Long-term deformation and surface change of the Heisei-Shinzan lava dome revealed by integrated monitoring

The Heisei-Shinzan lava dome at Unzen remains a potential hazard because deformation and surface modification have continued long after the end of lava extrusion. In this study, we integrate ground-based synthetic aperture radar (GBSAR; 2013–2025), electronic distance measurement (EDM; 2007–2025), and multi-temporal digital elevation models (DEMs; 2011–2024) to investigate long-term post-eruptive deformation and decadal-scale topographic change of the dome. The combined datasets document persistent but spatially heterogeneous line-of-sight (LOS) displacement, expressed as multiple temporal patterns across the monitored sectors, together with cumulative topographic modification and a net surface loss of approximately 11,600 m 3 between 2011 and 2024. Because both GBSAR and EDM measure only LOS displacement, interpretation of motion geometry requires caution. Localized positive or near-zero LOS trends are therefore interpreted as projection-dependent expressions of surface motion rather than as direct evidence of true uplift. When considered together with the DEM record, the observations suggest that long-term dome evolution at Heisei-Shinzan has involved progressive surface degradation coupled with internal structural adjustment. Although no catastrophic collapse was documented during the study period, gradual deformation and topographic change remained active over multiyear timescales. These results highlight the importance of integrated long-term monitoring for improving stability assessment and hazard evaluation at long-lived lava domes. • Long-term GBSAR and EDM records document persistent downslope motion of the lava dome. • Positive or near-zero LOS trends can be explained by projection effects and viewing geometry. • Deformation patterns diverge among dome blocks after 2017, suggesting structural heterogeneity. • The dome likely remains mechanically unstable three decades after eruption. • Integrated monitoring improves assessment of long-term dome instability and collapse hazard.