Earthquake cluster analysis reveals the complex response of microseismicity to the ongoing 2021-2023 inflation at Askja caldera, Iceland

Askja is an active volcano in Central Iceland that has experienced ~ 45 cm of uplift since August 2021, marking an abrupt end to decades of gradual deflation. We have operated a dense local seismic network around Askja since 2007, providing an exceptionally long time series of seismic data within which to search for patterns that relate to this sudden change in behaviour. Here we focus on spatiotemporal changes in microseismicity associated with the switch to inflation. Understanding what seismicity can tell us about the ongoing unrest at the volcano is crucial, because it is one of the few monitoring tools that is available year-round. Furthermore, joint interpretation of seismic and geodetic data is key to overcoming ambiguities in the interpretation of surface deformation measurements alone.

Our catalogue of microseismicity in Askja spanning July 2007 to August 2022 contains more than 25,000 events detected and located with QuakeMigrate¹. Increases in seismicity rate are clearly observed in August 2021, corresponding to the start of inflation as measured by a GPS station close to the centre of uplift. However, a strong spatial variation across the caldera is observed in the magnitude and duration of the seismicity rate increase. To investigate this further, we cross-correlate earthquake waveforms and calculate relative relocations. Combined with cluster analysis, this divides the seismicity into sharply resolved structures, with markedly different temporal evolution. We identify new clusters of events not seen in the 14 years preceding the current inflation, as well as previously persistent clusters which have now shut off, and areas of microseismicity which are seemingly unaffected by the inflation. Combined with analysis of tightly-constrained earthquake focal mechanisms covering the same time period, these results provide new insight into both the mechanism linking the observed deformation and seismicity rate changes, and the role of caldera fault slip in facilitating the ongoing inflation at Askja.

 

1: Winder, T., Bacon, C., Smith, J., Hudson, T., Greenfield, T. and White, R., 2020. QuakeMigrate: a Modular, Open-Source Python Package for Automatic Earthquake Detection and Location. https://doi.org/10.1002/essoar.10505850.1