CO2 production by mechanical stress on carbonate rocks and its implications for natural hazards assessment

La Jolla, California, USA

Natural CO2 discharges generally coincide with the in-land segments of major zones of seismicity throughout the world, showing the strong correlation between natural degassing and earthquakes. On the other hand, aftershocks of large earthquakes have been attributed to the coseismic release of trapped, high-pressure CO2-dominated fluids propagating through damaged zones created by the main shock thus underlining the role of the fluids as “agents” able to generate overpressures and reactivate fault segments inducing earthquakes.
Recent experimental results have demonstrated that CO2 can be produced by mechanical stress applied on carbonate rocks sometimes requiring a relatively low energy rates. As a result, crustal volatiles can be produced due to high-pressure, mechanical stresses at moderate levels within the crust. Laboratory experiments, whereby different types of carbonate rocks (natural and synthetic) have been milled, have shown that carbonates release CO2 systematically and reproducibly leaving little doubt that carbonate rock located in shallow crustal levels may undergo structural break-down to form CO2, particularly in the presence of accessory phases such as clays.
Isotopic ratios of CO2 and noble gases carried out on both the rocky samples, gases produced during the experimental work and gases from natural vents has demonstrated that, whereas carbonates can account for the origin of crustal-derived CO2, this is not the case for helium which is not retained in carbonates. Combining those results with data from natural vents of the Apennine Chain and gases extracted from the basement rocks, such as granite and gneiss, demonstrates that crustal helium can be released only from the basement thus implying that helium has a provenance distinct from that of CO2 and the seimogenesis involves deep crustal layers.
Temporal variations of the 13C CO2 might be interpreted either as a consequence of different intensity Gas-Water interactions or a CO2 production far from the isotopic equilibrium. That CO2 is released together with crustal-type helium marked by values as low as 0.02-0.2Ra
Moving the collected results from the laboratory to the natural systems means to improve the possibility of assessing the linkage between variations in geochemical tracers and the onset of seismic activity thus to gain an insight on the seismogenesis. Since crustal deformation can occur also aseismically, and rock deformation may produce CO2 as a response to mechanical stress accumulation and strain release, monitoring of CO2 discharges could be useful in estimating the probability increase of an impending earthquake in a potentially hazardous seismic region.