The West Bohemia and adjacent Vogtland are well known for quasi-periodical earthquake swarms persisting for centuries. The seismogenic area near Nový Kostel involved about 90 % of overall earthquake ...activity clustered here in space and time. The latest major earthquake swarm took place in August–September 2011. In 1994 and 1997, two minor earthquake swarms appeared in another location, near Lazy. Recently, the depth-recursive tomography yielded a velocity image with an improved resolution along the CEL09 refraction profile passing between these swarm areas. The resolution, achieved in the velocity image and its agreement with the inverse gravity modeling along the collateral 9HR reflection profile, enabled us to reveal the key structural background of these West Bohemia earthquake swarms. The CEL09 velocity image detected two deeply rooted high-velocity bodies adjacent to the Nový Kostel and Lazy focal zones. They correspond to two Variscan mafic intrusions influenced by the SE inclined slab of Saxothuringian crust that subducted beneath the Teplá-Barrandian terrane in the Devonian era. In their uppermost SE inclined parts, they roof both focal zones. The high P-wave velocities of 6,100–6,200 m/s, detected in both roofing caps, indicate their relative compactness and impermeability. The focal domains themselves are located in the almost gradient-free zones with the swarm foci spread near the axial planes of profound velocity depressions. The lower velocities of 5,950–6,050 m/s, observed in the upper parts of focal zones, are indicative of less compact rock complexes corrugated and tectonically disturbed by the SE bordering magma ascents. The high-velocity/high-density caps obviously seal the swarm focal domains because almost no magmatic fluids of mantle origin occur in the Nový Kostel and Lazy seismogenic areas of the West Bohemia/Vogtland territory, otherwise rich in the mantle-derived fluids. This supports the hypothesis of the fluid triggering of earthquake swarms. The sealed focal domains retain ascending magmatic fluids until their critical pressure and volumes accumulated cause rock micro-fractures perceived as single earthquake bursts. During a swarm period, the focal depths of these sequential events become shallower while their magnitudes grow. We assume that coalescence of the induced micro-fractures forms temporary permeability zones in the final swarm phase and the accumulated fluids release into the overburden via the adjacent fault systems. The fluid release usually occurs after the shallowest events with the strongest magnitudes M
L
> 3. The seasonal summer declines of hydrostatic pressure in the Cheb Basin aquifer system seem to facilitate and trigger the fluid escape as happened for the 2000, 2008, and 2011 earthquake swarms. The temporary fluid release, known as the valve-fault action, influences the surface aquifer systems in various manners. In particular, we found three quantities, the strain, mantle-derived
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He content in CO
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surface sources and ground water levels, which display a 3–5 months decline before and then a similar restoration after each peak earthquake during the swarm activities. The revealed structure features are particularly important since the main Nový Kostel earthquake swarm area is proposed as a site for the ICDP project, ‘Eger Rift Drilling’.
Deep-sea trace fossils of the Nereites ichnofacies occur in the uppermost Kimmeridgian-Berriasian deep-water turbidite deposits of the Kostel Formation in SW Bulgaria. The trace fossil assemblage ...contains Tithonian Ophiomorpha isp. and Ophiomorpha annulata, which are the oldest deep-sea representatives of this ichnogenus. They indicate much older invasion of Ophiomorpha into deep-sea, which was hitherto dated as to mid-Cretaceous. It is not clear whether the invasion failed or Ophiomorpha survived in deep-sea refuge and became widespread after improving oxygenation of the deep seas after the Cenomanian/Turonian anoxic event. Tithonian Scolicia, produced by irregular echinoids, exhibits a similar distribution pattern.
SZ segment, sektor 59. Situace v úrovni podloží (uloženina 6101) po vybrání části výkopů (1770, 1771, 1772, 1773). Výkop 1775 (vlevo dole) zůstává nevybrán. Pohled shora od severu.
SZ segment, sektor ...59. Situace v úrovni podloží (uloženina 6101) po vybrání části výkopů (1770, 1771, 1772, 1773). Výkop 1775 (vlevo dole) zůstává nevybrán. Pohled shora od severu.
SZ segment, sector 59. Situation at the level of the subsoil (deposit 6101) after the removal of part of the excavations (1770, 1771, 1772, 1773). The excavation of 1775 (bottom left) remains unselected. A view from the north.
SZ segment, uloženina 6100. Fragment keramické dlaždice pravděpodobně z původní gotické podlahy interiéru.
