The new in situ geodynamic laboratory established in the framework of the ICDP Eger project aims to develop the most modern, comprehensive, multiparameter laboratory at depth for studying earthquake swarms, crustal fluid flow, mantle-derived CO2 and helium degassing, and processes of the deep biosphere. In order to reach a new level of high-frequency, near-source and
multiparameter observation of earthquake swarms and related phenomena,
such a laboratory comprises a set of shallow boreholes with high-frequency
3-D seismic arrays as well as modern continuous real-time fluid
monitoring at depth and the study of the deep biosphere.
This laboratory is located in the western part of the Eger Rift at the
border of the Czech Republic and Germany (in the West Bohemia–Vogtland
geodynamic region) and comprises a set of five boreholes around the
seismoactive zone. To date, all monitoring boreholes have been drilled. This
includes the seismic monitoring boreholes S1, S2 and S3 in the crystalline
units north and east of the major Nový Kostel seismogenic zone,
borehole F3 in the Hartoušov mofette field and borehole S4 in the
newly discovered Bažina maar near Libá. Supplementary borehole P1 is being prepared in the Neualbenreuth maar for paleoclimate and biological
research. At each of these sites, a borehole broadband seismometer will be
installed, and sites S1, S2 and S3 will also host a 3-D seismic array
composed of a vertical geophone chain and surface seismic array. Seismic
instrumenting has been completed in the S1 borehole and is in preparation in the
remaining four monitoring boreholes. The continuous fluid monitoring site of
Hartoušov includes three boreholes, F1, F2 and F3, and a pilot monitoring phase is underway. The laboratory also enables one to analyze microbial activity at CO2 mofettes and maar structures in the context of changes in habitats. The drillings into the maar volcanoes contribute to a better understanding of the Quaternary paleoclimate and volcanic activity.
Main goals, overview processes and settings
What are the physical and chemical processes leading to earthquake
activity and fluid mobility? What are the pathways of fluids through the
crust and how are they influenced by tectonic stress variations? How can
geological processes influence the deep biosphere and the evolution of early
life at depth? These are the open questions tackled by the ICDP project
aimed at drilling in the Eger Rift region.
The West Bohemia–Vogtland geodynamic region in the westernmost
part of the Eger Rift: the epicenters of earthquakes from 1991 to 2021 are marked by
red circles; cyan circles represent CO2 degassing; blue squares represent mofettes; the white triangle marks the Nový Kostel (NK) focal zone; the positions of five Quaternary volcanoes are indicated.
The western Eger Rift, cutting the Bohemian Massif in the westernmost part
of the Czech Republic and the adjacent area in Germany, represents the West
Bohemia–Vogtland region, which is one of the most unique European
intracontinental geodynamic areas (Fig. 1). The geodynamic activity is
represented by active magmatic underplating, mid-crustal earthquake
swarms and massive diffuse degassing of mantle-derived CO2 (e.g.,
Bräuer et al., 2003; Horálek and Fischer, 2008). The region is also
characterized by numerous mineral springs, mofettes, Tertiary–Quaternary
volcanism, and neotectonic crustal movements located at the intersection of
major intraplate fault and tectonic zones around the Cheb Basin. The close
proximity and intensity of the different geodynamic processes in an
intracontinental massif far from plate boundaries are outstanding, and it is
likely that all of these phenomena are related to a common driver in the
lithospheric upper mantle. Currently, it is well accepted that many
earthquake swarms are driven by fluids in the crust. Nowadays, fluid migration and its
control by tectonics are recognized in many regions worldwide under
different tectonic and volcanic settings. The ICDP Eger Rift project
provides extraordinary observations that help to unravel the relations
between magmatic underplating, trans-crustal CO2 flux, earthquake
swarms and their relation to volcanism and microbiology.
The evolution of the Eger Rift, a 300 km long, 50 km wide, ENE–WSW-trending zone and an active element of the European Cenozoic Rift System
(Prodehl et al., 1995), is connected with the post-orogenic extension
and the alkaline magmatic activity during the Cenozoic. The Cheb
Basin developed at its southwestern end and dips to ∼ 300 m
depth towards the east, where it is delimited by the escarpment of the
Mariánské Lázně Fault. The basement of the Cheb Basin and
its surroundings include the crystalline schists of the Saxothuringian unit and
the granitoids of the Smrčiny Pluton, whereas the sedimentary fill of the
Cheb Basin consists of Paleogene to Quaternary sediments (Kvaček and
Teodoridis, 2007). Quaternary intraplate alkaline volcanism is
documented at the western flank of the Cheb Basin in small volcanoes dated to
0.78–0.12 Ma (Mrlina et al., 2007; Wagner et al., 2002), whereas the
CO2-dominated hydrothermal activity is dated to 0.23–0 Ma (Vylita et al.,
2007). They comprise two Quaternary scoria cones (Železná
hůrka/Eisenbühl and Komorní hůrka/Kammerbühl) and two
volcanic maars (Mýtina and Neualbenreuth), where the eruptions are
associated with phreatomagmatic and phreato-Strombolian activity (Geissler
et al., 2005; Mrlina et al., 2009; Flechsig et al., 2015; Rohrmüller et
al., 2017; Lied et al., 2020). The crustal structure of the area is complex
with a wide zone of increased reflectivity at the crust–mantle transition
(e.g., Tomek et al., 1997; Hrubcová et al., 2005, 2013; Hrubcová and
Geissler, 2009). These lower crustal features may be interpreted as
low-angle shear zones partly filled with fluids and/or small magmatic
intrusions or partial melting as confirmed by mantle xenoliths (Geissler et
al., 2005). In the upper crust, very distinct and highly reflective features
(“bright spots”) were identified (Klemt, 2013; Schimschal, 2013; Mullick
et al., 2015; Hrubcová et al., 2016), which can be related to the
spatial–temporal behavior of seismic swarm activity and its fluid-driven
origin.
The seismicity in the area is characterized by earthquake swarms, intensive
long-lasting and low-magnitude seismicity, which contrasts with more typical
mainshock–aftershock sequences. Such seismicity can be felt by the
population and sometimes causes damage to buildings. It has been recognized in different areas worldwide, such as volcanic and geothermal fields (e.g., Dahm and Brandsdottir, 1997; Wyss et al., 1997; Lees, 1998; Dreger et al., 2000) and at the
margins of tectonic plates (e.g., Einarsson, 1991), and can occur as a
precursor to larger earthquakes, such as during the recent L'Aquila 2009
earthquake in Italy. However, the controlling processes and possible
consequences of this kind of seismicity are still not fully disclosed.
In the western Eger Rift, the highest concentration of earthquake
activity and CO2 degassing occurs in the area of the Cheb Basin, at the
intersection of major tectonic lines with four Quaternary volcanoes. The
ENE–WSW-trending Eger Graben and the N–S-trending earthquake zone
between Vogtland and Leipzig (Grünthal et al., 2019), the
Cheb–Domažlice Graben, and the morphologically expressed
Mariánské Lázně Fault intersect at one location close to the
main seismically active Nový Kostel (NK) zone (Fig. 1). It seems that
the earthquake activity is related to the reactivation of a complex system
of faults and tectonic zones, triggered by the ascent of magmatic fluids caused
by ongoing magmatic process (Fischer et al., 2014; Bräuer et al., 2014)
in which fluid channels to depth (CO2 conduits) play an important role
(Kämpf et al., 2019). However, the understanding of magmatic activity,
fluid ascent and earthquake-stimulating processes as well as their
interconnections are not clear.
Fluid degassing of CO2 in the Cheb Basin and surrounding areas is in
the form of wet and dry mofettes and mineral springs. High portions of
mantle-derived helium and CO2 indicate a magmatic origin and fluid
transport from the depleted lithospheric mantle (Weinlich et al., 1999;
Bräuer et al., 2009), possibly related to active magmatic underplating
(Hrubcová et al., 2017). On their way to the surface, fluids penetrate
through faults, interact with the medium and are related to earthquake
activity (Kämpf et al., 2013, 2019; Nickschick et al., 2015, 2019;
Fischer et al., 2017; Liu et al., 2020). Subsurface life, protected from the
intense radiation in the atmosphere, represents an ambience of the Earth's
early biotopes; on the other hand, the microbial ecosystems abundant in the
subsurface react to changes in the composition of fluids or to their long-term exposure.
Because of these phenomena (and their concentration in a rather small region),
the West Bohemia–Vogtland (the western Eger Rift) area is a unique site
worldwide and offers an ideal possibility for interdisciplinary study.
The earthquake swarms and long-term degassing of mineral-rich waters and
gases in granitic and sedimentary layers makes this area perfectly suited to
study the fluid composition and fluid-induced source processes along with
the effect of CO2 on the deep biosphere and the development of early life at
depth. Long-term monitoring of the smallest signals and trend changes is
essential to understand these phenomena and their interactions. This points
to establishing a new level of multidisciplinary investigation. A modern,
comprehensive, high-resolution observatory at depth with unique
multiparameter observation can interconnect fields of primary research;
advance the interactions among earthquakes, fluids, rocks and the biosphere; and
contribute to answering related questions.
