Site characterization of the PIER-ICDP drill locations to understand the relation between earthquake swarms, mofette fields and crustal fluid pathways by imaging the electrical conductivity structure
Final Report Abstract
The West Bohemia/Vogtland region, characterized by the intersection of the Eger Rift and the Mariánské Lázne fault, is a geodynamically active area exhibiting repeated occurrence of earthquake swarms, massive CO2 emanations and mid-Pleistocene volcanism. The Eger Rift is the only known intra-continental region in Europe where such deep seated, active lithospheric processes currently take place. Consequently, this area is studied by the PIER-ICDP initiative which involve an interdisciplinary drilling program advancing the field of earthquake-fluid-rock-biosphere interaction. With the Magnetotelluric (MT) method we can image the electrical conductivity distribution of the subsurface on crustal scale and provide a site characterization. The electrical conductivity is a sensitive physical property to image the existence of and processes caused by fluids. We presented electrical resistivity models obtained from inversion of MT data along (i) a regional 2D profile across the Eger Rift, from (ii) a dense areal grid centred on the Bublák and Hartoušov mofettes and Radio-magnetotelluric (RMT) data across the Hartoušov mofette field. The dimensionality and directionality analysis of the data revealed a geo-electrical strike direction dominated by the Eger Graben and therefore the profile perpendicular to it was considered for 2D inversion. The authoctonous Palezoic metasediments, such as phyllites and slates, of the Saxo-Thuringian Zone are imaged as low to moderately low resistivity down to approximately 2 km. The Cheb Basin including the aquifer feeding the degassing centres of Bublák and Hartoušov mofettes can be correlated with a zone of very low resistivity within this near-surface structure. A sub-vertical narrow conductive channel extending from the near surface down to depths of > 20 km is revealed by the MT models. This conductive feature correlates spatially with the projected locations of the seismic events in the Nový Kostel focal area. In this context, the conductive channel could represent a fluid pathway from a mid-crustal reservoir into the surface, which is supported by seismic tomography results in the region which reveals a Vp/Vs anomaly of similar shape and location. The increase of the pore pressure produced by the fluid circulation would be the driving force that triggers the occurrence of the seismic events. A decrease in conductivity is observed roughly coinciding with the depth at which the H2 O-CO2 system changes from a single phase to two phases and where the earthquakes start occurring. The shallow 3D inversion model supports the existence of the sub-vertical conductive channel in the northern part of the Cheb basin and shows several shallow sub-horizontal conductive layers, which we interpret as shallow aquifers. The shallow conductivity structures were further studied with the RMT method which reveals images of first 50 m of the subsurface in the area of the Hartoušov mofette field. 2D and 3D inversions suggest that wet mofettes correlate with zones of high conductivity while dry degassing seems to be related with resistive soil. These results coincide with borehole logs of the resistivity and indicate that CO2 dissolved in water increases the electrical conductivity and serves as a marker to image the pathways and reservoirs for ascending fluids. Furthermore, another deep reaching conductive feature is imaged at the southern part of the profile. This conductive structure correlates at the surface with the locations of the Mýtina and the Neualbenreuth Maar. The enhanced conductivity could be associated with fossil hydrothermal alteration related to the volcanic activity; however, it cannot be ruled out that it corresponds with the electrical signature of the suture between the Saxo-Thuringian and Teplá-Barrandian zones, whose surface expression is located only a few kilometres away. Further areal investigations are required in this area to distinguish between these two possibilities.
Publications
- 2018. Regional twodimensional magnetotelluric profile in West Bohemia/Vogtland reveals deep conductive channel into the earthquake swarm region. Tectonophysics, 727, pp.1-11
Muñoz, G., Weckmann, U., Pek, J., Kováčiková, S. and Klanica, R.
(See online at https://doi.org/10.1016/j.tecto.2018.01.012)
