Volcano topography can distort ground deformation data

As magma flows into a chamber beneath a volcano, pressure builds and the ground inflates. Geophysicists often employ tiltmeters on the rims of calderas to measure the surface deformation and then translate that into a picture of the magma plumbing below. But data from tiltmeters do not always provide a complete picture. Now Jessica Johnson and colleagues at the University of East Anglia in the UK have demonstrated that sharp variations in a volcano’s surface topography can alter the way it deforms from magmatic activity. The researchers then applied their findings to Hawaii’s Kilauea volcano, whose surprising and damaging 2018 eruption focused attention on refining monitoring tools.

Most calderas, including Kilauea’s (shown in the photo), have steep, cliff-like features, the result of previous collapses of rock. Such features are not taken into account by the analytical models that are fed tiltmeter data: They assume a volcano’s surface is flat or gently sloped, as if it were a carpet sitting over an inflating balloon. Johnson and colleagues used numerical models to investigate the effects of abrupt variations in topography on surface tilt. The researchers proposed that deformation patterns along a cliff could be thought of as the combined influence of a shallow magma source and an underlying one whose depth is related to the cliff’s height. They found that the superposition of the two profiles results in a small secondary region of deformation that can arise a sizable distance from the region above the main inflating source. The extra vertical displacement can cause nearby tilt vectors to rotate up to 180° relative to those expected for a smooth topography.

The researchers applied their simulations to Kilauea, where decades’ worth of tilt measurements from one part of the caldera have persistently disagreed with other readings. They found that the 80-m-high caldera wall caused tilt at one position to rotate 10° away from the actual center of deformation, which helps explain the anomalous measurements. Such discrepancies could become even more pronounced because of new topographic features—cliffs up to 500 m high and terrace-like steps of 50–150 m each—that emerged during last year’s eruption. Johnson suggests that deploying new tiltmeters on caldera floors, rather than on jagged rims, could provide more meaningful data. (J. H. Johnson et al., Geophys. Res. Lett., 2019, doi:10.1029/2018GL081757 .)