Current state, scientific aims and experimental approach
The seismicity in West Bohemia–Vogtland, also documented in macroseismic
observations, occurs in the form of earthquake swarms, with the largest local
magnitudes not exceeding ML 5 (e.g., 1875 or 1908). The largest
instrumentally recorded earthquake occurred in the 1985–1986 swarm and reached
a magnitude of ML 4.6 (Vavryčuk, 1993). Since 1985,
seismicity has been concentrated in the Nový Kostel (NK) focal zone,
where more than 80 % of seismic energy has been released within the last
30 years (Fischer and Michálek, 2008). The seismicity is generally
shallow, with the event hypocenters occurring in the upper and middle crust,
mainly between 5 and 15 km (Horálek and Fischer, 2008). Since 1997, the
seismicity cluster beneath NK has been slowly but steadily growing,
laterally more to the north and upwards, along a nearly planar structure
(see Fig. 2).
The drilling sites and their prime scientific targets: S1 –
Landwüst, S2 – Tisová, S3 – Studenec (seismological monitoring), S4 – Libá (seismological monitoring, volcanology and the deep biosphere in
volcanic maar crater), F1–3 – Hartoušov (fluid, earthquake and deep-biosphere monitoring) and P1 (a planned borehole for paleoclimate and biosphere
investigation). The time of realization is indicated in parentheses. Red circles
represent the seismicity of the main Nový Kostel (NK) focal zone; blue
circles represent mofettes and mineral springs with a gas discharge of more
than 1 L min-1; Quaternary volcanoes are indicated by black triangles.
The surface earthquake recordings are of excellent quality (e.g., Fischer et al., 2014); however, they suffer from
high-frequency wave damping by the near-surface weathered layers and site
scattering, resulting in a smoothing of the wave signal details. A
monitoring depth of a few hundred meters will avoid high-frequency
attenuation and significantly improve the possibility of studying
low-magnitude earthquakes in the crust and of resolving small-scale
heterogeneities. This offers the possibility of analyzing the fluid-induced
source processes and the anatomy of earthquake swarms and their migration
in unprecedented detail. The fact that earthquake swarms in the Eger Rift
region occur regularly and persistently in known spots of activity with
known radiation patterns offers the opportunity to design and tune a
borehole-based monitoring network for optimized analysis.
Systematic fluid probing and analyses at springs and mofettes in the western
Eger Rift region have been performed for decades (Weise et al., 2001;
Bräuer et al., 2003, 2005a, b, 2008, 2014; Schuessler et al., 2016).
The results of the fluid isotope systematic monitoring have been capable of detecting
trend changes in 3He /4He, which may indicate silent magmatic
intrusions in the lower crust and upper mantle (e.g., Bräuer et al.,
2009; Kämpf et al., 2013). However, the data obtained have not been sufficient
to reach the objectives. The well-studied sites of massive CO2
degassing in mofettes offer the possibility of building a new generation of
continuous real-time fluid monitoring systems at different depth levels. Such monitoring can separate the effects of surface and deep processes related to the
composition and rate of fluids. It can also demonstrate the correlations
between isotope composition and seismic activity (e.g., Bräuer et al.,
2003, 2008, 2014) or reveal the link between an earthquake swarm and microbial
activities (e.g., Bräuer et al., 2008).
Microbiological studies show the existence of diverse and active microbial
ecosystems in the deep subsurface (e.g., Parkes et al., 1994, 2000; Lehman,
2007). This is a vast ambience, as between 75 % and 94 % of all microbes on
the Earth occur in deeply buried marine and terrestrial sediments (Kallmeyer et al., 2012). Moreover, the deep subsurface harbors
a huge carbon reservoir, equivalent to that of all plants on the Earth;
thus, deeply buried microbial communities are very important for driving
carbon and nutrient cycling as well as catalyzing a multitude of reactions among
sediments, rocks and fluids. The Eger Rift area hosts a diverse lithology
of surficial sediments overlying crystalline rocks as well as active
CO2 degassing and high flow rates of mineral-rich fluids and gases
(e.g., methane). The first studies of its mineral water and fluids (Alawi et
al., 2015; Schuessler et al., 2016; Krauze et al., 2017; Liu et al., 2018,
2020) have indicated that the active fault systems of the Eger Rift area can be
classified as a “hot spot” for microbial subsurface life. Microbial
ecosystems abundant in the subsurface may react to changes in the composition of
fluids. Thus, the long-term degassing of mineral-rich waters and gases in
granitic and sedimentary layers makes this area ideally suited to study the
effect of CO2 on the deep biosphere and the development of life at depth
(Bussert et al., 2017). The maar-diatreme volcanos, as paleoconduit
structures, are considered to be important pathways of magmatic fluids to study past activities under conditions in which the first biological molecules and
later the first life forms originated (Schreiber et al., 2012). Thus, the Eger
Rift area provides an environment for geo-microbiological studies and studies on the
origin of deep life.
Such goals can be fulfilled by the development of a modern, comprehensive
laboratory at depth to study the interconnected areas of primary research.
Specifically, such a laboratory comprises the novel concept of 3-D seismic
arrays with a set of shallow boreholes in order to reach a new level of
high-frequency, near-source and multiparameter observation of earthquake
swarms, real-time fluid monitoring at different depths and related
phenomena. Such a network brings a new high detection capability, which
improves the earthquake and fluid recordings. This offers the possibility to
study extremely low-magnitude earthquakes and analyze the fluid-induced
source processes. Repeated fluid probing at the surface can be complemented
by a new generation of continuous real-time fluid monitoring in a safe and
logistically accessible area. The variability in the local geological site
conditions can meet the interdisciplinary targets for volcanologic,
microbiological and paleoclimate research.
This initiative was introduced and discussed by Dahm et al. (2013) during
the second ICDP Eger Rift workshop, resulting in a conceptual drilling
approach to address the key scientific questions related to these processes.
It was discussed among approximately 50 scientists from Germany, the Czech
Republic, the USA, the UK and Poland; from these scientists, three scientific groups were identified
based on their interests: (i) a seismological group, (ii) a fluid group, and
(iii) a group interested in volcanology/petrology, paleoclimate and
microbiology. Although each group is responsible for its field, together they
comprise the unique interdisciplinary laboratory with a potential to better
understand the following:
fluid–rock interactions and the mechanism of fluid-induced earthquake swarms,
the structure of fluid pathways from the upper mantle to the surface,
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 impact of CO2-rich mantle-derived fluids on the geo–bio interaction in the western Eger Rift,
the Quaternary paleoclimate and volcanic activity in the western Eger Rift region.
Description of drillings, monitoring and scientific concepts
The in situ Eger comprehensive laboratory is currently being established by
the International Continental Scientific Drilling Program (ICDP) in the framework of the interdisciplinary project “Drilling the Eger Rift: Magmatic
Fluids Driving the Earthquake Swarms and the Deep Biosphere (EGER)”.
Specifically, this laboratory at depth comprises a set of five new,
distributed, shallow (less than 500 m deep) boreholes (Fig. 2). The
drilling sites were selected to be distributed around the Nový Kostel
(NK) focal zone; geophysical and geological surveys contributed to the
selection of the exact locations. The drill holes are denoted S1–S4
(seismological monitoring) and F1–F3 (fluid monitoring), indicating the primary
field of interest of each well (Fig. 2). The planned drill hole P1 will be
the main record for paleoclimate studies.
The drillings S1–S4 are designed for seismological monitoring in order to reach a new
level of high-frequency, near-source observations of earthquake swarms and
related phenomena, like seismic noise and tremors generated by fluid
movements. The drilling of S1 (Landwüst, depth 402 m), which is the only
drill hole located in German territory, was completed in August 2019 and
is supplemented by a 3-D high-frequency seismic array. The S2 site
(Tisová, depth 460 m) was finished in November 2017, and the S3 site
(Studenec, depth 408 m) was completed in December 2018; both S2 and S3 are
planned with borehole seismic arrays. The drilling of S4 (Libá, depth
400 m) was accomplished in December 2021 in the recently discovered maar
crater near the Czech–German border and will be equipped with a borehole
seismometer.
