Strain Localization at Volcanoes Undergoing Extension: Investigating Long-term Subsidence at Krafla and Askja in North Iceland

Localized ground deformation at volcanoes in extensional setting may occur because of strain localization. The magmatic system of a volcano with its liquid magma, magma mush, and hot crust will cause a rheological anomaly, where material properties may be very different from surrounding crust and mantle. Numerical models based on the Finite Element Method (FEM) are used to explore ground deformation at volcanoes in extensional environments, considering realistic volcano models with heterogeneous multi-layered structure, with both elastic and viscoelastic rheology. The effects of localized lateral and vertical variations in terms of geometry and material properties of the crust are explored, in a model domain undergoing stretching applied perpendicular to the lateral domain boundaries of one and two-layers model (at a rate of 17.4 mm/yr applied in our models). A one-layer model displays the same elastic feature throughout the whole domain except for a localized upper volume with lower elastic properties, compared to the surrounding crust, to simulate the shallow magmatic system. In a two-layer model, the top elastic layer overlies a viscoelastic layer that locally reaches shallower levels to symbolize the deep magmatic system beneath the shallow low-rigidity volume previously introduced. A localized surface subsidence signal is a characteristic feature of magmatic system with a large body of localized viscoelastic rheology at shallow depth. The subsidence signal is strongly dependent on the viscosity and volume of the up-doming viscoelastic material. A model with viscosity of 5 × 10¹⁹ Pa s in the up-doming material, and a 7 – 15 km-thick elastic layer, show a small subsidence rate, ~0.1 – 0.4 mm/yr. Our models show an increase of the localized subsidence rate, from 1.9 to 5.5 mm/yr, as the viscosity decreases from 10¹⁸ Pa s to 10¹⁶ Pa s in the up-doming material. Lower viscosities (<10¹⁶ Pa s) show no further change in subsidence rate when compared to the 10¹⁶ Pa s solution. We apply three-dimensional FEM models to improve understanding of the subsidence at the Krafla and Askja volcanic systems (1989-2018 and 1983-2018, respectively) in the Northern Volcanic Zone of Iceland. The two subsiding areas (roughly 9 × 10 km each) lie in about 50 km-wide zone which marks the North America-Eurasia divergent plate boundary. The rate of subsidence at Krafla was ~1.3 cm/yr in 1993-2000 and slowed down to 3-5 mm/yr in 2006-2015. The rate of subsidence at Askja decayed more slowly than Krafla. During the 1983-1998 the subsidence rate was ~5 cm/yr; in 2000-2009, geodetic monitoring showed that the subsidence slowed down to ~2.5 cm/yr. Comparison of FEM models to geodetic data in North Iceland suggests that plate divergence processes may account for part of the observed subsidence, dependent on how extensive rheological anomalies in relation to magma are beneath the volcanoes.