Mount Gamalama, a steep-sided stratovolcano on Ternate Island, North Maluku, represents one of the most critical volcanic systems in eastern Indonesia due to its compound hazard potential. Although currently exhibiting apparent stability, its tectonic setting, proximity to active subduction zones, dense coastal population, and confinement within shallow marine environments create conditions conducive to large-scale slope failure and landslide-induced tsunamis with severe socio-economic consequences. This study presents an integrated assessment of volcanic slope instability, landslide mobility, tsunami generation, and associated risks to coastal populations and critical infrastructure.
Slope stability was analysed using the Limit Equilibrium Method (LEM) under both static and dynamic conditions. Under gravitational loading alone, all slopes remain stable, with Factors of Safety (FoS) ranging from 1.945 to 3.361. However, the inclusion of external and internal triggers—namely seismic loading, magma pressure, and dyke intrusion—significantly reduces slope stability. Seismic loading, represented by horizontal acceleration (kh), produces substantial reductions in FoS, which is particularly relevant given the volcano’s location between the Sangihe and Halmahera thrust systems. At a peak ground acceleration of 0.37 g, FoS values approach critical thresholds, especially on the eastern flank. Magma pressure further destabilizes the system, with a triangular pressure distribution providing more realistic results than constant pressure assumptions. Increasing magma pressure (2–10 MPa) drives several slopes into unstable conditions (FoS < 1.0), with the southern flank showing the highest sensitivity. Dyke intrusion orientation also exerts a strong control, with specific geometries producing minimum FoS values and defining potential failure surfaces.
Estimated failure depths reach up to approximately 1120 m on the eastern flank and between 513 m and 570 m on other flanks, suggesting that potential collapse volumes may involve up to half of the volcanic edifice. These failure surfaces are primarily controlled by weak pyroclastic deposits overlying more competent andesitic units. Landslide mobility and tsunami generation were simulated using the multiphase numerical model Fluidity, which enables dynamic coupling between landslide motion and the water column without prescribing landslide kinematics. The results demonstrate that bathymetry exerts a first-order control on tsunami behaviour. Shallow and confined settings, such as the Halmahera Strait, amplify wave heights through shoaling and focusing effects, whereas deeper environments promote greater energy dissipation.
Simulated tsunamis reach adjacent coastlines within minutes, leaving limited time for evacuation. Coastal areas of Halmahera and Tidore Islands are identified as highly exposed due to their proximity and low elevation. Integration with spatial population and infrastructure data reveals significant vulnerability, as Ternate Island hosts over 186,000 inhabitants and functions as a regional transportation and economic hub. Critical facilities, including ports, an international airport, and submarine telecommunication cables, are concentrated along the coastline, amplifying potential impacts.
This study demonstrates that Mount Gamalama poses a significant multi-hazard threat arising from the interaction of volcanic, tectonic, and marine processes. The integrated approach provides a transferable framework for assessing landslide-induced tsunami hazards in volcanic island settings and highlights the importance of incorporating secondary hazards into volcanic risk management strategies.
