Six years of satellite radar suggest Masaya volcano draws on more than one magma reservoir

Every reservoir in this study is an inference from a single quantity. Sentinel-1's radar measures the distance between the satellite and the ground on each pass, and the change between passes is the deformation ^[8]. "And when we look at all the different satellite images across multiple years, we can see how the distance between the satellite and the ground changes, where it is uplifting or subsiding," said Young Cheol Kim, who completed his Penn State doctorate this summer ^[10][13]. With a pass about every 12 days, the 2018 to 2024 window gives on the order of 180 looks at the caldera ^[14].

Going from ground motion to magma is the interpretive step, and the description from lead author Lizzie Johnson is hedged accordingly ^[7]. "When it's subsiding, that means that there's probably magma draining away from the volcano, but once the ground starts uplifting again, that can mean that there's potentially new magma moving in," she said ^[11].

The stronger evidence for a more complicated system is timing. The caldera went from sinking to rising ^[3]. Through that whole switch, the area around Santiago crater and its lava lake kept deflating, as it did for the full six years ^[5]. Johnson gave the team's reading: "This indicates that the plumbing system is more complex and could be due to decreasing magma in a second, shallower reservoir" ^[6].

How many sources there are depends on who is describing it. The account published on phys.org opens with researchers finding "evidence of two new magma sources" ^[9]. The findings it then describes are a main reservoir that is inflating, possibly from a new deep source, and a shallower reservoir that may be draining ^[4][6]. Both new elements come with a 'possibly' or a 'could' attached.

For the 2 million people within 21 km, the hazard they meet most often is gas ^[1]. "Although Masaya last produced a major lava flow in 1772, its persistent degassing often produces dangerously high concentrations of sulfur dioxide volcanic gas," Johnson said ^[12]. The caldera is also a national park, and tourists come to see the lava lake ^[1][12].

Johnson's own claim for the work is careful. The observations "may improve monitoring and eruption forecasting in the future," she said ^[15]. I think that wording fits the evidence. The study gives monitors a reason to treat the crater and the wider caldera as separate signals. Building it into an eruption forecast would take reservoir depths, volumes and deformation rates, and the press account does not give them. Kim described what the monitoring is for. "Doing continuous monitoring allows us to understand the baseline activity and see if things are changing," he said. "And then if there is a need to warn people, we can do that in a timely manner" ^[16].