Seismic structure of the Cheb Basin from high resolution surveying – data quality assessment and first results

<p><span><span>In this study we present data and preliminary results from several shallow high-resolution seismic surveys in the Cheb Basin, CR, a small intracontinental basin in the North-West Bohemian Massif, located at the Western end of the Cenozoic Eger Rift. The area is well known for its intense earthquake activity, with the largest instrumentally recorded magnitude of M</span></span><span><sub><span>L</span></sub></span><span><span>=4.6. Macroseismic reports of local seismicity date back to the early 19</span></span><span><sup><span>th</span></sup></span><span><span> century, with magnitudes possibly above 5. Quaternary volcanoes, CO</span></span><span><sub><span>2</span></sub></span><span><span>-rich moffettes, and the swarm-like occurrence of the earthquakes suggest they are being triggered by crustal fluids. In contrast, most focal mechanisms show a dominant strike-slip component, indicative of tectonics. Investigating the role of fluids in triggering those earthquakes is one of the objectives of an ongoing ICDP program.</span></span></p><p><span>We expect high-resolution images of the basin structure to provide additional constraints regarding the importance of tectonic faulting. To that end, we surveyed several up to 3-km-long reflection and refraction profiles in the basin center across the putative Počátky-Plesná Fault, and at its edge, across the basin-bounding Mariánské Lázně Fault. The up to 350-m-thick basin sediments are mostly of Miocene and Quaternary origin, overlying Paleozoic Variscan units and post-Variscan granites. The main reflectors are around 200-400 ms. The data were collected with a 500-m-long split-spread of single geophones at 2 m spacing, and the raw shots are dominated by ground roll. In this presentation, we will show an overview of the field campaigns and present first results.</span></p>

New seismotectonic regionalization for Germany: comparison with existing regionalizations

<p>We have created a new seismotectonic regionalization for Germany including a 200 km zone around its borders, based on a new concept which initially processes geological information separately from modern seismicity. The identification of a region as a distinct seismotectonic unit is estimated from past deformation, not the present one as represented by earthquakes. This has been done by analyzing fault density and displacements separately for six time slices from 300 Ma to the Present. The final regionalization results from overlaying the six deformation intensity maps and contrasts regions that deformed either repeatedly or very strongly in the geological past with others that suffered very little deformation. The new regionalization is significantly different from existing regionalizations. The existing ones mostly relied on modern seismicity for defining areas while using geological contacts of varying type (surface traces of faults, but also erosional edges of stratigraphic units as represented on geological maps) to trace boundaries.</p><p>The new, geology-based regionalization comprises comparatively few regions. Ubiquitous small faults (cm- to m-displacements) in the geological record suggest that earthquakes of low magnitude can occur anywhere and need not be tied to large faults. Our regionalization concurs with earlier ones in identifying the Cenozoic rifts – Upper Rhine Graben, Lower Rhine Graben and Eger Rift – as zones of increased hazard. A 100-150 km wide, NW-SE-trending belt of intense Mesozoic deformation runs across northwestern and central Germany from the Emsland to the Erzgebirge where it bifurcates into two branches that continue along the borders of the Bohemian Massif. This belt coincides reasonably well with the relatively sparse earthquakes in central and northern Germany. The Tornquist Fault Zone running NW-SE from northern Denmark to Bornholm is another belt of increased past deformation and elevated seismic activity on the northeastern border of our region. Areas of particularly low past deformation comprise the Brabant Massif, the Rhenish Massif and Münsterland Basin east of the Lower Rhine Graben, the Alpine foreland south of the Danube river and the Bohemian Massif southeast of the Eger Rift. Earthquake clusters occurring in stable areas such as the Brabant Massif or the Swabian Jura highlight geologically unexpected events. They can be added to the regionalization as separate zones or accounted for via a logic tree. They should not be used to assign increased hazard to the larger regions of the geology-based regionalization.</p>

Magnetotellurics in the Eger Rift: An overview of subsurface imaging of different tectonic features

<p>The Bohemian Massif represents the easternmost part of the geodynamically active European Cenozoic Rift System. This region hosts the contact between three tectonic units of the Variscan Belt, the NE-SW trending Eger Rift and the NNW-SSE striking Marianské Lázne fault. It is characterised by ongoing magmatic processes in the intra-continental lithospheric mantle, repeated earthquake swarms, extensive CO<sub>2</sub> degassing in mineral springs and mofettes and the presence of Quaternary volcanoes. While the ICDP drilling programme utilizes information gathered within shallow boreholes in the region, we applied the Magnetotelluric (MT) method to obtain site characterizations in the vicinity of the proposed drill sites. The electrical conductivity has proven to be an important parameter to image the above-mentioned tectonic from the lower crust to the shallow subsurface as well as on a regional and a local scale. Here, we present 2D and 3D inversion models of different MT and Radio-MT (RMT) experiments to study e.g. the Hartouŝov mofette fields, the Quaternary scoria cones, the regional faults and their interplay. Thereby the experiments were designed that we can use lower frequency data from MT to support shallow 3D inversions of e.g. the scoria cones in the regions. The most prominent large-scale conductivity features map channels from the lower crust to the surface possibly forming pathways for fluids into the region of earthquake swarms, mofette fields and know spas. However, the locations of the scoria cones seem to be bound to regional fault zones.</p>

