Characterization of activity at Semeru volcano using high resolution radar and optical imagery

Semeru volcano, located in eastern Java, Indonesia, reactivated in December 2021 following the destabilization of a

summit lava dome that had been growing since 2009. Monitoring topographic changes and surface deformation at

Semeru is important for understanding eruptive processes and assessing associated hazards, but remains challenging

due to the inaccessibility of the summit area, frequent activity, and the cost and sparsity of ground-based instrumentation.

In this context, satellite remote sensing combining bi-static and repeat-pass Synthetic Aperture Radar interferometry

(InSAR) with high resolution optical photogrammetry provides observations of surface deformation and topographic

changes at high spatial resolution. However, steep topography, tropical climate, dense vegetation, and rapidly evolving

volcanic deposits strongly affect InSAR observations introducing noise associated with atmospheric delays, temporal

decorrelation, and residual topographic errors. These external contributions can obscure low-amplitude deformation

signals, especially during periods of moderate or persistent activity. A set of seven high-resolution digital elevation

models (DEMs) is produced from TanDEM-X bistatic acquisitions and Pleiades stereo images. These DEMs allow

detailed characterization of the summit dome and proximal deposits prior and posterior to the December 2021 eruption.

Between 2015 and July 2021, the lava dome grew heterogeneously, reaching a volume of about 1.35 million m³. Over

the same period, and pyroclastic deposits accumulated with thicknesses locally exceeding 75 m, progressively filling

existing eastward channels and contributing to a redirection of eruptive activity toward the eastern flank after 2018.

The major 2021 eruptions produces a large pyroclastic density current reshaping the summit and the Besuk Kobokan

valley with a total volume of material mobilized during the eruption of 29.1 Mm³. The analysis of ground deformation

using TerraSAR-X InSAR data, corrected for atmospheric delays using ERA-5 reanalysis, reveals spatially coherent

patterns of subsidence affecting older lava flows and pyroclastic deposits on the southeastern flank of Semeru. These

signals are interpreted as post-emplacement compaction, with line-of-sight displacement rates of 5 cm/yr. However,

low and spatially variable interferometric coherence within the summit crater and the main deposition channel prevents

reliable measurement of post-eruptive magmatic deformation in these areas. Volcanoes capable of rapid transitions from

Strombolian to Plinian activity in tropical environments affected by intense rainfall, as observed at Semeru in December

2021, remain hazardous and insufficiently understood, highlighting the need for long-term, integrated monitoring of both

topographic changes and ground deformation to better characterize eruptive processes and associated hazards.