Strain-rate variations as response to surface uplift and subsidence during a volcanic crisis measured with DAS on a dark fiber?

Tectonic deformation spans several orders of magnitude in both duration and spatial extent. However, instruments and acquisition techniques to date are capable to quantify only parts of this wide spectrum. Seismometers, for example, acquire dense temporal data but are sparsely distributed, resulting in spatial aliasing. Conversely, remote sensing techniques have a wide aperture but coarse temporal resolution and accuracy (mm range). Distributed fiber optic sensing converts fiber optic cables, which can be tens of kilometers long, into arrays of regularly spaced virtual sensors that measure the strain tensor component tangential to the fiber. As such, this measurement technique has the potential to complement conventional seismological instrumentation and remote sensing. It samples the Earth's surface deformation in a spatially and temporally un-aliased manner, opening up new possibilities for permanent monitoring at regional scales, for example to document the interplay between aseismic and seismic events or the uplift and subsidence in response to magma intrusions in the subsurface.

 

In this contribution we analyze distributed dynamic strain recordings acquired in early 2020 during the geothermal unrest preceding the 2021 Fagradalsfjall volcano eruption on the Reykjanes Peninsula, South West Iceland. Measurements were accomplished with a telecommunication optic fibre connecting the town of Gridavik with the Svartsengi geothermal power plant 5 km to the North and the western most tip of the Reykjanes Peninsula approximately 15 km to the west. Located approximately between 10 and 20 km west of the volcanic eruption to follow, the fiber crosses areas subjected to significant surface uplift and subsidence during the preparatory phase. Long continuous strain-rate recordings show a response at low frequencies during the onset of uplift and subsidence. We approximate the tectonic situation with the Mogi-model to obtain first-order predictions of surface strain and to compare them with our observations. Prospectively, our efforts aim at investigating the feasibility of employing distributed optical strain-rate measurements along telecommunication infrastructure to track slow deformations.