High frequency array observations of December 2020 swarm at surface and borehole stations at ICDP Eger Rift site Landwüst (Vogtland)

<p>Within the ICDP project “Drilling the Eger Rift”, we focus on the German-Czech border region West Bohemia/ Vogtland which is known for its earthquake swarms. These swarms are clusters of small magnitude (ML<4) earthquakes which are supposed to be linked to the rise of fluids with mainly mantle origin. We aim to improve the seismological observation of these small magnitude earthquakes and related processes especially at <span>frequencies above 100 Hz</span> by installing three dense small aperture 3D arrays. Each single 3D array will consist of a 400 m deep vertical array borehole installation and a small aperture (400 m) surface array.</p><p>The drill site S1 in Landwüst and its surroundings serve as pilot site for the first installation. The borehole chain consists of eight 3-component 10 Hz geophones and the continous recordings are sampled with 1000 Hz. In parallel, twelve surface stations are installed which are equiped with 4.5 Hz geophones. The data were recorded with 400 Hz sampling rate at most locations, but at some selected stations we additionally record data with 1000 Hz sampling rate being the desired sampling rate for the final array configuration. Due to the high sampling rates and the high frequency content of the recorded earthquake signals, local site conditions may lead to non-coherent recordings for different parts of the array which have a major influence on the overall array performance. However, preliminary results from broad band frequency wave number analysis (5-180 Hz) in a moving time window (0.2 s) with first test installation data also indicate that the coherency across the array site is still high enough to clearly identify P and S waves from local earthquakes.</p><p> </p><p>In <span>the period December 2020 – January 2021,</span> an earthquake swarm took place with two activity clusters in Nový Kostel (Czech Republic) and Obertriebel/ Oelsnitz (Vogtland, Germany) about 20 km apart. This swarm was recorded by both borehole stations and surface stations in Landwüst. Preliminary results show that more than 14000 events can be identified at the borehole stations and that about 70-80% of these events are also observed at the surface stations. For small earthquakes, mainly the S wave can be identified, but also impulsive P waves are clearly visible at the surface stations. These high frequency waves (up to 230 Hz at the surface) show a good coherency across the surface array. At the borehole stations, we observe an even higher frequency content up to 300 Hz and more. We present recordings from selected events to analyse frequency content and coherency across the 3D array.</p>

Multi-level fluid monitoring to understand the origin of transients

<p>Anomalies in timeseries are frequently reported in the context of earthquake precursor studies. The state of knowledge can be summarized as follows: (i) significant anomalies exist, (ii) seismo-tectonically induced anomalies might exist, (iii) anomalies of non-tectonic origin exist and may look very similar to tectonic ones. Thus, presumably only a fraction of all reported precursors is real in the sense that they are of seismo-tectonic origin. A key problem in earthquake prediction research is to understand the origin of an anomaly and thus the separation of internal and external drivers like e.g. rainfall.  </p><p>State-of-the-art fluid monitoring techniques allow for a high temporal resolution compared to the low-resolution discrete sampling approach used in the last decades. A unique approach will allow to monitor ascending fluids along a vertical profile in a set of drillings from a depth of a few hundred metres to the surface. This setup can provide hints on the origin of temporal variations related to the opening of fault-valves, admixture of crustal fluids to a background mantle-flow or the release of hydrogen during fault rupturing. Gas migration velocities can thus be measured directly from the arrival times of anomalies at different depth levels. In addition, potential admixtures of mantle fluids with crustal or meteoric fluids during the ascent to the Earth’s surface can be quantified.</p><p>A prototype of a multi-level gas monitoring system has been implemented at a mofette. Mofettes are gas emission sites where CO2 ascends through long-lived, narrow channels from the deep crust and possibly the Earth’s upper mantle and thus provide natural windows to magmatic processes at depth. The primary objective of our research on mofettes is to clarify physical links between fluid properties, their pathways and the relation to swarm earthquakes. The Hartoušov mofette field with an estimated daily CO<sub>2</sub> flux between 23 and 97 t over an area of about 350,000 m<sup>2</sup> has been chosen as a key site in the frame of the ICDP project: “Drilling the Eger Rift: Magmatic fluids driving the earthquake swarms and the deep biosphere.” It is located in the Cheb Basin, which terminates the Czech part of the Eger Rift to the West and is known for recurring earthquake swarms and mantle degassing. Gas and isotope compositions will be continuously analyzed in-situ at different depth levels (30 m, 70 m, 230 m) reached by three adjacent boreholes.</p>

