Earthquakes and helium: evidences of the impulsive nature of earth degassing

In seismic regions, fluids play active roles during the preparatory phases of large earthquakes and, through their chemical and isotopic signature, transport to the surface information about deep processes within the fault zones.

In this scenario, noble gases are useful to investigate earth degassing, and their isotopic ratios help to decipher the dynamics of natural processes such as volcanic eruptions and earthquakes. The lightest of noble gases is helium (He), and in natural fluids, it is present with two isotopes, ³He and ⁴He. The former being mainly primordial and stored in the mantle, the latter continuously produced by U and Th decay in the earth interior. In stable continental region the He flux is dominated by the radiogenic ⁴He that is produced into the crust (mantle He <1%).  In contrast, primordial ³He escape to the atmosphere in regions of active tectonic (from extensive to compressive).

Experimental studies highlighted that during rocks deformation micro-fracturation increases as an effect of dilation, and consequently, He is liberated from rocks and it escapes towards the pore fluids and successively to the atmosphere. Hence, it indicates a direct link between seismicity and the crustal ⁴He degassing. However, it is mandatory to know the volume of the rocks involved in earthquakes-induced rock-fracturation to quantify the amount of He released in seismic processes.

Fault zones are complex systems whose mechanical properties evolve over time. Field observations and experimental works allow to schematically simplify these zones into two main structural regions: (1) the fault core and (2) the damage zone. However, the lack of direct observations limits the knowledge of their architecture at depth. Thus, in order to understand the multi-scale, physical/chemical processes responsible for the faulting that earthquakes occur on, it is fundamental to consider phenomena that intersect different scientific research fields. Near Fault Observatories (NFOs) are grounded on multidisciplinary infrastructures, collecting near fault high resolution scientific data that allows generation of innovative observations (Chiaraluce et al., 2022).

Here, we analysed a 12-year earthquake catalogue (M<4) in the IRPINIA NFO (Italy), a region affected by high-magnitude disastrous earthquakes (i.e. M= 7.0 in 1857 and M= 6.9 in 1980).

The analysis of this earthquakes catalogue allows reconstructing year by year the volumes of both the fault core and the damage zone. We computed the ⁴He output from the two faults zone observing that the low-magnitude earthquakes (M < 4) efficiently contribute to variations of the crustal helium output into the atmosphere. Our results support the impulsive nature of He degassing in tectonically active continental regions (Caracausi et al., 2022). We recognized a quantitative relationship between crustal helium outputs and the volume of fault zones, and  we suggest that variations in helium flux may represent a gauge of changes in the stress field that are related to the nucleation of earthquakes.

 

References

Caracausi et al. (2022). doi:10.1038/s43247-022-00549-9.

Chiaraluce et al. (2022). doi:10.4401/ag-8778.