Carbon dioxide controlled earthquake distribution pattern in the NW Bohemian swarm earthquake region, western Eger Rift, Czech Republic – gas migration in the crystalline basement

Abstract

The ascent of magmatic carbon dioxide in the western Eger (Ohře) Rift is interlinked with the fault systems of the Variscian basement. In the Cheb Basin, the minimum CO2 flux is about 160 m3 h−1, with a diminishing trend towards the north and ceasing in the main epicentral area of the Northwest Bohemian swarm earthquakes. The ascending CO2 forms Ca‐Mg‐HCO3 type waters by leaching of cations from the fault planes and creates clay minerals, such as kaolinite, as alteration products on affected fault planes. These mineral reactions result in fault weakness and in hydraulically interconnected fault network. This leads to a decrease in the friction coefficient of the Coulomb failure stress (CFS) and to fault creep as stress build‐up cannot occur in the weak segments. At the transition zone in the north of the Cheb Basin, between areas of weak, fluid conductive faults and areas of locked faults with frictional strength, fluid pressure can increase resulting in stress build‐up. This can trigger strike‐slip swarm earthquakes. Fault creep or movements in weak segments may support a stress build‐up in the transition area by transmitting fluid pressure pulses. Additionally to fluid‐driven triggering models, it is important to consider that fluids ascending along faults are CO2‐supersaturated thus intensifying the effect of fluid flow. The enforced flow of CO2‐supersaturated fluids in the transitional zone from high to low permeability segments through narrowings triggers gas exsolution and may generate pressure fluctuations. Phase separation starts according to the phase behaviour of CO2‐H2O systems in the seismically active depths of NW Bohemia and may explain the vertical distribution of the seismicity. Changes in the size of the fluid transport channels in the fault systems caused, or superimposed, by fault movements, can produce fluid pressure increases or pulses, which are the precondition for triggering fluid‐induced swarm earthquakes.

CO2 Inflow and Elements Desorption Prior to a Seismic Sequence, Amatrice‐Norcia 2016, Italy

“…The involvement of deep CO 2 ‐bearing fluids in the triggering of earthquake sequences has been widely debated (Miller, ; Weinlich, ). A carbon mass balance of the involved water describing the origin of total dissolved inorganic carbon (TDIC) can be represented as follows (amounts expressed as m = molality; see the Supporting Information S1 for details) (Chiodini et al, ): Ccarb is the amount of carbon derived from the interaction of groundwater with carbonate aquifer rocks, while Cext is carbon derived from processes other than these (i.e., from external sources).…”

The October 2008 Nový Kostel earthquake swarm and its gas geochemical precursor

“…The strain measurements between Kynšperk nad Ohří and Luby are cumulative values of the entire distance. Along the Mariánské Lázně fault south of Nový Kostel, there are weakened fault segments which have been kaolinized by CO 2 (Weinlich ). Small fault motions along this section induce only small‐scale strain variations and do not necessarily have an effect on the fault permeability in the Nový Kostel area and consequently on the CO 2 data.…”

“…Nový Kostel is right at the edge of the magmatic CO 2 occurrences of the Cheb Basin (Weinlich et al . ), that is in the transition zone from weak, conductive fault segments with CO 2 driven kaolinization of the fault planes to unaltered, locked fault segments with high frictional strength (Weinlich ). In this zone, movements along the kaolinized weak faults in the Cheb Basin can induce and transmit increasing fluid pressure or pressure pulses and can lead to a stress build‐up triggering these earthquake swarms at the northern edge of the Cheb basin at Nový Kostel.…”

“…The time-space seismicity pattern during the January 1997 swarm indicates step-by-step penetration of fluids into the fault system in the Nov y Kostel area; this suggests that fluid intrusions triggered the seismic activity ( Spi c ak & Hor alek 2001; Fischer et al 2014). Nov y Kostel is right at the edge of the magmatic CO 2 occurrences of the Cheb Basin (Weinlich et al 1998), that is in the transition zone from weak, conductive fault segments with CO 2 driven kaolinization of the fault planes to unaltered, locked fault segments with high frictional strength (Weinlich 2014). In this zone, movements along the kaolinized weak faults in the Cheb Basin can induce and transmit increasing fluid pressure or pressure pulses and can lead to a stress buildup triggering these earthquake swarms at the northern edge of the Cheb basin at Nov y Kostel.…”

Locating mofettes using seismic noise records from small dense arrays and matched field processing analysis in the NW Bohemia/Vogtland Region, Czech Republic

“…The CO 2 isotope signature indicates an upper mantle origin and denotes the ongoing magmatic activity in the NW part of the Bohemian Massif ( Fig. 1) (Weinlich et al 1999(Weinlich et al , 2013Weinlich 2014;Bräuer et al 2004Bräuer et al , 2009Bräuer et al , 2011.…”