The response of microseismicity to re-inflation at Askja caldera, Iceland: new insights from a dense nodal deployment

Askja is an active volcano in Central Iceland that has experienced ~ 70 cm of uplift since August 2021, marking an abrupt end to decades of gradual deflation. We have operated a 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. 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. Between July-September 2023 we deployed a dense network of SmartSolo 3-component 5 Hz nodal seismometers within the caldera, to augment the long-running regional array. Their compact form factor allowed for deployment within areas previously inaccessible for deployment of broadband sensors, both due to access and local deployment conditions. Here we focus on investigating the improvement this provided to earthquake catalogue completeness and location accuracy, as well as discussing the data quality and lessons learned for future deployments. Using the data from the nodal network results in an improvement of the magnitude of completeness by around 1 magnitude unit (to ML ~ -1.0), further elucidating known fault structures, and allowing new classes of events to be successfully detected and located. This includes a previously unknown family of long-period earthquakes, which occur at the approximate depth and location of the recent uplift signal, possibly providing direct evidence of magma ascent responsible for re-inflation of the caldera.