The drill holes F1–F3 are primary designed for fluid monitoring in the framework
of a multilevel gas monitoring system built in the Hartoušov mofette
field. This mofette represents a gas emission site where CO2 ascends
through crustal-scale conduits from as deep as the upper mantle; thus, the site can
provide a natural window into ongoing magmatic processes at the mantle depth
level. It is located at the crossing of the Eger Rift with the
Počátky–Plesná zone (PPZ) tectonic lineament, which is the fault possibly
related to the main Nový Kostel focal zone (Fig. 1). In particular,
two existing monitoring wells, F1 and F2 (Bussert et al., 2017; Fischer et al.,
2020), were complemented by the F3 drill hole; these three adjacent boreholes,
F1 (30 m), F2 (70 m) and F3 (230 m), provide continuous monitoring of fluids at high sampling rates to acquire fluid parameters (gas flow, water
temperature and water level/pressure) as well as chemical (CO2, Ar, N2,
O2, He, H2 and CH4) and isotopic (δ13CCO2, δ18OCO2 and 222Rn) gas content (Woith et al., 2020).
Additionally, samples for laboratory analysis of He, Ne and Ar isotopes are
taken repeatedly (roughly every 2 months), as theses isotopes are
useful tracers for constraining the fluid origins and mixing ratios of mantle
components. Moreover, the fluid monitoring at different depths separates the
effects of surface and deep processes related to the composition and ascent
rate of fluids. The drill site is also prepared for seismological monitoring
to complement the monitoring network.
All boreholes were cored; the coring in Tertiary–Quaternary sedimentary
sequences (S4, F2 and F3) is utilized for paleoclimate research and
microbiological investigation (Bussert et al., 2017). The coring of solid
rocks outcropping at the surface (S1 and S3, phyllites) is utilized for
structural and tectonic investigation. Moreover, the core of drill hole
S4, located in the maar crater, also targets volcanology and the evaluation of the
neotectonic evolution of the maar. The final supplementary borehole, P1
(∼ 150 m), in another maar volcano near Neualbenreuth, Germany, at
the Czech–German border is planned to support paleoclimate research and
is scheduled for early 2023.
Drilling and specific characteristics of individual drill holesPre-drilling site surveys
Several geophysical experiments were conducted to map deeper and shallow
crustal structure. From
reflection and seismic source data, distinct and highly reflective features (bright
spots) were found in the upper crust close to the main NK focal zone (Mullick et al., 2015; Hrubcová et
al., 2016). Local earthquake tomography showed clear indications of a
mid-crustal intrusive body beneath the NK focal zone from increased P-wave / S-wave (Vp/Vs)
ratios (Alexandrakis et al., 2014; Mousavi et al., 2015). Magnetotelluric investigations found highly conductive channel-like structures above the focal zone (Muñoz et al., 2018) that were complemented by highly attenuating
bodies beneath and north of NK to 11 km depth (Mousavi et al., 2017). All of
these features point to fluid pathways and interconnections between
seismicity and fluid degassing.
The exact positions of the drill holes were investigated by local
geophysical surveying to control the quality of waveforms, the
signal-to-noise ratio (SNR), and to provide structural and geological
constraints for fluid pathways and their movements. This comprised electric
resistivity tomography and high-resolution reflection and refraction
seismic surveying along the resistivity profile, as well as seismic noise
measurements (Umlauft and Korn, 2019) at the Hartoušov site, both
completed in late 2017 (Nickschick et al., 2019).
Drilling and coring
The drill sites are located in a natural mineral spring
and spa resource protection zone and required specific permissions to meet the strict
governmental requirements prior to the commencement of work. Drilling
works were performed by the German drilling company Pruy KG with the
HD110 drilling rig (the S1 site and the pre-drill of F3) and the Czech
drilling company Geoněmec – vrty, s.r.o. with the
Christensen 140C drilling rig (sites S2–S4). The drilling of F3 was conducted within the
Swedish national research infrastructure for scientific drilling (Riksriggen)
at Lund University, Sweden, with the Atlas Copco CT20C drilling rig (crawler
mounted). Due to potential CO2 blowouts in the region, the drillings
were performed through a blowout preventer to overcome the problem of
pressurized CO2 in the drilling shaft; however, none of the sites faced such
issues during drilling. The drillings were conducted under strict site
contamination control conditions; after the termination of works, all sites and access
roads were restored (Fig. 3). Furthermore, the wellheads were secured by a
concrete head casing, and the cased peduncle was secured by a lock (Fig. 4).
The Eger Rift drilling setup, showing (a) the drilling rig with drilling rods at the S3 site, (b) the wellhead after the termination of work at the S3 site and (c) an example of a diamond drill head designed for drilling in hard rocks.
Panel (a) shows the Hartoušov mofette field, and panel (b) presents the F3 borehole in Hartoušov after the completion of drilling works, with the concrete head casing secured by a lock.
Parameters of the drill holes.
The parameters of the drilling sites are summarized in Table 1. Some
innovative approaches were applied during drilling and subsequent logging to
meet the specific requirements of multidisciplinary research. All boreholes
are nearly vertical, reaching depths < 460 m, and they were all
steel cased. After casing, the drillings were cleared (redrilled within the
casing) to ensure clear passage from the head to the bottom, which is necessary
for subsequent successful installation of fluid and seismic monitoring
instrumentation. Seismological drill holes S1, S3 and S4 were cemented to
ensure seismic coupling. The quality of the cementing was controlled by
well logging, and an innovative approach using a fiber-optic cable
was additionally applied to monitor the cementing at S1. Wireline coring was applied
to all boreholes, and the cores were retrieved and organized in wooden boxes (Fig. 5) before being stored (except for S2) at the Research INfrastructure for Geothermal ENergy (RINGEN) center in Litoměřice and the Federal Institute for Geosciences and Natural Resources (BGR) core repository in Spandau, Berlin
(S1). The core of drill hole S2 is not available to the scientific
community, as the drill hole was an in-kind contribution from the
Golden Pet s.r.o. exploration company to the ICDP Eger project.
Panel (a) presents an example of drill cores stored in wooden boxes that have been marked and labeled, showing phyllitic core from S1. Panel (b) presents an example of different lithologies in hard rocks, showing (from top to bottom) granite from S4, granite from S4, basalt from S4 and phyllite from S3.
Well logging was provided for all drilling sites except for S2 (the
in-kind contribution from the Golden Pet s.r.o. exploration company). It
comprised a complex of methods, including borehole geometry, caliper, sonic
logging with full waveforms, and acoustic images to localize cracks,
fractures, and/or tectonic and geological features. Gamma–gamma logging
was applied to sample rock densities, neutron–neutron logging was applied for porosity
and water content, natural gamma logging was used for the detection of unstable isotopes,
and resistivity logging was used for the degree of rock deformation. For the records of
well logging in individual boreholes, see Sect. 5.
Due to the specific requirements of microbiological research, drillings
F3 and S4 were conducted under strict contamination control conditions, following
the approach of Bussert et al. (2017). In the case of F3, the cores were
retrieved in 3 m long polyvinyl chloride (PVC) liners to protect them from biological
contamination; in the case of S4, the cores were encapsulated in aluminum. In both
cases, the microbiological samples were frozen at -80 ∘C and sent for
further analyses.
Seismological monitoring requires the drillings to be distributed and
optimized for detection, location, source mechanism and seismic wave
scattering studies. The technical objectives of being able to analyze
ML>-1 earthquakes (and nonvolcanic tremors) need to be
addressed by taking a step up from the current short-period seismic
monitoring network to a high-frequency 3-D seismic array. Boreholes
S1–S3 involve deploying vertical seismic arrays combined with a surface
small-aperture high-frequency array; a pilot observatory has already been
deployed at S1. Such a configuration allows for detailed high-resolution study (at a
1 kHz sampling rate) of earthquake migration, short-term anomalies in
the beginning phase of swarms, mixed-mode rupture processes, near-source
scatterers, the depth distribution of events and the detection of microearthquakes
along fluid channels. The location in unaltered rocks not affected by fluid
ascent assures the recording of high-frequency signals of the smallest
earthquakes (ML≥-1) and is supported by the pioneering tests of
Hiemer et al. (2012) from a small-aperture array of short-period stations at
the surface (the 6-month eight-sensor test-array deployment near S1 at Rohrbach
borehole V01–V08 detecting microearthquakes from ML>-1.2
from the NK focal zone).
Surface and borehole 3-D high-frequency seismological arrays and the
types of sensors realized at the S1 site. Red color denotes the S1 borehole
with a surface sensor.
Borehole S1 (depth 402 m, inner diameter (i.d.) 92 mm) is located in
Landwüst (Germany) about 10 km northwest of the Nový Kostel (NK)
focal zone in a forest area with basement rocks outcropping at the surface
(metamorphosed Cambrian sediments – phyllites with quartzite layers). The
test array installations and test measurements in the nearby Bad Brambach 80 m hole indicated the appropriate site conditions and excellent
signal-to-noise ratios (SNRs) for weak microearthquakes with a significantly
reduced S-wave damping effect. The S1 instrumentation comprises the ASIR
bottom-hole broadband seismometer and a vertical array of 3C borehole
sensors between 180 and 400 m; additionally, a surface array of 3C seismic
sensors is installed around the borehole (Fig. 6). The fiber-optic cable
was cemented behind the borehole casing to monitor the microearthquakes as
well as the quality of cementing.
