Isotopic evidence (³He/⁴He, of fluid‐triggered intraplate seismicity

Abstract

The aim of this study is to find a link between the occurrence of mantle‐derived CO 2 exhalations and earthquake swarms beneath the western Eger rift, central Europe. We monitored the gas and isotope composition of the free gas of the Eisenquelle mineral spring and the Bublak mofette weekly for 2 years. These time series include periods before, during, and after an earthquake swarm event. Shifts in δ 13 C and 3 He/ 4 He were found for several months after the occurrence of the earthquake swarm on 4 and 5 December 1994. The isotopic shifts were explained by the admixture of crustal fluids released at the hypocenter to the “permanent” mantle volatile flux. From the beginning of the earthquake swarm, as starting point for the gas release from the hypocenter and the moment of arriving fluid with changed isotopic signature at the locations, we estimated that the fluid transport velocity in the upper crust ranges between 400 m/day near a center of CO 2 emanation (Bublak mofette) and 50 m/day in the periphery (Eisenquelle mineral spring). Fault rupture induced by the buildup of fluid overpressure in the brittle crust (associated with sealing processes surrounding the ascending channels of the permanent mantle flux) seems to be the main reason for periodic/episodic earthquake swarm activity in this area.