Can changes in the frequency-magnitude distribution of earthquakes be used as an eruption precursor?

The emplacement of magmatic bodies within the crust induces significant changes in chemical and physical properties of the bedrock, which can be remotely measured to monitor the state of volcanoes and assess potential eruptions. In addition to geodetic (Dzurisin, 2003, 2007; Bruno et al., 2022) and geochemical (Aiuppa et al., 2007; Paonita et al., 2021) methods, seismology is widely used for volcano monitoring, as magma movement within the conduits and storages produces low-frequency vibrations of the volcanic edifice (i.e., volcanic tremor and long-period events; Eaton et al., 1987; Sciotto et al., 2022), while intruding magma loads the bedrock triggering volcano-tectonic earthquakes due to fracturing processes or the reactivation of pre-existing faults (McNutt et al., 2005; Firetto Carlino et al., 2022).

Magma movement along volcanic plumbing systems has been shown to also modify the rheology of the crust, influencing the attitude of a crustal volume at storing and releasing elastic energy (Firetto Carlino et al., 2022 and references therein). This aspect can be investigated by detecting changes in the slope b of the Gutenberg & Richter Frequency-Magnitude Distribution of earthquakes (Gutenberg and Richter, 1944; FMD; logN = a − bM, where N is the cumulative number of seismic events with magnitude above or equal to M and a represents the productivity), commonly referred to as the b-value.

The b-value expresses the proportion of small to large earthquakes, and time changes of this parameter should be considered a proxy for crustal stress variation (Scholz, 1973; Goebel et al., 2013). To examine whether the b-value can track magma movement from deep crustal sectors to the surface and potentially serve as an eruption precursor, we use Mount Etna (southern Italy) as a test site. Variation of the b-value over time has been computed on the 1 January 2005 - 31 December 2024seismic catalogue, but we restricted the period of observation from mid-2016 to December 2024, to ensure a significant number of earthquakes to be considered.

Our results show significant variations along the Etna plumbing system, which can be attributed to magma recharge from depth, increased fluid pressure within the magma storage and dike propagation, leading to eruptive activity.