SUMMARY
The West Bohemia/Vogtland region is one of the seismically most interesting areas in Europe because of its swarm‐like occurrence of seismicity. The installation of the local West Bohemian ...seismological network (WEBNET) has made the recording of small magnitude seismicity (detection threshold ML≈−0.5) possible. We investigated if microseismicity exists below the detection threshold of WEBNET. A microseismic field campaign was carried out in the focal area Nový Kostel. The measurement was performed with three small arrays lasting for 6 d in a seismically quiet, interswarm period. We were able to detect and locate 13 microearthquakes in the magnitude range −1.5 ≤ML≤−0.1 and achieved a detection threshold about one magnitude lower than the local network. A relative location suggests that the recorded seismicity is rather related to a specific fault segment than randomly distributed. The determined fault zone is aligned NW–SW and confirms the viability of mapping active faults with short‐term measurements. The results demonstrate that a linear extrapolation of the b‐value, determined by the network bulletin, down to ML=−0.5 fits well with the amount of our recorded events.
The information on seismic loading and vibration effect is significant when assessing stability of underground spaces located in the vicinity of seismic active areas. Underground structures are well ...known to be earthquake resistant. Nevertheless, the existence of discontinuities makes them vulnerable to collapses, particularly in case of shallow underground openings. This study presents results of long-term seismic monitoring performed in the shallow medieval mine. This mine is located at a distance of about 25 km southeast of Nový Kostel focal zone (West Bohemia, Czech Republic), where seismic activity in the form of seismic swarms occurs. The most intensive earthquakes were documented here in the past with local magnitude
M
L
=5.0. During two intensive seismic swarms from the Nový Kostel focal zone in 2008 (
M
L
≤3.8) and 2011 (
M
L
≤3.7), almost 2000 earthquakes were recorded at the seismic station in the mine. Vibration effect underground was analysed together with results of geomechanical monitoring. Based on the measured data, vibration effect in the mine was extrapolated for an expected more intensive earthquake with the local magnitude
M
L
=5.0.
▶ 20 years seismohydrological investigations at Bad Brambach (Vogtland, Germany). ▶ For the first time, semi-quantitative comparison of two strong swarm earthquake series (2000 and 2008). ▶ Different ...migrations of the earthquake foci through time, horizontally and in depth. ▶ Fluid triggering not only the in earthquake initiating phases. ▶ Long-term gas flow increases at several gas outlets in the Vogtland/NW Bohemia region between 1998 and 2008 ▶ Systematic relationship between anomaly occurrence and seismic activity cannot be derived at present.
For more than 20 years, seismohydrological investigations have been undertaken at the mineral aquifer system of Bad Brambach (Vogtland, Germany). Two strong swarm earthquake series in 2000–2001 and 2008–2009 at the Nový Kostel epicentre (Czech Republic, 10
km E of BB) have enabled for the first time a comparison of seismological and groundwater hydraulic features in a semi-quantitative way. In spite of their similar spatial distribution in 2001 and 2008, the earthquake foci of each swarm migrated differently through time, horizontally as well as in depth. The seismic energy of the 2008–2009 events was released predominantly within 1 month, in contrast to 2000–2001 when it occurred over 3 months. The main distinctive features of each are seen in the hydraulic pressure anomalies which accompanied the earthquake swarms: number, shape, and progression (duration) of the anomalies. The comprehensive hydraulic data, with high temporal resolution, suggest that fluid triggering dominated not only the earthquake initiating phases. In particular, the long-lasting seismicity of the 2008–2009 swarm can be attributed to a continued triggering of weak earthquakes by over-pressured deep fluids. Here, the remaining static strain was obviously not sufficient to generate strong earthquakes as at the beginning of the earthquake swarm periods. Furthermore, the enduring high fluid pressure in 2009 could also indicate a continuation of the long-term gas flow increase observed at several gas outlets in the Vogtland/NW Bohemia region between 1998 and 2008. However, it is not possible at present to derive a systematic relationship between anomaly occurrence and seismic activity, as generally proposed in the context of earthquake prediction discussion.
We present the pattern of seismic activity in the period between 2001 and 2007 for the Nový Kostel focal zone, which is recently the most active zone of the West-Bohemia/Vogtland earthquake swarm ...region. While the year 2001 was characterized by dying out of the 2000-swarm activity in the form of a few microswarms, almost no seismicity occurred in the period between 2002 and 2003. Since 2004 an elevated seismic activity occurs in the form of repeating microearthquake swarms. We used a relative location method to relate the hypocenter positions of the post-swarm activity to the geometry of the 2000-swarm cluster. We found that the activity has concentrated in several clusters, which have been repeatedly activated. Some clusters coincide with the position of the previous activity; the others have activated so far inactive deep segments at the southern edge of the Nový Kostel fault. Besides the shift of the hypocenters to the edges of the previously active area we observe a southward migration of the activity and an increase of maximum depths of earthquakes from 10 to 13 km. The waveform similarity analysis disclosed that some fault patches consist of only a single, repeatedly activated fault plane, while the others consist of multiple, differently oriented fault planes activated almost simultaneously. Most of the focal mechanisms are consistent with the geometry of hypocenters showing NNW-SSE trending steep fault planes with left-lateral strike-slip mechanisms and varying dip-slip component.
Interiér kostnice, vstupní část. Výkop provedený stavební firmou po začištění (později označeno jako sektor 65). Jihozápadní roh s patrným hrobem (hrob 1832). Pohled od východu.
Interiér kostnice, ...vstupní část. Výkop provedený stavební firmou po začištění (později označeno jako sektor 65). Jihozápadní roh s patrným hrobem (hrob 1832). Pohled od východu.
Interiér kostnice, sektor 65B v úrovni podloží (uloženina 6101) s výkopy 1790 a 1792. Pohled od východu.
Interiér kostnice, sektor 65B v úrovni podloží (uloženina 6101) s výkopy 1790 a 1792. Pohled ...od východu.
Interior of the ossuary, sector 65B at the level of the bedrock (deposit 6101) with excavations 1790 and 1792. A view from the east.
Interiér kostnice, sektor 65C. Situace v úrovni podloží (uloženina 6101) a po vybrání hrobu 1835 (výkop 1793). Pohled shora od jihu.
Interiér kostnice, sektor 65C. Situace v úrovni podloží (uloženina ...6101) a po vybrání hrobu 1835 (výkop 1793). Pohled shora od jihu.
Interiér kostnice, sektor 65B v úrovni podloží (uloženina 6101) s výkopy 1790 a 1792. Pohled od severu.
Interiér kostnice, sektor 65B v úrovni podloží (uloženina 6101) s výkopy 1790 a 1792. Pohled od ...severu.
Interior of the ossuary, sector 65B at the level of the bedrock (deposit 6101) with excavations 1790 and 1792. A view from the north.
Interiér kostnice, sektor 62. Severní strana se základem nosného pilíře (zeď 962). Pohled od jihu.
Interiér kostnice, sektor 62. Severní strana se základem nosného pilíře (zeď 962). Pohled od jihu.
...Interior of the ossuary, sector 62. North side with the base of the supporting pillar (wall 962). A view from the south.
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
3
He content in CO
2
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’.
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