Martinelli G.¹, Yüce G.², Gherardi F.³, Medioli G.⁴, Pierotti L.³, Facca G.³, Pinti D.L.⁵

(¹INGV, National Institute of Geophysics and Volcanology, Palermo, Italy; ²Hacettepe University, Geological Engineering Department, Ankara, Türkiye; ³CNR-IGG, Institute of Geosciences and Earth Resources, Pisa, Italy; ⁴Independent Researcher; ⁵Geotop & Dépt des sciences de la Terre et de l’atmosphère, Université du Québec à Montréal, Canada)

The presence and spatial distribution of geofluids provides valuable information on zones of enhanced crustal permeability generated by long-term tectonic deformation and fault activity. Global mapping of thermal springs and associated helium isotopic ratios has identified areas with relatively high tectonic activity, often associated with extensional regimes. These areas will continue to experience seismic events until substantial global geodynamic changes modify the stress field. Geofluid occurrences marked by anomalous mantle helium content have also been detected in locations not often associated with contemporary seismicity.

Geochemical indicators can therefore complement conventional seismological datasets by identifying deep, permeable fault systems connected to lithospheric fluid pathways. Deep-seated geofluids migrate preferentially through fault zones that remain hydraulically connected over geological time. Such faults represent long-lived crustal-scale permeability structures that facilitate upward transport of mantle-derived volatiles, including helium and carbon dioxide. Consequently, the occurrence of thermal waters and anomalous ³He/⁴He ratios provide independent evidence for persistent lithosphere-to-surface fluid pathways that may not be recognizable from historical seismic catalogs alone.

The approaches mentioned above can more accurately identify sites where comparatively high crustal permeability values may influence deep faults with lithospheric roots. It’s possible to identify peculiar areas of the Earth that experience relatively frequent seismicity, as indicated by seismic events with magnitudes between 5 and 6. These areas are chiefly characterized by normal faults, which usually host thermal springs and could be identified as representative of a first crustal permeability quantum.

Some of these areas host dry or bubbling CO₂ degassing. If helium carried by CO₂ shows ³He/⁴He isotopic signatures higher than the regional average, deepest and persisting faults must be considered as representative of a second crustal permeability quantum. At these sites, seismic events with 6<M<9.2 or higher may occur, although seismic catalogs have recorded no evidence of them. Therefore, seismic hazard analyses that incorporate geochemical observations can better identify potentially hazardous seismogenic structures and provide more realistic long-term seismic hazard assessments.

Author Profile (corresponding author):

PINTI, D.L. Male, Full Professor, PhD, mainly engaged in isotope geochemistry. E-mail: pinti.daniele@uqam.ca

Keywords: Geofluids; Thermal springs; Helium Isotopes; Crust Permeability; Seismogenic Faults.