5:15 PM - 7:15 PM
[MIS01-P04] Monitoring Micro-Displacements of the Unzen-Fugendake Lava Dome Using DEM Differencing, GBSAR, and Optical Distance Measurements
Keywords:lava dome, GBSAR, Structure from Motion - Multi-View Stereo
Lava domes typically form and collapse repeatedly during periods of volcanic activity, with most of the dome collapsing during such active phases, resulting in high pyroclastic flow-related hazards. In this study, we conducted a comprehensive evaluation of the gravitational deformation and erosion processes of the Unzen-Fugendake lava dome by combining three methods: (1) Ground-Based Synthetic Aperture Radar (GBSAR), (2) optical distance measurements, and (3) digital elevation model (DEM) differencing, to capture micro-scale displacement phenomena over time.
[Background and Objectives]
The formation and collapse of lava domes is a major phenomenon during volcanic unrest. However, even after the end of an eruptive phase, a remnant dome can undergo slope failures and erosion, which may lead to small-scale debris flows or large-scale pyroclastic flows under certain conditions. At Unzen-Fugendake in particular, a large volume of the dome formed in the final stage of eruptive activity has remained near the summit, raising concerns about further slope instability and potential recurrence of pyroclastic flows (a form of gravitational flow). This study aims to quantify the dome’s minor displacements through the integration of multiple measurement techniques, thus estimating both the mechanisms of slope collapse and the potential for gravitational flows.
[Methods]
First, DEM differencing was performed using DEMs generated from SfM and LiDAR surveys in 2007, 2016, and 2023, to characterize long-term topographic changes (elevations). We then conducted continuous, high-frequency, high-accuracy (millimeter- to centimeter-order) monitoring of dome slope displacements using GBSAR and optical distance measurements, capturing short-term dynamics over time. Additionally, we investigated system biases and differences in projection components arising from distinct measurement principles and observation angles, verifying the need for geometric corrections to improve the accuracy of displacement estimates.
[Results]
According to DEM differencing, a notable decrease in elevation (negative change) was observed around the dome’s valley between 2007 and 2016, suggesting that erosion was a dominant process of topographic change. In contrast, between 2016 and 2023, a pronounced increase in elevation (positive change) was identified near the dome margin, implying ongoing talus formation due to slope collapse and gravitational movement. Cumulative displacement data from GBSAR and optical distance measurements showed displacements of up to several centimeters to tens of centimeters around the dome’s edges, pinpointing areas with high collapse potential. Nevertheless, discrepancies in absolute displacement values between DEM differencing and short-term observations indicate possible influences of differences in observational timescales (short- vs. long-term), as well as system-specific measurement errors or biases.
[Discussion and Implications for Gravitational Flows]
Instability of a remnant dome can trigger a rapid transition from slope failure to pyroclastic or debris flows. The distinct dominant processes identified for each period (erosion-driven vs. collapse-driven) are crucial for understanding the overall evolution of dome topography, and they also provide valuable insights into the mechanisms that may initiate pyroclastic flows. Detailed time-series displacement data obtained from multiple observation methods can directly contribute to refining gravitational flow prediction models and improving hazard assessments.
[Future Prospects]
Moving forward, we aim to elucidate the critical conditions under which minor dome displacements escalate into large-scale collapses and pyroclastic flows, while also examining the fundamental mechanisms in comparison with similar phenomena such as landslides and debris flows. Notably, the Unzen lava dome offers an excellent research environment with existing GBSAR and optical measurement installations, which provides a unique opportunity for long-term, multifaceted monitoring. By leveraging this field site to capture even subtle displacements in detail, we expect further advancements in assessing dome instability–related collapse risks and clarifying the mechanisms responsible for such collapses.
[Background and Objectives]
The formation and collapse of lava domes is a major phenomenon during volcanic unrest. However, even after the end of an eruptive phase, a remnant dome can undergo slope failures and erosion, which may lead to small-scale debris flows or large-scale pyroclastic flows under certain conditions. At Unzen-Fugendake in particular, a large volume of the dome formed in the final stage of eruptive activity has remained near the summit, raising concerns about further slope instability and potential recurrence of pyroclastic flows (a form of gravitational flow). This study aims to quantify the dome’s minor displacements through the integration of multiple measurement techniques, thus estimating both the mechanisms of slope collapse and the potential for gravitational flows.
[Methods]
First, DEM differencing was performed using DEMs generated from SfM and LiDAR surveys in 2007, 2016, and 2023, to characterize long-term topographic changes (elevations). We then conducted continuous, high-frequency, high-accuracy (millimeter- to centimeter-order) monitoring of dome slope displacements using GBSAR and optical distance measurements, capturing short-term dynamics over time. Additionally, we investigated system biases and differences in projection components arising from distinct measurement principles and observation angles, verifying the need for geometric corrections to improve the accuracy of displacement estimates.
[Results]
According to DEM differencing, a notable decrease in elevation (negative change) was observed around the dome’s valley between 2007 and 2016, suggesting that erosion was a dominant process of topographic change. In contrast, between 2016 and 2023, a pronounced increase in elevation (positive change) was identified near the dome margin, implying ongoing talus formation due to slope collapse and gravitational movement. Cumulative displacement data from GBSAR and optical distance measurements showed displacements of up to several centimeters to tens of centimeters around the dome’s edges, pinpointing areas with high collapse potential. Nevertheless, discrepancies in absolute displacement values between DEM differencing and short-term observations indicate possible influences of differences in observational timescales (short- vs. long-term), as well as system-specific measurement errors or biases.
[Discussion and Implications for Gravitational Flows]
Instability of a remnant dome can trigger a rapid transition from slope failure to pyroclastic or debris flows. The distinct dominant processes identified for each period (erosion-driven vs. collapse-driven) are crucial for understanding the overall evolution of dome topography, and they also provide valuable insights into the mechanisms that may initiate pyroclastic flows. Detailed time-series displacement data obtained from multiple observation methods can directly contribute to refining gravitational flow prediction models and improving hazard assessments.
[Future Prospects]
Moving forward, we aim to elucidate the critical conditions under which minor dome displacements escalate into large-scale collapses and pyroclastic flows, while also examining the fundamental mechanisms in comparison with similar phenomena such as landslides and debris flows. Notably, the Unzen lava dome offers an excellent research environment with existing GBSAR and optical measurement installations, which provides a unique opportunity for long-term, multifaceted monitoring. By leveraging this field site to capture even subtle displacements in detail, we expect further advancements in assessing dome instability–related collapse risks and clarifying the mechanisms responsible for such collapses.