SZ segment, uloženina 6100. Fragment keramické dlaždice pravděpodobně z původní gotické ...podlahy interiéru.
SZ segment, deposit 6100. Fragment of a ceramic tile probably from the original Gothic floor of the interior.
The Cheb Basin, located in the western Eger (Ohře) Rift, is part of the European Cenozoic Rift system. Although presently non-volcanic, it is the most active area within the European Rift with signs ...of recent geodynamic activity like emanations of mantle derived CO
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, and the repeated occurrence of swarm earthquakes, which are common features in active volcanic regions. It is assumed that the fluids, uprising in permeable channels, play a key role for the genesis of these earthquake swarms.
An image of the distribution of the electrical conductivity (resp. resistivity) in the upper crust can give information about the fluid distribution since the electrical patterns reflect pathways of fluids and fluid properties like ionic content. This was the motivation to start both a regional-scale direct current (DC) geoelectrical test covering the Cheb Basin area and several local-scale near surface investigations inside the basin at the seismically active faults Počátky-Plesná Zone (PPZ) and Mariánské Lázně Fault Zone (MLF) near Nový Kostel. It was the research idea to test electrical tomography’s ability to detect faults and tectonic deformation in a complex geological environment and to identify characteristic features of these faults.
The more methodically justified regional field test mainly aimed at clarifying the opportunity to trace DC electrical signals in an area with known high industrial noise in a sufficient quality. A field set-up with a range of 15–20 km is necessary for an investigation depth of 4–5 km in case of DC geoelectrics. A new developed inversion strategy for sparse electrical data sets allows for a first (coarse) model of resistivity distribution. The near surface investigations in prominent fault zones of the Cheb Basin give more detailed information about the structure of fault zones, divide the sedimentary units in different resistivity zones and detect vertical displacements in the quaternary formations.
We analyze refraction measurements along a short profile in western Krušné hory crystalline unit. The profile passed close to the seismically active zone of Nový Kostel. The measurements were carried ...out to distances of about 15 km using quarry blasts near the village of Horní Rozmyšl, located at the eastern margin of the crystalline unit. Smoothed P-wave travel times were interpreted using the Wiechert-Herglotz method, which yielded a 1-D velocity model of the shallow crustal structure of the crystalline unit down to a depth of 1.7 km. The P-wave velocity of the model increases from about 4.0 km/s at the surface to 5.9 km/s at a depth of 1.7 km. The superficial velocities of our model are somewhat higher than the superficial velocities of the model that is routinely used for earthquake location in the region.
GPS observations in the Western Bohemia/Vogtland earthquake swarm region revealed indications of horizontal displacements of low amplitude, and no clear long-term trend in 1993–2007. On the other ...hand, in 1998–2001 there was relatively significant active movement along NNE-SSW oriented line that we called the “Cheb-Kraslice GPS Boundary” (ChKB), identical with an important limitation of earthquake activity. The most impressive were dextral (right-lateral) movements in the 1998–1999 period followed by reverse sinistral (left-lateral) movements in 1999–2000 that correlate with prevailing motion defined by fault plane solutions of the Autumn 2000 earthquake swarm. Before the February 2004 micro-swarm, two points located on opposite sides of the Mariánské Lázně fault showed extension in the order of about 7 mm in the same NNE-SSW direction of ChKB. The new NOKO permanent GPS station in Nový Kostel showed the peak-to-peak vertical changes up to 10 mm before and during the February 2007 micro-swarm.
Annual precise levelling campaigns in the local network around Nový Kostel revealed regular vertical displacements during the 1994, 1997 and 2000 earthquake swarms. The points around the Nový Kostel seismological station showed uplift during the active periods, including the micro-swarm February 2004. However, no such indication was observed on levelling points in the period of the February 2007 swarm. Long-term vertical displacements depend on the same direction NNE-SSW (ChKB) as the GPS displacements. Both geodetic techniques have revealed oscillating displacements, GPS horizontal, and levelling vertical, rather than any long-term trends in the study period 1993–2007. The displacements exhibited significant spatial and temporal relation to tectonic activity (earthquake swarms) including their coincidence with the seismologically determined sense of motion along the fault plane during earthquakes.
Interiér kostnice, sektor 65B. Východní řez se zdivem krypty (zeď 963). Pohled od západu.
Interiér kostnice, sektor 65B. Východní řez se zdivem krypty (zeď 963). Pohled od západu.