Microbial Signatures in Deep CO2-Saturated Miocene Sediments of the Active Hartoušov Mofette System (NW Czech Republic)

The Hartoušov mofette system is a natural CO2 degassing site in the central Cheb Basin (Eger Rift, Central Europe). In early 2016 a 108 m deep core was obtained from this system to investigate the impact of ascending mantle-derived CO2 on indigenous deep microbial communities and their surrounding life habitat. During drilling, a CO2 blow out occurred at a depth of 78.5 meter below surface (mbs) suggesting a CO2 reservoir associated with a deep low-permeable CO2-saturated saline aquifer at the transition from Early Miocene terrestrial to lacustrine sediments. Past microbial communities were investigated by hopanoids and glycerol dialkyl glycerol tetraethers (GDGTs) reflecting the environmental conditions during the time of deposition rather than showing a signal of the current deep biosphere. The composition and distribution of the deep microbial community potentially stimulated by the upward migration of CO2 starting during Mid Pleistocene time was investigated by intact polar lipids (IPLs), quantitative polymerase chain reaction (qPCR), and deoxyribonucleic acid (DNA) analysis. The deep biosphere is characterized by microorganisms that are linked to the distribution and migration of the ascending CO2-saturated groundwater and the availability of organic matter instead of being linked to single lithological units of the investigated rock profile. Our findings revealed high relative abundances of common soil and water bacteria, in particular the facultative, anaerobic and potential iron-oxidizing Acidovorax and other members of the family Comamonadaceae across the whole recovered core. The results also highlighted the frequent detection of the putative sulfate-oxidizing and CO2-fixating genus Sulfuricurvum at certain depths. A set of new IPLs are suggested to be indicative for microorganisms associated to CO2 accumulation in the mofette system.

ICDP project Drilling the Eger Rift – present status and further plans

<p><span>Within the ICDP-Eger drilling project we are developing one of the most modern and comprehensive laboratories at depth worldwide to study the interrelations between the flow of mantle-derived fluids through the crust and their degassing at the surface, the occurrence and characteristics of crustal earthquake swarms, and the relation to the geo-biosphere. The Cheb basin located in the western Eger Rift at the Czech-German border provides an ideal natural laboratory for such a purpose. In October 2016 the ICDP proposal was accepted for complementing two existing shallow monitoring wells with five new, distributed, medium depth (<400 m) drill holes F3 and S1-S4. </span></p><p><span>The resulting natural laboratory at depth will comprise five drilling sites for studying above mentioned phenomena. The F1-F3 drillings form a unique facility of three wells at one site within an active CO<sub>2</sub> mofette of Hartou</span><span>šov </span><span>for continuous recordings of fluid composition and fluid flow rate, as well as for intermittent GeoBio fluid sampling. Drillings S1-S4 are planned for seismological monitoring to reach a new level of high-frequency, near source observations of earthquake swarms and related phenomena such as seismic noise and tremors generated by fluid movements. Instrumentation of the seismic wells S1-S3 will include 8-element geophone chains and a bottom-hole broadband sensor. The borehole sensors will be complemented at S1 by small-scale surface array of approximately 400 m diameter to obtain truly 3D-array configurations. If possible, broadband surface stations and other sensors will be added to each drill location. </span></p><p><span>So far, we have completed drillings at sites S1, S2 and S3, with depth of </span><span>402, 480 and 400 m. </span><span>The drilling of S4 is planned in 2020 at one of the recently discovered Maars at the Czech-German border region. Drilling F3 was completed in September 2019 at a depth of 239 m. It has reached several over-pressurized, CO</span><span>2 </span><span>bearing layers. The three boreholes have been connected by underground tubes system to the nearby field laboratory equipped by flowmeters and mass spectrometers allowing for long time precise monitoring of the degassing process. The S1 borehole (Landwust) will be instrumented in January 2020 by a test geophone chain allowing, along with the DAS fibre-optic cable installed behind the casing, to carry out a VSP measurement.</span></p><p><span>In our presentation we provide information on the status of drillings, sensor installation and plans for the complete monitoring and data handling concept.</span></p>

3D imaging of the subsurface electrical conductivity structure in West Bohemia covering mofettes and Quaternary volcanic structures by using magnetotellurics