Borehole S2 (depth 464 m, i.d. 76 mm) is located near Tisová (Czech
Republic) about 15 km north of the Nový Kostel (NK) focal zone in weakly
metamorphosed Ordovician sediments (phyllites). The thickness of the
overlying weathering products (debris with rock fragments) did not exceed
several meters. The noise characterization from a test array deployed
indicates good SNR conditions. The instrumentation, array design and
configuration are planned to be the same as for borehole S1, except for the
fiber-optic cable.
Borehole S3 (depth 400 m, i.d. 76 mm) is located in Studenec (Czech Republic)
about 7 km northeast of the Nový Kostel (NK) focal zone in a dynamic
landscape with a minimally weathered uppermost crust on metamorphosed
Cambrian sediments (phyllites). The site is in a remote area with a good SNR; this is
confirmed by long-term monitoring, as the site also coincides with one
seismic station (STC) of the WEBNET surface monitoring network (Horálek
et al., 2000). Moreover, this coincidence provides the opportunity to compare
results from both networks and use the existing operation hut. A
Güralp Radian bottom-hole seismometer (Güralp Co.) is currently installed
and tested at the hole bottom.
Maar drilling – seismic monitoring, paleovolcanic and microbiological research
Borehole S4 (depth 406 m, i.d. 77 mm) is located in Bažina near Libá
(Czech Republic), about 17.5 km southwest of the Nový Kostel (NK) focal
zone, in a newly discovered volcanic maar structure penetrating the surrounding
granitic rocks. The borehole sits in a conic maar crater and penetrates the
alternating Quaternary siliciclastic and highly organic sediments of the
crater. At 60 m depth, it reaches the basaltoids inside the crater; at a
depth of 170 m, it reaches the contact of the crater with the surrounding
granitoids (the host rocks), which continue until the bottom at 400 m depth. Because of its
uppermost volcanic character, borehole S4 provides the record for
combined paleovolcanic, magmatic, paleoclimate and deep-biosphere studies.
As a detailed geophysical pre-site survey indicated good SNR
conditions in granites, a bottom-hole seismometer is planned to be deployed
in the solid granitic rocks at the bottom of the hole, along with a surface
reference station.
Fluid and seismic monitoring and microbiological research – continuous sampling at different depths
Boreholes F1–F3 are located in the Hartoušov mofette field (Figs. 4, 7)
in the Tertiary–Quaternary sedimentary successions of the Cheb Basin. The
site appears well suited to exploring the relation between the swarm seismicity
and CO2 degassing, as a massive coseismic increase in CO2 release
has been observed here twice – in the case of the 2008 and 2014
earthquake swarms (Fischer et al., 2017). Three adjacent boreholes, F1 (depth
28 m, i.d. 115 mm), F2 (depth 108 m, i.d. 100 mm) and F3 (depth 239 m, i.d. 78 mm),
supplemented by measurements in the nearby mofette (Fig. 7) allow
continuous fluid monitoring at different depth levels within the basin
sediments or in the CO2-permeated weathered crystalline basement (F3).
The site survey comprised seismometers and a weather station installed
on-site in order to quantify the impact of earthquakes as well as the
environmental effects on the fluid regime. The multilevel gas monitoring
system is being installed at three wells tapping the CO2 horizons at
20, 65 and 229 m. Continuous radon measurements while drilling revealed a
promising CO2 horizon, which was later chosen for perforations of the
steel casing. Further hydraulic tests at F3 are needed to confirm whether
the perforation was successful.
Fluid and seismic monitoring in the Hartoušov mofette field (after
Woith et al., 2020), showing boreholes F1–F3 and a mofette site with the sensors and
types of monitoring. BB denotes a broadband seismometer (borehole and/or surface), and
SP represents a short-period 4.5 Hz 3C borehole sensor.
The F2 borehole already hosts the ASIR broadband SiA seismometer at 70 m
depth. Ultimately, a borehole seismometer will be installed at the bottom of
F3 and will be complemented by a capillary tube to collect “fresh” gases from the
CO2 horizon at depth, directly at the point where the fluids enter the
borehole to avoid possible contamination or impact from external
processes. Further details on the instrumentation of this mofette field with
massive CO2 degassing (up to 97 t d-1) as well as the first monitoring results are summarized in Fischer et al. (2020), Woith et
al. (2020) and Daskalopoulou et al. (2021). Once the novel monitoring
system is fully operational, fluid transients will be able to be observed in great
detail. We expect new insights into the physical processes that control the
complex interplay between earthquakes, deep degassing and permeability
variations along the path to the surface.
Seismic monitoring in F1–F3, located about 9 km south of the Nový Kostel (NK) focal zone (Fig. 2), complements the network of shallow boreholes. Borehole F2 (108.5 m depth) is equipped with a broadband borehole seismometer at 70 m depth; a similar sensor will be installed at the bottom of F3 (239 m depth). Moreover, a broadband surface seismic station is installed at F1 (Fig. 7).
The microbiological investigation at the Hartoušov mofette field was
accomplished at the F2 and F3 drill holes. The pilot hole, F2 (HJB-1), was drilled
in spring 2016, after extensive pre-drill surveys to optimize the well
location (Bussert et al., 2017). The drilling through a thin caprock-like
structure triggered a CO2 blowout, indicating a CO2 pathway. Pumping tests revealed a Na–Ca–HCO3–SO4-type groundwater with a
total mineralization of 5870 mg L-1, which is typical for the mineral waters
of Františkovy Lázně Spa that is located about 8 km to the west of the
drill site (Bussert et al., 2017). The first microbiological investigations
included activity tests for microbial methane production, DNA extractions
and cultivation experiments as well as testing of the microbial DNA extracted
from samples (Bussert et al., 2017). These steps were supplemented by the
investigation of the F3 borehole which involved testing for microbiological life on samples
from different depths. Further analyses of F3 microbial samples are ongoing.
Drilling for paleoclimate and microbiological research
Borehole P1 is planned in the Neualbenreuth Quaternary maar structure (in
Germany, at a site located about 3 km
southeast of the Mýtina maar) down to ∼ 150 m depth. It will penetrate a succession of lake sediments
of at least 100 m depth with varying lithologies, surrounded by
Paleozoic metamorphic rocks and underlain by the diatreme. Borehole P1 will
be the main record for paleoclimate studies, as it will overlap an existing
100 m drill core obtained in 2015 (Rohrmüller et al., 2017), thereby
enabling the development of continuous time series. Apart from paleoclimate,
the site will be exploited for microbiological and deep-biosphere studies. Due
to groundwater protection issues, the well has to be closed after drilling.
The geophysical pre-site survey of maar structures indicates that shallow
sedimentary successions of maar craters are not suitable for high-frequency
seismic monitoring; thus, the deployment of a seismic sensor in this
borehole is questionable.
Lithologies and well logging of boreholes
The downhole logging measurements for the Eger Rift project have been
performed in four of five boreholes (S1 Landwüst, S3 Studenec, S4
Libá and F3 Hartoušov). In this paper, we present the results of
three seismological boreholes: S1 Landwüst, well logged by BLM Company in
2019 (Fig. 8); S3 Studenec, well logged by Aquatest in 2018 (Fig. 9); and
S2 Tisová, which has not been well logged and only a stratigraphy profile is
provided (Fig. 10). The following downhole logging measurements have been
acquired: gamma ray, neutron–neutron, density, resistivity, temperature, P-
and S-wave velocity, focus electrical resistivity (FEL), electrical
conductivity, caliper, borehole deviation, and borehole azimuth direction.
All logging measurements have been depth matched using the gamma ray as the
reference logging present in all sondes. As the other two boreholes, S4
and F3, are the subject of further focused research, their profiles are presented in individual studies. Moreover, all records of stratigraphy and
well logging in boreholes S1–S4 and F3 are depicted in Fischer et al. (2022) and in the upcoming Operational Report.
Logging data of the S1 Landwüst borehole and a stratigraphy profile based on the initial core description. The abbreviations used in the figure are as follows: Neutron neutr. – neutron–neutron; WU –
water unit; Electr. cond. – electrical conductivity; FEL – focus electrical
resistivity; Vp and Vs – P- and S-wave velocity, respectively.
Logging data of the S3 Studenec borehole and a stratigraphy profile
based on the initial core description. The abbreviations used in the figure are as follows: TVD – total vertical depth; Resist. –
resistivity; Vp and Vs – P- and S-wave velocity, respectively.
Stratigraphy profile based on the initial core description of
borehole S2 near Tisová in weakly metamorphosed Ordovician sediments
(phyllites).
