When image correlation is needed: unravelling the complex dynamics of a slow-moving landslide in the tropics with dense radar and optical time series.

Slow-moving landslides exhibit persistent but non-uniform motion at low rates which makes them25 exceptional natural laboratories to study the mechanisms that control the dynamics of unstable26 hillslopes. Here we leverage 4.5+ years of satellite-based radar and optical remote sensing data to27 quantify the kinematics of a slow-moving landslide in the tropical rural environment of the Kivu Rift,28 with unprecedented high spatial and temporal resolution. We measure landslide motion using sub29pixel image correlation methods and invert these data into dense time series that capture weekly to30 multi-year changes in landslide kinematics. We cross-validate and compare our satellite-based results31 with very-high-resolution Unoccupied Aircraft System topographic datasets, and explore how rainfall,32 simulated pore-water pressure, and nearby earthquakes control the overall landslide behaviour. The33 landslide exhibited seasonal and multi-year velocity variations that varied across the landslide34 kinematic units. While rainfall-induced changes in pore-water pressure exerts a primary control on35 the landslide motion, these alone cannot explain the observed variability in landslide behaviour. We36 suggest instead that the observed landslide kinematics result from internal landslide dynamics, such37 as extension, compression, material redistribution, and interactions within and between kinematic38 units. Our study provides, a rare, detailed overview of the deformation pattern of a landslide located39 in a tropical environment. In addition, our work highlights the viability of sub-pixel image correlation40 with long time series of radar-amplitude data to quantify surface deformation in tropical41 environments where optical data is limited by persistent cloud cover and emphasize the importance42 of exploiting synergies between multiple types of data to capture the complex kinematic pattern of43 landslides.