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Primary reference(s)

Dzurisin, D., 2007. Volcano Deformation. Springer.

Additional scientific description

Uplift and subsidence may occur before, during and after volcanic eruptions (Dzurisin, 2007). Before eruptions, uplift and/ or subsidence may be among the first signs that a magmatic system is restless, so monitoring and understanding ground deformation is critical to attempts to understand magmatic systems, forecast eruptions and mitigate volcanic risk (Dzurisin, 2007; Acocella et al., 2015).

‘Volcanic unrest’ is defined as any deviation of ground deformation, seismicity, gas emission, and/or other geophysical and geochemical indicators from normal baselines, increasing the probability of eruption (Acocella, 2019). Volcanic unrest may typically last from hours to months but at some caldera volcanoes, unrest episodes may last for years to decades (Acocella et al., 2015). Some volcanoes that have not erupted for tens to hundreds of years may experience repeated episodes of unrest over several years before a critical threshold is reached and an eruption occurs (e.g., Sigmundsson et al., 2010).

During unrest at volcanoes, ground deformation is usually on the order of millimetres to centimetres per year and it is not uncommon for the centre of uplift to move (e.g., Di Vito et al., 2016). Some caldera volcanoes may show deformation rates of metres per year (e.g., Acocella, 2019) and some calderas show very long-term ground deformation (‘resurgence’) which may cause uplift of up to 1 km over hundreds to thousands of years (e.g., Galetto et al., 2017; Acocella, 2019).

Volcanic calderas are some of the most dangerous volcanoes on Earth and many have large populations living in and around the caldera (Acocella et al., 2015). They have surface depressions from ~1 km to tens of kilometres across, and up to several kilometres in topographic change from rim to floor (Acocella et al., 2015). Some contain lakes (e.g., Taal, Philippines) and some are semi-submarine (e.g., Santorini, Greece; Krakatau, Indonesia). Most calderas have large (over 1000 km3), long-lived, heterogeneous and active magmatic systems and about 20 caldera volcanoes show unrest each year, most driven by magma intrusion (Acocella et al., 2015).

For example, the Campi Flegrei caldera (Italy) is 12 km across and lies under the outskirts of Naples. At least 5 m of uplift was observed in the hours to days before the last eruption at Campi Flegrei in 1538 (from Monte Nuovo) resulting in the seaward retreat of the shoreline by ‘200 paces’ (Parascandola, 1947; Dvorak and Gasparini, 1991). Campi Flegrei experienced major uplifts in 1950–1951, 1969–1972 and 1982–1984 which cumulatively raised the town of Pozzuoli by 4 m. Pozzuoli experienced a maximum of 1.8 m of uplift during unrest in 1982–1984 (Berrino et al., 1984).

Metrics and numeric limits

Not identified.

Examples of drivers, outcomes and risk management

Uplift is typically associated with pressurisation (‘inflation’) of a shallow (few kilometres below the surface) magmatic system caused by the injection of magma but may also be caused by volatile degassing of a magma body (e.g., Lowenstern et al., 2006; Dzurisin, 2007; Sparks et al., 2012; Reath et al., 2020). Subsidence is associated with depressurisation (‘deflation’) of a magmatic system and may be caused by magma cooling and solidification, or the outflow of magma (during eruption or lateral migration into dykes or sills, e.g., Sigmundsson et al., 2015). Subsurface hydrothermal systems at volcanoes may also cause ground deformation (Gottsmann et al., 2006), as may tectonic events such as large earthquakes (Battaglia et al., 1999; Pritchard et al., 2019).

Near real-time ground deformation monitoring may enable scientists to anticipate the start of eruptions and key hazardous events during eruptions (e.g., Sparks et al., 2012; Sigmundsson et al., 2015; Fernández et al., 2017; Pallister et al., 2019). Satellite technologies such as InSAR have the potential to make a significant contribution to volcano ground deformation monitoring, especially in the form of regional surveys and for remote volcanoes with limited monitoring infrastructure (Ebmeier et al., 2018). Numerical simulations of ‘inflation’ and ‘deflation’ at volcanoes are generally carried out to understand and interpret observed ground deformation in terms of the dynamics and shape of the pressure source (e.g., Gottsmann et al., 2006).

Damage can be caused to buildings (Pingue et al., 2011), transport networks, critical infrastructure and facilities hampering response and mitigation efforts. Coastal regions affected by uplift/subsidence may be unable to use harbours and ferries for evacuation purposes (e.g., Alberico et al., 2012). Most damage during unrest at Campi Flegrei in 1982–1984 occurred within 2 km of the centre of uplift where total vertical movement exceeded about 60% of its maximum value of about 1.8 m but there was also intense volcanic earthquake activity (Barberi et al., 1984; Berrino et al., 1984; Charlton et al., 2020). Multiple hazards will occur before, during and after eruptions leading to cascading impacts, so risk mitigation measures need to account for this (e.g., Charlton et al., 2020).

Volcanic unrest associated with uplift and/or subsidence may cause significant distress to residents, with associated evacuations causing permanent displacement for some and loss of livelihoods (e.g., Barberi et al., 1984; Longo, 2019). A risk perception study at Campi Flegrei showed that residents who remembered the unrest episodes of the 1970s and 1980s were more concerned about unrest than an eruption (Ricci et al., 2013). Testing and practicing evacuation procedures for future response may enhance the awareness and preparedness of populations (e.g., Commune di Napoli, 2019).