The S1 Landwüst borehole (Fig. 8) was drilled to 402 m depth and
shows a southward deflection with a small deviation up to 5 ∘ at
the bottom. The borehole encounters monotone silt–phyllite rocks with
different stages of fracturing. All logs show a constant trend except at two
main fractured intervals: one zone was identified from around 12.0 to 57.8 m,
and the second zone was identified between 134 and 165 m. In these fractured zones, gamma
ray, FEL, density, neutron–neutron, P- and S-wave velocity, and caliper logs
show abrupt changes. In particular, in the second fracture interval, the
electrical conductivity shows constant values of around 0.2 µS cm-1 up to 115 m that slightly increase from 0.2 to 1.8 µS cm-1 between 115 and 140 m. The fracturing becomes more intense between 140 and 155 m, where the electrical conductivity reaches the maximum values of around 1.8 µS cm-1; the values
then gradually decrease to 0.7 µS cm-1 at 165 m before reaching a constant value of 0.2 µS cm-1 at the bottom. These two main fracture zones were also
detected by the caliper, which indicates an opening from 155 to 200 mm.
The temperature in the borehole increases continuously from 7 ∘C
at 25 m to 13.5 ∘C at 400 m depth.
The S3 Studenec borehole (Fig. 9) reached a total depth of 402.5 m,
with the deviation increasing smoothly up to 9 ∘ at the bottom and
with deflection to the southeast. The borehole encounters a sequence of
phyllite rocks up to 221 m, and a quartzitic phyllite formation is then
predominant in the bottom part. The variation in lithology along the
borehole is well recorded by the gamma ray log. The gamma ray shows average
values of 50 µR h-1 for the phyllite rock formation, mainly related to the higher content of clay minerals compared with the quartzitic phyllite formation which shows average values around 39 µR h-1. A slight enrichment of
the clay component is observed between 330 and 402.5 m. Around 139–141 m, the borehole encountered a tectonic fault that fragmented the rock of the borehole wall and caused the collapse of the section between 140 and 142.5 m. Only
downhole logging measurements with nuclear sources (such as neutron–neutron
and density measurements) have been run inside the pipe from 0 m to the bottom of the hole,
whereas P- and S-wave velocity, caliper and resistivity logs were provided
only in shallow part between 30 and 140 m. Several fracture intervals have
been identified around 59, 71.0, 100, 111 and 135 m. They are
detected by resistivity, P-wave velocity and gamma ray logs, showing a
decrease in values compared with the general trend, whereas the caliper records
show an increase in the hole size (spikes).
Temperature measurements and the heat flow
Due to the persistent tectonic activity and the existence of thermal
springs, places with increased heat flux (Čermák, 1994) can be
expected in the area. However, due to the absence of deeper boreholes
penetrating sedimentary successions of the Cheb Basin to the basement
crystalline rocks, detailed information is missing. For this reason, precise
temperature well logging was performed in all boreholes (Fig. 11). The
temperature recordings allowed for the determination of the temperature gradients,
and topographic corrections were subsequently calculated for each well
using 3-D numerical models. To determine the heat flow density, knowledge of
the thermal conductivity of the rocks intersected by the boreholes was
necessary. The thermal properties (thermal conductivity, thermal diffusivity
and volumetric heat capacity) were measured using a high-resolution optical
scanning method on several samples from each borehole. The anisotropy of the
thermal properties was also assessed, which turned out to be very
significant especially in the case of boreholes (S1, S2 and S3) intersecting
the metamorphic rocks. Based on the above, we calculated the heat flow,
which ranged from 75 mW m-2 for borehole S2 near Tisová to more than 100 mW m-2 for S1 in Landwüst.
Temperature logs of the individual boreholes.
Seismic monitoring concept – the 3-D high-frequency seismic array at the S1 site
The 3-D high-frequency seismic array at the S1 site has been accomplished as a pilot
array for the seismic part of the ICDP Eger Rift project. The S1
instrumentation comprises the ASIR bottom-hole broadband seismometer and a
vertical array of 3C borehole sensors between 180 and 400 m; additionally, a
surface array of 3C seismic sensors is installed (Fig. 6). A fiber-optic
cable was cemented outside of the borehole casing to monitor the
microearthquakes and will provide comparison with the borehole chain.
Moreover, it was also used to check the quality of the casing cementation.
Borehole array
The downhole array consists of two major parts: a 10-level high-frequency
borehole chain and one additional bottom-hole seismometer. The borehole
chain is assembled with high-frequency geophones for one vertical and two
horizontal directions at 10 depth levels. We identified 10 Hz HG-7 geophones
as the optimum sensors (little or no tilt sensitivity, high spurious and low
corner frequencies and, thus, a large usable bandwidth). In order to increase
the output voltage, the geophones are in sets of two sensors connected in
series for each direction and depth level, which results in a total of 30
analogue channels to be handled. The analogue signals are converted by five
six-channel Earth Data EDR-209 digitizers at a rate of (up to) 1 kHz. The
bottom-hole equipment consists of a newly developed ASIR SiA broadband seismometer
with a passband of 200 s to 1.5 kHz bundled with a 3C 4.5 Hz
geophone. The analogue signals are converted by one six-channel Earth Data EDR-209 digitizer at a rate of (up to) 1 kHz. The example of waveforms of an
ML 0.1 microearthquake recorded with a borehole chain at S1
Landwüst is presented in Fig. 12.
Example of microearthquake waveforms recorded with an 8-level
borehole chain at S1 Landwüst (test installation before the deployment
of the final 10-level chain) for the event on 23 January 2020, 22:21:11 UTC at 8 km depth, ML 0.1, at an epicentral distance of 11 km (near Luby). Note the good
signal-to-noise ratios (SNRs) of the recordings and their improvement with
depth. The red box in panel (a) indicates the zoomed area in panel (b).
Surface array
A small-aperture array of short-period stations at the surface consists of
three rings around a central borehole station. The aperture of the first
ring is 10 m, and it comprises three 3 m deep postholes that are not yet fully equipped.
The second ring has an aperture of 100 m and comprises five sensors. Ring 3 is
equipped with seven sensors with an aperture of 400 m. The surface sensors are
positioned in shallow (50 cm deep) holes and powered by solar panels. The
sensors of rings 2 and 3 are of the same type (4.5 Hz HG-6 geophones with a
gain of 27.7 Vs m-1). The sampling rate of 1 kHz is used with seismological data loggers (EDR-209) and near-real-time transmission of the data. The data are transmitted via mobile connection.
Fiber-optic cable – the quality of cementing
After drilling S1, a fiber-optic cable was installed behind the casing
down to a depth of 397.5 m on the western side of the well. The fiber-optic cable had a tight buffer design. It contained one single-mode optical fiber
(ITU-T G.652.D/657.A1) preserved in a stainless-steel metal tube and embedded in a polyamide sheath with a structured surface (outer diameter 3.2 mm) to increase the mechanical coupling between cement and cable. The cable was attached to the casing using cable ties and tape. Two fixations were used per tube joint. To improve the cement deposition and reduce the risk of micro-channels between cable and cement, 4 mm separators were installed between the cable and the casing. Figure 13a shows the fiber-optic cable (and its
fixing with separators) prepared for the deployment in the well.
Panel (a) shows the fiber-optic cable (blue) fixed at the 4.5 in. (115 mm) borehole casing of S1. Blue separators can be seen beneath the tape to improve the cable coupling. Panel (b) presents the fiber-optic distributed strain rate measurements during cementing of the 4.5 in. (115 mm) casing. Panel (c) provides a sketch of cementing (flow indicated by arrows); the cement suspension (green) and spacer fluid (blue) are schematically depicted for the time of 26 min.
To verify cable integrity during installation, optical time-domain
reflectometry was performed after cable installation and prior to
cementation. Afterwards, the well cementation was performed to ensure the
best possible coupling of the casing and the surrounding rocks. The quality of cementation was controlled by the distributed fiber-optic strain measurements along the cable with an optical backscatter reflectometer (Luna OBR 4400) previously used in borehole applications (e.g., Lipus et al., 2018). The OBR interrogator is based on the optical frequency domain
reflectometry (OFDR) principle, following Froggatt and Moore (1998). The
measurement resulted in a strain section recorded every 60 s over a time span
of 2 h during the cement pumping. An example of such measurement is depicted
in Fig. 13b, showing the strain change between two successive measurements.
Data management
The ICDP Eger project comprises a large number of continuously operating
sensors for recording seismic, gas-related and meteorological data streams.
In future, additional sensors also are planned for the detection of infrasonic
and/or rotational seismic signals. As the project represents a long-term
monitoring effort and shall be operated for at least 10–15 years, a vast
amount of data has to be considered.
