Ongoing unrest at Icelandic volcanoes: What deformation and seismicity patterns to expect leading up to future eruptions

Precursors to volcanic eruptions vary widely between volcanic systems and their individual eruptions. Volcanic systems in Iceland undergoing unrest include the Reykjanes, Svartsengi, Fagradalsfjall, and Krísuvík systems on the obliquely spreading Reykjanes Peninsula. Main precursors prior to the Fagradalsfjall eruptions in 2021 and 2022 were signals associated with the formation of dikes releasing stored tectonic stress over weeks and days, respectively. If volcanic activity occurs at Fagradalsfjall in coming years it may be associated with shorter warning time, as less stored tectonic stress remains. In contrast, the nearby Svartsengi system experienced cumulative uplift of about 15 cm in multiple inflation episodes during 2020 to 2022, modeled as repeating sill intrusions. Prior to, in-between, and following the intrusive events, the surface subsided. We find that the onset of diking accompanied by a sudden increase in seismicity and deformation rates is a likely scenario prior to future eruptions on the Reykjanes Peninsula. A decline in seismicity and/or deformation may occur as unrest activity progresses, as experienced prior to the 2021 and 2022 eruptions. In other areas of Iceland, since 2020 magma storage areas with increasing pressure have been identified at the Askja, Grímsvötn, Krafla, and Bárðarbunga calderas, as well as at Hekla volcano. Increasing pressure buildup in the roots of these volcanoes, is expected to a varying degree prior to next eruption, with different amounts of inflation and seismicity. Tectonic stress release as observed during the 2014/15 Bárðarbunga rifting event may occur or not. The largest capacity for pressure increase is expected at the Askja caldera, where the surface over the magma chamber subsided by more than 1 m from 1983 to 2021, but since August 2021 over 45 cm of uplift has occurred and deformation continues. The amount of subsidence prior to present uplift may indicate the scale of further inflation needed to reach critical conditions, assuming that the current inflation is sourced in a similar crustal volume as the deflation, and the strength of the surrounding material remains similar (e.g., no new faulting/fracturing). Examples of intermittent flow of magma to shallow depth, or pressure increase beneath calderas, occurred during 2017-2018 at Öræfajökull, where a slight increase in seismicity has been detected in recent months, and inflation 2018-2019 at Torfajökull caldera. It remains a challenge to promptly identify seismic swarms that may be indicative of formation of magma feeding conduits versus those indicating intermittent increases in seismic activity due to high stress levels, e.g., caused magma recharging, changes in geothermal activity, or glacial retreat. Experience from the Northern Volcanic Zone and the Reykjanes Peninsula oblique rift, suggest precursory activity may take place simultaneously over wide parts of plate boundary areas, indicating to some extent coupled activity of nearby volcanic systems.