Renewed Inflation of Krafla Caldera, North-Iceland, since 2018: Magma Inflow or Hydrothermal Changes?

The Krafla caldera, N-Iceland, shows signs of inflation in 2018, after deflation lasted three decades. Krafla was the centerpiece of a rifting episode including multiple inflation-deflation events and eruptions in 1975-1984. After this event, a magma body at 3 km depth ended in an inflated state, with inflation proceeding until 1989. Since then, the caldera deflated until 2018. The difference in surface ground velocity fields 2015-2018 and 2018-2020 reveals uplift (relative to prior subsidence) centered on the area between Leirhnjukur and IDDP-1 well at a rate of 10 13 mm/yr in 2018-2020, continuing to present. The differential velocity field broadly fits deformation caused by spherical source of pressure within a uniform elastic half-space, with a volume change of 2.6 - 3.8 x 105 m3/y and centre depth 2.1-2.5 km. This is similar to the local maximum depth of the earthquakes in the area, i.e. close to the brittle-ductile boundary. Rather than being due to renewed magma inflow, the possibility of inflation due to geothermal changes is also evaluated. We test if the inflation can possibly relate to about 0.1 MPa/yr pressure increase in the geothermal system, as measurements in monitoring well KG-10 in the area indicate. Modelling shows that inferred volume change can be due to a spherical source with radius = 1.4 km and 0.1 MPa/yr pressure change if the surrounding crust has Youngs modulus E 7 GPa. However, the average regional Youngs modulus for the upper crust in Iceland has been estimated to be E = 30 GPa. We use the Finite Element Method to evaluate how deviations from a uniform elastic half-space may affect deformation. We test the influence on displacement of the presence of a local crustal volume around the source with lower Youngs modulus value in near-field and higher value in the far field. Such a model can reproduce significant features of the observed deformation, indicating that both hydrothermal and magmatic processes should be further explored as an explanation for the present inflation. Changes in seismic activity is observed after the onset of the inflation, showing fewer and larger events while the seismic release increases. This indicates crust strengthening and increased energy input in the crust released in earthquakes. No noticeable changes have been observed in the chemical composition of fumarole discharge.