The major part of these data is represented by seismic monitoring at the S1–S4
and F1–F3 sites, where the 3-D arrays at the S1–S3 sites play a particularly
crucial role. Each of these sites, equipped with 25
geophones/seismometers (3C instruments in all cases) with both borehole (9 geophones and 1
seismometer) and surface installations (15 geophones), produces a data
stream of 75 seismic channels. With the continuous sampling rate of 1 kHz,
the amount of data produced at each 3-D 3C array installation at S1–S3 will
be in the order of 16 GB d-1 (75 channels × 1000 samples / (channel × s) × 86 400 s d-1× 2.5 B per sample = 16.2 GB d-1). This sums to 50 GB d-1 (including S4), i.e., ∼ 1.5 TB every month accumulating to
180 TB over a 10-year operating period. Data streams will be transmitted to
Potsdam and/or partner institutions in Prague and Leipzig via 4G
mobile phone standard to an open VPN server with SeedLink. The amount of
data produced from gas sensors and meteorological data will be much smaller
and can safely be estimated to be in the order of 1 % of the seismic
data.
Summary
The new in situ geodynamic laboratory established in the framework of the ICDP
Eger project involves five borehole sites aimed at long-term monitoring of
seismic activity and CO2 degassing in the West Bohemia–Vogtland
geodynamic region (western Eger Rift). At each of these sites, a borehole
broadband seismometer will be installed, and sites S1, S2 and S3 will
also host a 3-D seismic array composed of a vertical geophone chain and
surface seismic array. To date, all of the monitoring boreholes have been drilled.
This includes the seismic monitoring boreholes S1, S2 and S3 in the
crystalline units north and east of the major seismogenic zone of Nový
Kostel (NK), borehole F3 in the Hartoušov mofette field south of NK
and borehole S4 in the newly discovered Bažina maar near Libá
west of NK. An additional borehole, P1, is being prepared in the Neualbenreuth
maar, Germany, and is aimed at paleoclimate research. Seismic instrumenting has been
completed in the S1 borehole and is under preparation at the four remaining
monitoring borehole sites (S2, S3, S4 and F3). The continuous fluid
monitoring site Hartoušov includes three boreholes, F1, F2 and F3, and a
pilot monitoring phase is underway.
It is expected that the Eger Rift laboratory will result in an increasing
sensitivity and discrimination capability with respect to seismic monitoring. Thanks
to fluid sampling at different depths, which removes near-surface
contamination, it will allow for enhanced monitoring of ongoing deep
magmatic processes. This will improve our understanding of the relationships
among mofette degassing, gas composition and swarms. The Eger Rift borehole
laboratory also enables one to analyze microbial activity at CO2 mofettes
and maar structures in the context of changes in habitats. Last but not least,
drillings into the maar volcanoes contribute to a better understanding of the
Quaternary paleoclimate and volcanic activity.
Data availability
After a 3-year embargo period, beginning on the date of sampling party, the downhole logging and stratigraphy data will be available at the ICDP repository database (https://nextcloud.gfz-potsdam.de, last access: 7 June 2022; Fischer et al., 2022).
Author contributions
TF, TD, PH and HW designed, planned and executed the drillings and raised funding. TV executed the drillings and the coring. JH and JeK designed the drillings. MO, JV, PD, HK, MZ, MPL, FK, KH, TR, JaK, DV and KD performed the measurements. PH, TF, TD and HW wrote the manuscript, SP wrote the well-logging section, and all authors reviewed and edited the manuscript.
Competing interests
The contact author has declared that neither they nor their co-authors have any competing interests.
Disclaimer
Publisher’s note: Copernicus Publications remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
Acknowledgements
The ICDP Eger Rift project was funded by the International Continental
Scientific Drilling Program (ICDP); the Deutsche Forschungsgemeinschaft
(DFG, German Research Foundation) – project nos. 419880416, 419459207 and
419909358; the Czech Science Foundation; GFZ Potsdam; a large project of the Czech infrastructure CzechGeo (CZ.02.1.01/0.0/0.0/16_013/0001800);
and CzechGeo/EPOS (LM2015079). We also received co-funding from the Swedish
National Research infrastructure for scientific drilling (Riksriggen) at Lund
University, Sweden. The in-kind contribution of S2 drilling by the
Golden Pet s.r.o. exploration company is gratefully acknowledged.
For technical support before and during drilling, we are grateful to Ulrich
Harms, Thomas Wiersberg, Santiago Aldaz and Jochem Kück of the ICDP
Operational Support Group. We thank the international team of
drillers from Geoněmec – vrty, s.r.o., Protek Norr AB and PRUY KG, with
special thanks to František Kalenda and Milan Němec (Czech
Republic), Danilo Pruy (Germany), Johan Kullenberg (Sweden) and
Friðfinnur K. Danielsson (Iceland). We thank also Ralf Bauz (Germany)
for support during the drilling of S1 and F3.
We acknowledge Christian Cunow and Marius Isken for support during the S1
field and cable installations, and we are grateful to Marius Kriegerowski and Henning Lilienkamp for seismic noise measurements. Funding for the sensors and equipment for the S1 surface array was provided by the Saxionan State Office for Agriculture, Environment and Geology
and the University of Leipzig. Special thanks go to LIAG (Germany) for
well logging of F3 and Aquatest/SG Geotechnika (Czech Republic) for
well logging of the other sites.
We also thank the editor Tomoaki Morishita, Ulrich Harms (the editor in chief) and the two anonymous reviewers for their comments that improved the paper.
Financial support
This research has been supported by the International Continental Scientific Drilling Program (ICDP, project no. 5008); the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation, project nos. 419880416, 419459207 and 419909358); the Czech Science Foundation; GFZ Potsdam; a large project of the Czech infrastructure CzechGeo (grant no. CZ.02.1.01/0.0/0.0/16_013/0001800); and CzechGeo/EPOS (no. LM2015079). We also received co-funding from the Swedish National Research infrastructure for scientific drilling (Riksriggen) at Lund University, Sweden, and an in-kind contribution of S2 drilling from the Golden Pet s.r.o. exploration company.
Review statement
This paper was edited by Tomoaki Morishita and reviewed by two anonymous referees.
ReferencesAlawi, M., Nickschick, T., and Kämpf, H.: Mikrobiologische Prozesse in
CO2-Aufstiegskanälen, System Erde, 5, 28–33, 10.2312/GFZ.syserde.05.01.5, 2015.Alexandrakis, C., Calò, M., Bouchaala, F., and Vavryčuk, V.: Velocity structure and the role of fluids in the West Bohemia Seismic Zone, Solid Earth, 5, 863–872, 10.5194/se-5-863-2014, 2014.Bräuer, K., Kämpf, H., Strauch, G., and Weise, S. M.: Isotopic
evidence (3He /4He, 13CCO2) of fluid triggered intraplate seismicity, J. Geophys. Res., 108, 2070, 10.1029/2002JB002077, 2003.Bräuer, K., Kämpf, H., Faber, E., Koch, U., Nitzsche, H.-M., and
Strauch, G.: Seismically triggered microbial methane production relating to
the Vogtland NW Bohemia earthquake swarm period 2000, Central Europe, Geochem. J., 39, 441–450, 10.2343/geochemj.39.441, 2005a.Bräuer, K., Kämpf, H., Niedermann, S. and Strauch, G.: Evidence for
ascending upper mantle-derived melt beneath the Cheb basin, central Europe, Geophys. Res. Lett., 32, L08303, 10.1029/2004GL022205, 2005b.Bräuer, K., Kämpf, H., Niedermann, S., Strauch, G., and Tesař,
J.: The natural laboratory NW Bohemia – Comprehensive fluid studies between
1992 and 2005 used to trace geodynamic processes, Geochem. Geophy. Geosy., 9, Q04018, 10.1029/2007GC001921, 2008.Bräuer, K., Kämpf, H., and Strauch, G.: Earthquake swarms in
non-volcanic regions: What fluids have to say, Geophys. Res. Lett., 36, L17309, 10.1029/2009GL039615, 2009.Bräuer, K., Kämpf, H., and Strauch, G.: Seismically triggered
anomalies in the isotope signatures of mantle-derived gases detected at
degassing sites along two neighbouring faults in NW Bohemia, Central Europe, J. Geophys. Res.-Sol. Ea., 119, 5613–5632, 10.1002/2014JB011044, 2014.Bussert, R., Kämpf, H., Flechsig, C., Hesse, K., Nickschick, T., Liu, Q., Umlauft, J., Vylita, T., Wagner, D., Wonik, T., Flores, H. E., and Alawi, M.: Drilling into an active mofette: pilot-hole study of the impact of CO2-rich mantle-derived fluids on the geo–bio interaction in the western Eger Rift (Czech Republic), Sci. Dril., 23, 13–27, 10.5194/sd-23-13-2017, 2017.
Čermák, V.: Results of heat flow studies in Czechoslovakia, in
Crustal structure of the Bohemian Massif and the West Carpathians, in:
Exploration of the Deep Continental Crust, edited by: Bucha, V. and
Blížkovský, M., Berlin, Springer-Verlag, 85–120, ISBN 3540579869, 1994.
