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

Abstract

[1] 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 d 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.

A model of deep crustal fluid flow following the Mw = 8.0 Antofagasta, Chile, earthquake

“…A fracture system could support a high fluid velocity of several hundred meters per day while keeping low intrinsic permeability k of around 1 Â 10 À16 m 2 [Zimmermann et al, 2000]. Recent evidence suggests earthquake triggering due to high pore pressure pulses [Baisch and Harjes, 2003;Bräuer et al, 2003;Miller et al, 2004], also in case of the 1995 M w = 8.0 Antofagasta earthquake [Shapiro et al, 2003]. If the aftershock clusters show observed velocities of several hundred meters per day [Toda et al, 2002], we propose particle flow along an open fracture system [Lee and Cho, 2002] extending not only in the plane of the rupture but also in the surrounding rock volume [Cox, 1995;Davies, 1999].…”

Intraplate seismicity in the western Bohemian Massif (central Europe): A possible correlation with a paleoplate junction

“…Tullis and Yund (1980) found that the addition of 0.2 wt% of water to granite deformed under the dry conditions produced a pronounced weakening, enhanced ductility and reduced the temperature of the brittleductile transition by about 150-200 o C for both quartz and feldspar. Although no gas and fluid exhalations are directly above the NK hypocenters, possibly due to a low-permeable cap above the active hydraulic system (Bräuer et al, 2003), the NK focal area is at the periphery of the CHB escape center (Fig. 1).…”

“…The Tertiary Eger (Ohře) Rift (ER), a 300 km long ENE-WSW striking structure characterized by high heat flow and Cenozoic volcanism is part of the European Cenozoic Rift System (ECRIS; Prodehl et al, 1995) and its formation is thought to be related to Alpine collision (Ziegler, 1992; see also review of the ER development in Geissler et al, 2005). Active tectonics is primarily manifested by frequent weak to moderate earthquake swarms (Horálek et al, 2000), emanations of CO 2 dominated gases of mantle origin (Weinlich et al 1999;Bräuer et al, 2003), and by neotectonic crustal movements (Bankwitz et al, 2003). The post-rift uplift of the Krušné hory Mts.…”

Drilling into an active mofette: pilot-hole study of the impact of CO<sub>2</sub>-rich mantle-derived fluids on the geo–bio interaction in the western Eger Rift (Czech Republic)

“…The strongest registered earthquakes reached local magnitudes of M L 4.5 (Fischer et al, 2014). Active fault zones very likely represent migration pathways for the degassing of mantle-derived CO 2 that causes intense mofette activity (Kämpf et al, 2013;Nickschick et al, 2015Nickschick et al, , 2017, while the ascent of magmas and the fluid activity probably constitute the forcing mechanisms of the seismic activity (Bräuer et al, 2003(Bräuer et al, , 2008(Bräuer et al, , 2011Dahm et al, 2008;Fischer et al, 2014).…”

“…1), strong subcontinental mantle-dominated CO 2 degassing occurs in the Bublák and Hartoušov mofette fields (Bräuer et al, 2003Kämpf et al, 2013;Nickschick et al, 2015Nickschick et al, , 2017. Mofettes are places where geogenic CO 2 ascends through conduits from the mantle to the surface (Fig.…”