<p>The West Bohemian Massif represents the easternmost part of the geo-dynamically active European Cenozoic Rift System. This region hosts different tectonic units, the NE-SW trending Eger Rift, the Cheb Basin and a multitude of different faults systems. Furthermore, the entire region is characterised by ongoing magmatic processes in the intra-continental lithospheric mantle. These processes take place in absence of active volcanism at surface, but are expressed by a series of phenomena, including e.g. the occurrence of repeated earthquake swarms and massive degassing of CO<sub>2</sub> in the form of mineral springs and mofettes. Active tectonics is mainly manifested by Cenozoic volcanism represented by different Quaternary volcanic structures e.g. the Eisenbühl, the Kammerbühl and different maars. All these phenomena make the Eger Rift a unique target area for European intra-continental geo-scientific research. Therefore, an interdisciplinary drilling programme advancing the field of earthquake-fluid-rock-biosphere interaction was funded within the scope of the ICDP. Magnetotelluric (MT) measurements are applied to image the subsurface distribution of the electrical conductivity from shallow surface down to depths of several tens of kilometres. The electrical conductivity is a physical parameter that is particularly sensitive to the presence of high-conductive phases such as aqueous fluids, partial melts or metallic compounds. First MT measurements within this ICDP project were carried out in winter 2015/2016 along two 50 km long perpendicular profiles with 30 stations each and a denser grid of 97 stations close to the mofettes with an extension of 10 x 5 km<sup>2</sup>. Muñoz et al. (2018) presented 2D images along the NS profile of one regional profile. They reveal a conductive channel at the earthquake swarm region that extends from the lower crust to the surface forming a pathway for fluids into the region of the mofettes. A second conductive channel is present in the south of the model. Due to the given station setup, the resulting 2D inversion allows ambiguous interpretations of this feature. 3D MT data and inversions are required to distinguish between different scenarios and to fully describe the 3D structure of the subsurface. Therefore, we conducted a large MT field experiment in autumn 2018 by extending the study area towards the south. Broad-band MT data were measured at 83 stations along three 50-75 km long profiles and some additional stations across the region of the maars, the Tachov fault and the suture zone allowing for 2D as well as 3D inversion on a crustal scale. To improve the data quality, advanced data processing techniques were applied leading to good quality transfer functions. Furthermore, the previously collected MT data were reprocessed using the new approaches. This entire MT data set across the Eger Rift environment together with old MT data collected within the framework of the site characterisation in the surrounding of the KTB drilling are used to compute 3D resistivity models of the subsurface, with combining different transfer functions. These 3D inversion results will be introduced and discussed with regard to existing geological hypotheses.</p><p> </p>

Variations of gas compositions during a drilling process: A key study on the Hartoušov Mofette, Czech Republic

<p>The Eger Rift (Czech Republic) is an intraplate region without active volcanism but with emanations of magma-derived gases and the recurrence of mid-crustal earthquake swarms with small to intermediate magnitudes (M<sub>L</sub> < 5) in the Cheb Basin. To understand the anomalous earthquake activity and CO<sub>2</sub> degassing, an interdisciplinary well-based observatory is built up for continuous fluid and earthquake monitoring at depth.</p><p>The fluid observatory is located at the Hartoušov Mofette (Cheb Basin), an area characterized by intense mantle degassing with a subcontinental lithospheric mantle (SCLM) contribution of He that increased from 38% in 1993 to 89% in 2016. Two drillings with depths of 30 and 108 m (F1 and F2, respectively) are being monitored since August 2019 for the composition of ascending fluids. Additionally, the environmental air composition is monitored. Gas concentrations were determined in-situ at 1-min intervals, while direct sampling campaigns took place periodically and samples were analyzed for their chemical and isotope composition. Samples of gases emerging in the mofette were also collected. During this period, a third borehole (F3) with a depth of 238 m was drilled.</p><p>At Hartoušov, carbon dioxide is the prevailing gas component (concentrations above 99.5%), with helium presenting a mantle origin (up to 90% considering a SCLM-type source). The atmospheric contribution is negligible, even though during drilling of F3 enrichments in atmospheric components such as Ar and N<sub>2</sub> have been observed. An increase in both CH<sub>4</sub> and He has been noticed in F2 (108 m borehole) at 40 m depth, whilst a decrease in He has been observed at 193 m depth in both F1 and the natural mofette. Enrichments in less soluble gases (eg. He and N<sub>2</sub>) at various depths accompanied by a minor CO<sub>2</sub> decrease have also been noticed. Such variations may have been caused by the different solubilities of gases in aquatic environments. Moreover, a decrease in CO<sub>2</sub> followed by a subsequent enrichment of CH<sub>4</sub> and C<sub>x</sub>H<sub>y</sub> during the first days after the initial drilling could promote the hypothesis of the generation of microbialy derived CH<sub>4</sub>. Diurnal variations were observed for the majority of the gas components during the last phase of the F3 drilling, when the well reached a depth >200 m.</p><p>This research is a part of the MoRe - “Mofette Research” project, which is included in the ICDP project “Drilling the Eger Rift: Magmatic fluids driving the earthquake swarms and the deep biosphere”). This work was supported by the DFG grant# WO 855/4-1 and BA 2207/19-1.</p>