Dahm, T. and Brandsdottir, B.: Moment tensors of micro-earthquakes from the
Eyjafjallajökull volcano in South Iceland, Geophys. J. Int., 130, 183–192, 1997.Dahm, T., Hrubcová, P., Fischer, T., Horálek, J., Korn, M., Buske, S., and Wagner, D.: Eger Rift ICDP: an observatory for study of non-volcanic, mid-crustal earthquake swarms and accompanying phenomena, Sci. Dril., 16, 93–99, 10.5194/sd-16-93-2013, 2013.Daskalopoulou, K., Woith, H., Zimmer, M., Niedermann, S., Barth, J. A. C.,
Frank, A. H., Vieth-Hillebrand, A., Vlček, J., Bağ, C. D., and Bauz,
R.: Insight into Hartoušov Mofette, Czech Republic: Tales by the Fluids, Front. Earth Sci., 9, 615766, 10.3389/feart.2021.615766, 2021.
Dreger, D. S., Tkalcic, H., and Jonston, M.: Dilational processes
accompanying earthquakes in the Long Valley Caldera, Science, 288, 122–125, 2000.
Einarsson, P.: Earthquakes and present-day tectonism in Iceland, Tectonophysics, 189, 261–279, 1991.Fischer, T. and Michálek, J.: Post 2000-swarm microearthquake activity
in the principal focal zone of West Bohemia/Vogtland: space-time
distribution and waveform similarity analysis, Stud. Geophys. Geod., 52, 493–511, 10.1007/s11200-008-0034-y, 2008.Fischer, T., Horálek, J., Hrubcová, P., Vavryčuk, V.,
Bräuer, K., and Kämpf, H.: Intra-continental earthquake swarms in
West-Bohemia and Vogtland: A review, Tectonophysics, 611, 1–27, 10.1016/j.tecto.2013.11.001, 2014.Fischer, T., Matyska, C., and Heinicke, J.: Earthquake-enhanced permeability
– evidence from carbon dioxide release following the ML 3.5 earthquake in
West Bohemia, Earth Planet. Sci. Lett., 460, 60–67, 10.1016/j.epsl.2016.12.001, 2017.Fischer, T., Vlček, J., and Lanzendörfer, M.: Monitoring crustal CO2 flow: methods and their applications to the mofettes in West Bohemia, Solid Earth, 11, 983–998, 10.5194/se-11-983-2020, 2020.Fischer, T., Hrubcová, P., Dahm, T., Woith, H., Vylita, T., Ohrnberger, M., Vlček, J., Horálek, J., Dedeček, P., Zimmer, M., Lipus, M. P., Pierdominici, S., Kallmeyer, J., Krüger, F., Hannemann, K., Korn, M., Kämpf, H., Reinsch, T., Klicpera, J., Vollmer, D., and Daskalopoulou, K.: ICDP Drilling of the Eger Rift Observatory: Operational Data Sets, GFZ Data Services [data set], 10.5880/ICDP.5008.001, 2022.Flechsig, C., Heinicke, J., Mrlina, J., Kämpf, H., Nickschick, T.,
Schmidt, A., Bayer, T., Günther, T., Rücker, C., Seidel, E., and
Seidl, M.: Integrated geophysical and geological methods to investigate the
inner and outer structures of the Quaternary Mýtina maar (W-Bohemia,
Czech Republic), Int. J. Earth Sci., 104, 2087–2105, 10.1007/s00531-014-1136-0, 2015.Froggatt, M. and Moore, J.: High-spatial-resolution distributed strain measurement in optical fiber with Rayleigh scatter, Appl. Optics, 37, 1735–1740, 10.1364/AO.37.001735, 1998.Geissler, W. H., Kämpf, H., Kind, R., Klinge, K., Plenefisch, T.,
Horálek, J., Zedník, J., and Nehybka, V.: Seismic structure and
location of a CO2 source in the upper mantle of the western Eger
(Ohře) Rift, central Europe, Tectonics, 24, TC5001, 10.1029/2004TC001672, 2005.Grünthal, G., Stromeyer, D., Bosse, C., Cotton, F., and Bindi, D.:
Erdbebengefährdung Deutschlands – neu bewertet für aktuelle
Baunorm, System Erde, 9, 1, 26–31, 10.2312/GFZ.syserde.09.01.4, 2019.Hiemer, S., Roessler, D., and Scherbaum, F.: Monitoring the West Bohemian
earthquake swarm in 2008/2009 by a temporal small-aperture seismic array, J. Seismol., 16, 169–182, 10.1007/s10950-011-9256-5, 2012.Horálek, J. and Fischer, T.: Role of crustal fluids in triggering the
West Bohemia/Vogtland earthquake swarms: just what we know (a review), Stud. Geophys. Geod., 52, 455–478, 10.1007/s11200-008-0032-0, 2008.Horálek, J., Fischer, T., Boušková, A., and Jedlička, P.:
The Western Bohemia/Vogtland Region in the light of the Webnet network, Stud. Geophys. Geod., 44, 107–125, 10.1023/A:1022198406514, 2000.Hrubcová, P. and Geissler, W. H.: The crust-mantle transition and the
Moho beneath the Vogtland/West Bohemian region in the light of different
seismic methods, Stud. Geophys. Geod., 53, 275–294, 10.1007/s11200-009-0018-6, 2009.Hrubcová, P., Środa, P., Špičák, A., Guterch, A., Grad,
M., Keller, G.R., Brückl, E., and Thybo, H.: Crustal and uppermost
mantle structure of the Bohemian Massif based on CELEBRATION 2000 data, J. Geophys. Res., 110, B11305, 10.1029/2004JB003080, 2005.Hrubcová, P., Vavryčuk, V., Boušková, A., and Horálek,
J.: Moho depth determination from waveforms of microearthquakes in the West
Bohemia/Vogtland swarm area, J. Geophys. Res.-Sol. Ea., 118, 120–137, 10.1029/2012JB009360, 2013.Hrubcová, P., Vavryčuk, V., Boušková, A., and Bohnhoff, M.:
Shallow crustal discontinuities inferred from waveforms of microearthquakes:
Method and application to KTB Drill Site and West Bohemia Swarm Area, J. Geophys. Res.-Sol. Ea., 121, 881–902, 10.1002/2015JB012548, 2016.Hrubcová, P., Geissler, W. H., Bräuer, K., Vavryčuk, V., Tomek,
Č., and Kämpf, H.: Active magmatic underplating in western Eger
Rift, Central Europe, Tectonics, 36, 2846–2862, 10.1002/2017TC004710,
2017.
Kallmeyer, J., Pockalny, R., Adhikari, R. R., Smith, D. C., and D'Hondt, S.: Global distribution of microbial abundance and biomass in subseafloor sediment, P. Natl. Acad. Sci. USA, 109, 16213–16216, 2012.Kämpf, H., Bräuer, K., Schumann, J., Hahne, K., and Strauch, G.:
CO2 discharge in an active, non-volcanic continental rift area (Czech
Republic): characterisation (δ13C, 3He /4He) and
quantification of diffuse and vent CO2 emissions, Chem. Geol., 339, 71–83, 10.1016/j.chemgeo.2012.08.005, 2013.Kämpf, H., Broge, A. S., Marzban, P., Allahbakhshi, M., and Nickschick,
T.: Nonvolcanic carbon dioxide emission at continental rifts: the Bublak
Mofette Area, Western Eger Rift, Czech Republic, Geofuids, 2019, 1–19, 10.1155/2019/4852706, 2019.
Klemt, C.: Seismic imaging of the crustal structure in the central European
Variscan orogen by reprocessing of the deep seismic reflection profiles
GRANU9501 und GRANU9502, MS thesis, TU Bergakademie, Freiberg, 2013.Krauze, P., Kämpf, H., Horn, F., Liu, Q., Voropaev, A., Wagner, D., and
Alawi, M.: Microbiological and geochemical survey of CO2-dominated
mofette and mineral waters of the Cheb Basin, Czech Republic, Front. Microbiol., 8, 2446, 10.3389/fmicb.2017.02446, 2017.
Kvaček, Z. and Teodoridis, V.: Tertiary macrofloras of the Bohemian
Massif: a review with correlations within Boreal and Central Europe, Bull. Geosci., 82, 383–408, 2007.
