Earthquake focal mechanisms before and after the onset of re-inflation at Askja caldera, Iceland

Askja, an active volcano in Iceland’s Northern Volcanic Zone, has experienced over 70 centimetres of uplift since August 2021, following several decades of ground surface deflation. In the same month that surface uplift began, there was also an overall increase in shallow microseismicity. This overlap suggests that there is a link between the shallow seismicity and re-inflation, but much more information is needed to constrain an exact mechanism.

 

To gain more insight into the stresses driving seismicity beneath Askja, and how they may have changed with the onset of uplift, moment tensor solutions were constructed from a subset of events within the Askja caldera, both before and after the start of re-inflation. The goal is to uncover any dominant earthquake slip orientation patterns, and how they have changed post August 2021. Cambridge’s Volcano Seismology Group has maintained a dense seismic network surrounding and within Askja from July 2007 to the present day, which provides sufficient data to produce well constrained moment tensor solutions, even for events as small as magnitude 0.5.

 

Moment tensor solutions constructed from the first year of inflation suggest that the direction of slip along faults encircling the crater lake Öskjuvatn did not change when Askja switched from deflation to re-inflation; however, data from a longer recording period is needed to provide a definitive conclusion. With another 12 months of data from the main network now downloaded, as well as additional data from a supplementary dense network of seismic nodes within the Askja caldera from July 2023 to September 2023, a more in-depth analysis of the moment tensor solutions for shallow microearthquakes underneath the region of uplift through time is now possible. The preliminary results of this analysis will be presented, along with plans for future interrogation of this enhanced dataset.