route. By conducting repeated surveys, vertical displacement at each benchmark over the given period can be determined. Although this method requires considerable time and effort, making its temporal resolution inferior to satellite geodetic methods such as GNSS and InSAR, it offers superior accuracy in measuring vertical deformation (Aoki, 2016).
Around Mt. Usu, there are several leveling routes: one passing through the western foot from Toyako Town (WF Route), another running along the coast of Uchiura Bay at the southern foot (UB Route) and a third connecting these two routes, encircling the volcano from northwestern to southeastern foot through Sobetsu Town. Repeated leveling surveys have been conducted along these routes by the Geospatial Information Authority of Japan (GSI). Prior to the 2000 eruption, leveling surveys captured signals of preparatory processes of the eruption in the deep magma reservoir (Murakami, 2003).
The surveys were conducted by Hokkaido University along both Encircling Route around Mt. Usu and the western shore of Lake Toya in 2006. In this study, surveys were conducted in 2023 and 2024 along both the 2006 route and UB Route to the first-order benchmark 7188. This presentation examines the spatial patterns of vertical deformation around Mt. Usu during both eruptive and quiescent periods from both the latest and past leveling survey results. The datasets used in this study include past survey results from GSI and Hokkaido University.
During the 1977–1978 and 2000 eruptions, deformation patterns exhibited similar trends. In these active periods, significant uplift of approximately 10 cm was observed on the northwestern and northeastern flanks, while large subsidence of several tens of centimeters was recorded on the northern flank.
During quiescent periods, only minor deformation of a few millimeters per year were observed. On the western foot, a distinct displacement pattern was noted: subsidence was observed near the uplift zone of the 2000 eruption, while uplift was observed slightly away from this zone to the northwest and southwest. On the northwestern foot, subsidence rates increased significantly after the 2000 eruption, suggesting a combination of steady long-term subsidence and relaxation of the uplift zone. Along the UB Route, between 1985–1986 and 1991–1992, subsidence was observed eastern benchmarks from benchmark J6. While uplift was recorded western up to the first-order benchmark 7189, approximately 5 km northwest. This displacement pattern is suggested to indicate magma accumulation in the deep magma reservoir before the 2000 eruption. Between 2004 and 2009, a similar pattern of eastern subsidence and western uplift is observed, but the uplift beyond benchmark 7189 showed little change. This could be attributed to post-seismic deformation after major earthquakes like Tokachi-Oki Earthquake in 2008. To account for this effect, a second-degree polynomial fitting method (Munekane, 2021) is applied to remove the crustal deformation due to tectonic stresses with the GEONET coordinate time-series. After this correction, the uplift west of benchmark 7189 decreased, but compared to the elevation change from 1985-1986 to 1991-1992, the area showing uplift relative to benchmark J6 expanded.