Lees, J. M.: Multiplet analysis at Coso geothermal, Bull. Seismol. Soc. Am., 88, 1127–1143, 1998.Lehman, R. M.: Microbial distribution and their potential controlling factors
in terrestrial subsurface environments, in: The spatial distribution of
microbes in the environment, edited by: Franklin, R. B. and Mills, A. L., Springer, 135–178, 10.1007/978-1-4020-6216-2, 2007.Lied, P., Kontny, A., Nowaczyk, N., Mrlina, J., and Kämpf, H.: Cooling
rates of pyroclastic deposits inferred from mineral magnetic investigations:
a case study from the Pleistocene Mýtina Maar (Czech Republic), Int. J. Earth Sci., 109, 1707–1725, 10.1007/s00531-020-01865-1, 2020.Lipus, M. P., Reinsch, T., Schmidt-Hattenberger, C., Henninges, J., and Reich, M.: Gravel pack monitoring with a strain sensing fiber optic cable, Oil Gas-Eur. Mag., 44, 179–185, 10.19225/181202, 2018.Liu, Q., Kämpf, H., Bussert, R., Krauze, P., Horn, F., Nickschick, T.,
Plessen, B., Wagner, D., and Alawi, M.: Influence of CO2 degassing on
the microbial community in a dry mofette field in Hartoušov, Czech
Republic (Western Eger Rift), Front. Microbiol., 9, 2787, 10.3389/fmicb.2018.02787, 2018.Liu, Q., Adler, K., Lipu, D., Kämpf, H., Bussert, R., Plessen, B.,
Schulz, H.-M., Krauze, P., Horn, F., Wagner, D., Mangelsdorf, K., and
Alawi, M.: Microbial Signatures in Deep CO2-Saturated Miocene Sediments
of the Active Hartoušov Mofette System (NW Czech Republic), Front. Microbiol., 11, 543260, 10.3389/fmicb.2020.543260, 2020.Mousavi, S., Bauer, K., Korn, M., and Hejrani, B.: Seismic tomography
reveals a mid-crustal intrusive body, fluid pathways and their relation to
the earthquake swarms in West Bohemia/Vogtland, Geophys. J. Int., 203, 1113–1127, 10.1093/gji/ggv338, 2015.Mousavi, S., Haberland, C., Bauer, K., Hejrani, B., and Korn, M.:
Attenuation tomography in West Bohemia/Vogtland, Tectonophysics, 695, 64–75, 10.1016/j.tecto.2016.12.010, 2017.
Mrlina, J., Kämpf, H., Geissler, W. H., and van den Bogaard, P.: Proposed
Quaternary maar structure at the Czech/German boundary between Mýtina
and Neualbentreuth (western Eger Rift, Central Europe): geophysical,
petrochemical and geochronological indications, Z. Geol. Wiss., 35, 213–230, 2007.Mrlina, J., Kämpf, H., Kroner, C., Mingram, J., Stebich, M., Brauer, A.,
Geissler, W. H., Kallmeyer, J., Matthes, H., and Seidl, M.: Discovery of the
first Quaternary maar in the Bohemian Massif, Central Europe, based on
combined geophysical and geological surveys, J. Volcanol. Geoth. Res., 182, 97–112, 10.1016/j.jvolgeores.2009.01.027, 2009.Mullick, N., Buske, S., Hrubcová, P., Růžek, B. Shapiro, S.,
Wigger, P., and Fischer, T.: Seismic imaging of the geodynamic activity at
the western Eger rift in central Europe, Tectonophysics, 647–648, 105–111, 10.1016/j.tecto.2015.02.010, 2015.Muñoz, G., Weckmann, U., Pek, J., Kováčiková, S., and Klanica, R.: Regional two-dimensional megnetotelluric profile in West Bohemia/Vogtland reveals deep conductive channel into the earthquake swarm region, Tectonophysics, 727, 1–11, 10.1016/j.tecto.2018.01.012, 2018.Nickschick, T., Kämpf, H., Flechsig, C., Mrlina, J., and Heinicke, J.:
CO2 degassing in the Hartousov mofette area, western Eger Rift, imaged
by CO2 mapping and geoelectrical and gravity surveys, Int. J. Earth Sci., 104, 2107–2129, 10.1007/s00531-014-1140-4, 2015.Nickschick, T., Flechsig, C., Mrlina, J., Oppermann, F., Löbig, F., and Günther, T.: Large-scale electrical resistivity tomography in the Cheb Basin (Eger Rift) at an International Continental Drilling Program (ICDP) monitoring site to image fluid-related structures, Solid Earth, 10, 1951–1969, 10.5194/se-10-1951-2019, 2019.
Parkes, R. J., Cragg, B. A., Bale, S. J., Getliff, J. M., Goodman, K., Rochelle, P. A., Fry, J. C., Weightman, A. J., and Harvey, S. M.: Deep bacterial biosphere in Pacific Ocean sediments, Nature, 371, 410–413, 1994.
Parkes, R. J., Cragg, B. A., and Wellsbury, P.: Recent studies on bacterial
populations and processes in subseafloor sediments: a review, Hydrogeol. J., 8, 11–28, 2000.
Prodehl, C., Mueller, S., and Haak, V.: The European Cenozoic Rift System,
in: Continental rifts: evolution, structure, tectonics, in: Dev. Geotecton, edited by: Olsen, K. H., Elsevier, 133–212, ISBN 9780080529837, 1995.Rohrmüller, J., Kämpf, H., Geiß, E., Großmann, J., Grun, I.,
Mingram, J., Mrlina, J., Plessen, B., Stebich, M., Veress, C., Wendt, A.,
and Nowaczyk, N.: Reconnaissance study of an inferred Quaternary maar
structure in the western part of the Bohemian Massif near Neualbenreuth,
NE-Bavaria (Germany), Int. J. Earth Sci., 107, 1381–1405, 10.1007/s00531-017-1543-0, 2017.Schimschal, S.: Seismic imaging of the crustal structure in the
Münchberg/Vogtland/Erzgebirge area by reprocessing of the deep seismic
reflection profile MVE90', MS thesis, TU Bergakademie, Freiberg, 2013.
Schreiber, U., Locker-Grütjen, O., and Mayer, C.: Hypothesis: origin of
life in the deep-reaching tectonic faults, Orig. Life Evol. Biosph., 42, 47–54, 10.1007/s11084-012-9267-4, 2012.Schuessler, J. A., Kämpf, H., Koch, U., and Alawi, M.: Earthquake impact
on iron isotope signatures recorded in mineral spring water, J. Geophys. Res.-Sol. Ea., 121, 8548–8568, 10.1002/2016JB013408, 2016.
Tomek, Č., Dvořáková, V., and Vrána, S.: Geological
interpretation of the 9HR and 503M seismic profiles in Western Bohemia, in:
Geological Model of Western Bohemia Related to the KTB Borehole in Germany, edited by: Vrána, S. and Štedrá, V., J. Geol. Sci. Prague, 47, 43–50, 1997.Umlauft, J. and Korn, M.: 3-D fluid channel location from noise tremors
using matched field processing, Geophys. J. Int., 219, 1550–1561, 10.1093/gji/ggz385, 2019.
Vavryčuk, V.: Crustal anisotropy from local observations of shear-wave
splitting in West Bohemia, Czech Republic, Bull. Seism. Soc. Am., 83, 1420–1441, 1993.
Vylita, T., Žák, K., Cílek, V., Hercman, H., and
Mikšíková, L.: Evolution of hot-spring travertine accumulation
in Karlovy Vary/Carlsbad (Czech Republic) and its significance for the
evolution of Teplá valley and Ohře/Eger rift, Z. Geomorphol. N.F., 51, 427–442, 2007.
Wagner, G. A., Gögen, K., Jonckhere, R., Wagner, I., and Woda, C.:
Dating of Quaternary volcanoes Komorní hůrka (Kammerbühl) and
Železná hůrka (Eisenbühl), Czech Republic, by TL, ESR,
alpha-recoil and fission track chronometry, Z. Geol. Wiss., 30, 191–200, 2002.
Weinlich, F. H., Bräuer, K., Kämpf, H., Strauch, G., Tesař, J.,
and Weise, S. M.: An active subcontinental mantle volatile system in the
western Eger rift, Central Europe: Gas flux, isotopic (He, C, and N) and
compositional fingerprints, Geochim. Cosmochim. Ac., 63, 3653–3671, 1999.
Weise, S. M., Bräuer, K., Kämpf, H., Strauch, G., and Koch, U.:
Transport of mantle volatiles through the crust traced by seismically
released fluids: A natural experiment in the earthquake swarm area
Vogtland/NW Bohemia, central Europe, Tectonophysics, 336, 137–150, 2001.Woith, H., Daskalopoulou, K., Zimmer, M., Fischer, T., Vlček, J.,
Trubač, J., Rosberg, J.-E., Vylita, T., and Dahm, T.: Multi-Level Gas
Monitoring: A New Approach in Earthquake Research, Front. Earth Sci., 8, 585733, 10.3389/feart.2020.585733, 2020.Wyss, M., Shimazaki, K., and Wiemer, S.: Mapping active magma chambers by b
values beneath the off-Ito volcano, Japan, J. Geophys. Res., 102, 20413–20422, 10.1029/97JB01074, 1997.