by Torsten Dahm, Alexandre M. Anesio, Alexandre M. Anesio, Stefan Buske, David Dolejs, Josef Horalek, Pavla Hrubcova, Michael Korn, Frank Krüger, Jiri Málek, David Shelly, Dirk Wagner

The Bohemian Massif (500-250 Ma), the easternmost part of the Variscan orogenic belt, is one of the largest stable outcrops of pre-Permian rocks in Central and Western Europe. This region has persistent geodynamic activity that is clearly linked to upper mantle, and offers a globally unique location for studying intra-continental earthquake swarm (ES) seismicity in combination with deep crust and mantle degassing as well as their interaction with the deep biosphere. The main questions regarding seismicity, microbial life and origin, and heat flow are all linked by the common questions of fluid flow, pathways, and composition. The ICDP project 'Drilling the Eger Rift' aims to develop the most modern, comprehensive laboratory at depth worldwide for the study of ES, crustal fluid flow, mantle-derived CO2 and He degassing, and processes of the deep biosphere. In order to reach a new level of high-frequency, near source, and multi-parameter observation of ES and related phenomena, such a laboratory will comprise of a high-frequency 3D seismic array with a set of four shallow boreholes, combined with modern continuous real-time fluid monitoring at depth (the shallow boreholes) and the study of deep biosphere. Existing studies on ES so far have relied on surface observations, such as discontinuous gas and fluid monitoring and collection at springs and mofettes and surface seismic stations. These types of studies are now approaching the edge of their capabilities. In particular, the gas analysis suffers from contamination of meteoric fluids and gases, which are present in the surficial sediment layers and partially mask the signatures of mantle-generated gas. From the microbiological viewpoint active fault zones could be seen as -Hot Spots-of microbial life in the deep subsurface compared to other continental deep biosphere ecosystems due to an intensified substrate support by the CO2-dominated fluid flow. Therefore active fault zones provide unique conditions for studying microbial life at depths, which can only be done in situ. The regular occurrence of persistent earthquake swarms with known radiation effects offers the unique possibility to design and tune a borehole-based monitoring network for optimized analysis of the high frequency content of weak swarm-like seismicity, which appears clearly related to fluid-flow in the crust between 5 and 10 km depth. Although the existing seismic network provides excellent broadband data recordings, the high-frequency data, which is most useful for resolving small magnitude events and investigating small-scale subsurface structures, such as fluid pathways, is strongly attenuated by the weathered bedrock and the sedimentary layer and lost in the noise generally recorded by surface stations.

• With this proposal we suggest the development of the most modern, comprehensive laboratory at depth worldwide for the study of three interconnected areas of primary research:
• i) flow of mantle-derived CO2 through the crust and its degassing at the surface,
• ii) earthquake swarms,
• iii) the composition and processes of the deep biosphere. We aim to study each of these individual topics, and investigate the interconnections between them. Specifically, such a laboratory will comprise a novel concept of 3D seismic arrays with a set of shallow boreholes in order to reach a new level of high-frequency, near source, and multi-parameter observation of earthquake swarms and related phenomena. One site of three boreholes will be also equipped with modern continuous real-time fluid monitoring at different depth levels, combined with the monitoring and sampling of the deep microbial biosphere. The proposed laboratory has a unique interdisciplinary potential to better understand,
• - the fluid-rock interaction and the mechanism of fluid-induced earthquake swarms,
• - the structure of fluid pathways from the upper mantle to the surface,
• - the physical, chemical and biological interrelations between geological processes, mantle-derived fluids and the biosphere down to 400 m depth,
• - the fault-valve mechanism and its relevance for earthquake triggering, seismic hazard, degassing and the activity of the deep biosphere,
• - the Quaternary palaeoclimate, volcanic activity and palaeoseismicity in the Eger rift region.

Czechia, Earthquake Swarms, EGER, Europe, Germany, Magmatic Fluids, West Bohemia

Latitude: 50.07306, Longitude: 12.36917