Earthquake focal mechanisms reveal a complex response to re-inflation at Askja caldera, Iceland

Askja, an active basaltic caldera volcano in Iceland’s Northern Volcanic Rift Zone, has experienced more than 85 centimetres of surface uplift since August 2021, following several decades of subsidence. Geodetic modelling of the observed uplift suggests an inflating sill type source at around 3 km below the surface (Parks et al., 2024), and recent tomography work by Han et al (2024) and Fone et al. (2025) image a shallow low-velocity anomaly, centred on the area of maximum uplift. In the same month that uplift began, there was a clear increase in the rate of shallow microseismicity, observed primarily in clusters surrounding the youngest lake-filled caldera Öskjuvatn. 

To gain more insight into how the change in rate of microseismicity relates to the observed reversal in surface deformation, moment tensor solutions were constructed for a subset of events beneath Askja, both before and after the start of re-inflation. The Cambridge Volcano Seismology Group has maintained a dense seismic network around Askja since July 2007, which provides sufficient azimuthal coverage to produce well constrained moment tensor solutions. An expanded network deployed within Askja caldera in summer 2023 improves this azimuthal coverage significantly, extending the smallest well constrained events from magnitude 0.5 to just below magnitude 0. 

Our results provide new constraints on the ring fault geometry beneath Öskjuvatn – where the microseismicity rate increase was most prominent – complementing previous insights from mapping of surface faults. Surprisingly, there is no evidence for a reversal in earthquake slip direction associated with the start of re-inflation, and only the modelled stress changes during the re-inflation period favour slip that aligns with our moment tensor solutions. We therefore propose that the microseismicity prior to the onset of re-inflation may have been driven primarily by regional deformation processes, not the long-term subsidence within Askja caldera. Our future work will exploit this expanded dataset of manually picked earthquake phase arrivals to improve our resolution of the velocity structure at the shallowest depths beneath Askja. This will contribute to a full structural model linking surface deformation, ring faulting and the underlying magma storage region. 

Citations: 

Han, J., N. Rawlinson, T. Greenfield, R. White, B. Brandsdóttir, T. Winder, and V. Drouin (2024),  

Evidence of a shallow magma reservoir beneath askja caldera, iceland, from body wave  tomography, Geophysical Research Letters, 51 (9), e2023GL107,851 

 

Parks, M. M., F. Sigmundsson, V. Drouin, S. Hreinsdóttir, A. Hooper, Y. Yang, B. G. Ófeigsson, E.  

Sturkell, Á. R. Hjartardóttir, R. Grapenthin, et al. (2024), 2021–2023 unrest and geodetic  

observations at askja volcano, iceland, Geophysical Research Letters, 51 (4),  

e2023GL106,730. 

 

Fone, J., Winder, T., Rawlinson, N., White, R., Brandsdóttir, B., and Soosalu, H. (2025), Imaging the  

shallow structure beneath Askja volcano, Iceland, with ambient noise tomography, Journal of  Geophysical Research: Solid Earth, 130 (12), e2025JB031,905.