Science1 publisher3 min readPublished
Two radars looking from opposite sides caught Erta Ale sinking during Hayli Gubbi's eruption
Wenbin Xu's team combined NISAR's left-looking radar with Sentinel-1's right-looking passes over Ethiopia's Afar Depression. The largest ground motion during November's Hayli Gubbi eruption sat next door, at Erta Ale.
The Scientist · Science desk
What happened
- A study in Geodesy and Geodynamics used satellite radar to examine the November 2025 eruption of Hayli Gubbi, in Ethiopia's Afar Depression, where three tectonic plates are pulling apart.
- The authors attribute the Erta Ale pattern to two dikes centred about 1 km underground that resemble structures involved in earlier eruptions at the same volcano.
- Stress changes associated with the eruption also appear to have reactivated nine faults near Erta Ale, according to the researchers.
Compiled by The ScientistSomething wrong?How this is made
Why it matters
- capability Groups watching remote volcanoes have had to infer sideways ground motion from line-of-sight measurements. Two missions looking from opposite sides give them the horizontal components directly, at centimetre scale.
- constraint The result is only available where both missions cover the same ground in the same window. A site imaged by one geometry still yields one projection of the motion, not three components.
- decision Watching only the erupting edifice would have missed this one. Hazard teams in rift zones now have a documented case where the deformation peaked under a volcano that was not the vent.
Each radar pass measures one thing per patch of ground: how much the distance changed along the line between satellite and surface. Compare passes from different dates and movements of a few centimetres show up [4]. Motion at right angles to that line does not register, so one satellite cannot fix the direction a piece of ground has travelled [5]. NISAR looks left of its track, Sentinel-1 looks right. The team used that pair of viewing geometries to separate east-west, north-south and vertical movement over the Afar Depression [6].
"By viewing the volcanoes from complementary directions, we reconstructed the three-dimensional deformation field and measured all three components of ground movement at centimeter scale," said Wenbin Xu of Central South University in China, the paper's corresponding author [7].
What the field showed at Erta Ale was subsidence combined with inward motion [10]. That is the signature the authors attribute to two steep magma-filled cracks losing their magma: drain a dike and the rock around it can contract, pulling the surface down and in [11]. Both dikes sit about 1 km below the surface in their model. Both resemble structures implicated in earlier eruptions there, which the authors read as evidence that these are standing features of the local plumbing [12].
The dikes are an inference from the surface pattern, not something the radar sees. The phys.org account did not report displacement magnitudes or the number of radar tracks behind the three-dimensional decomposition.
The forward-looking part of the paper is thinner than the deformation field. The November event was more localised than a larger eruption earlier in 2025 and drew mainly on shallow storage [14]. From that contrast the authors propose a general division: deep magma involvement with longer dike propagation and stronger tectonic response, shallow reservoirs with smaller and more concentrated events [15]. Two eruptions in one rift zone support a hypothesis of that shape [20], but they are not a test of it.
The nine faults near Erta Ale are described as reactivated by stress changes associated with the eruption [13]. What the data establish is coincidence in time between the eruption and movement on those faults, with a physical story that would explain it. In a rift where three plates are pulling apart, faults have other reasons to move [2].
The claim to novelty here is narrower than it first sounds. The study is among the first to use the NASA-ISRO radar mission on an eruption at all [3]. The value the authors claim is a viewing angle they did not previously have for volcanoes with little or no ground instrumentation [16]. The eruption itself left two new collapse pits and cracks at Hayli Gubbi, and a partially collapsed summit crater at Erta Ale [9].
"Combining multiple radar missions could improve our ability to distinguish how different eruptions develop," Xu said [17].
What to watch
- Publication of the fitted dike parameters and per-component uncertainties, which would show how tightly two viewing geometries constrain each direction of motion.
- Whether the nine faults near Erta Ale keep moving after the eruption, separating a one-off stress pulse from continuing rift extension.
- Whether other groups repeat the NISAR plus Sentinel-1 pairing at a volcano with independent ground measurements to check the three-dimensional field.