The geochemical signature caused by earthquake propagation in carbonate-hosted faults

Abstract

Friction laboratory experiments have been performed at sub-seismic (≈ 0.01 m/s) to seismic slip rates (N1 m/s) on dolomite gouges of the Triassic evaporites, which hosted the five mainshocks (5bMw b6) of the 1997 Colfiorito earthquakes in the Northern Apennines (Italy). Experimental faults are lubricated as marked falls of the steady state sliding friction coefficients, μss≈0.2, are observed at seismic slip rates, as opposed to values of μss≥0.6 attained for sub-seismic slip rates. At seismic slip rates decarbonation reactions, triggered by frictional heating in the experimental slip zone, produced: 1) new fluid (CO2) and mineral phases (e.g. Mg-calcite, periclase/brucite, lime/portlandite); 2) isotopic fractionation between the reaction products and the reactant mineral phases. The variations of total dissolved inorganic carbon (TIDC) in concentration Δ(TDIC) and isotopic composition Δ(δ13CTIDC) in a carbonate aquifer, with geochemical parameters similar to those of an aquifer located in the seismic belt of the Northern Apennines, have been modelled after an input of earthquake-produced CO2. Modelling results show that variation in Δ(δ13CTIDC) can be detected in volumes of groundwater which are about three times larger than those calculated for the variations in Δ(TDIC). For amounts of CO2 produced by coseismic decarbonation of ≤5 wt.% of the slip zone gouge, modelling results show that a detectable geochemical anomaly is obtained if the produced CO2 is dissolved into volumes of water comparable to those of the shallower aquifers feeding the springs in the 1997 Colfiorito earthquakes area. We conclude that the integration of results from laboratory experiments, performed at seismic condition, and geochemical analyses can potentially aid in the calibration of monitoring strategies of geochemical properties of water in seismically active areas and provide insights into seismic fault zone processes (e.g. constraints on the temperature rise during earthquake propagation).