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1.
The western part of the Bohemian Massif is characterized by repeated occurrences of intraplate earthquake swarms. To study surface deformations of this anomalous region, a network covering about 2000 square kilometres for repeated geodetic measurements was established in 1993 - 1994. The positions of the individual points of the network were carefully picked with respect to local tectonic structure and earthquake foci distribution. GPS and precise levelling measurements were performed 1-2 times a year. The GPS data were processed by Bernese GPS software.
No tendency to any displacement - either horizontal or vertical - of geological blocks was derived from the geodetic data for the period 1993 - 96. Only displacements of less than 5 mm/year in average could occur in the whole region in that period; larger displacements would have been revealed by our measurements. 相似文献
2.
The amplitude response of a LCR gravimeter with the SRW-E feedback was determined on a vertical vibrating platform. The ink-pen recorder was connected parallel with the digital voltmeter input to obtain an analog response of the gravimeter to the harmonic motion of the base with a peak-to-peak amplitude of 10 m and periods excited in an interval of 4 - 10 s as by the ground motion of meteorological microseisms, and in the interval 10 -100 s as by surface waves of distant strong earthquakes.
In the first interval, an unexpected maximum of the amplitude response was observed with the double amplitude of apparent 6,5 Gal (6.5 × 10
–8
m/s
2
) at a period of 4.8 s, and a baseline shift with the amplitude of–64Gal was observed at the same period. The value of this direct component cannot be separated from the effect of the Earth's gravity field. In the second interval, the amplitude response of the gravimeter displayed one expected maximum at a period of 40 s with the double amplitude of 8.1Gal. At the usual level of microseisms with a peak-to-peak amplitude of up to 2 m the fluctuation of the gravimeter on the direct baseline shifted by –4.9 Gal was estimated at 1 Gal. With typical Rayleigh surface waves with periods of 20 s and double amplitudes of up to 100 m, the fluctuation reached 67 Gal. 相似文献
3.
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 Novy 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 Novy Kostel revealed regular vertical displacements during
the 1994, 1997 and 2000 earthquake swarms. The points around the Novy 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. 相似文献
4.
Former geophysical surveys performed in the region of the volcanic centre of the České Stř edohoří Mts. in North Bohemia (the
Ohře Rift zone) showed that anomalous volcanic bodies and features can be effectively identified within sedimentary environment.
For this reason we carried out new geophysical measurements in the area of the main mafic intrusion of essexitic character.
The target was the exact location and geometry of the intrusion and its relation to other components of the volcanic centre.
We used gravity, magnetic, shallow seismic and electromagnetic techniques. The new gravity and magnetic data were tied to
the old databases so that we could investigate the area as a whole complex. Electromagnetic measurements were applied in the
area of the expected extent of the intrusion, and the seismic measurements in the central part of the intrusion.
Based on all the data, mainly on gravity modelling, we delineated not only the surface and subsurface extent of the intrusion,
but we also defined the hidden relief of the intrusion. It was found that the intrusion is formed by a single body that has
a few protrusions, and not by a set of separate individual intrusions, as indicated by surface outcrops. However, the body
of the intrusion is affected by a major fault that caused lithological differences on both sides (essexite/monzodiorite).
In detail we show the depth of the debris cover and the thickness of the weathered zone in the central part of the essexite
body. We also derived indications of tectonic elements in the area of the intrusion in the main structural/tectonic direction
in the region. The results will be utilized to establish a 3D geological model of the whole volcanic centre.
This investigation may serve as an example of non-seismic geophysical exploration applied to the study of volcanic centres
surrounded by sedimentary rocks. 相似文献
5.
Miroslav Novotný Zuzana Skácelová Jan Mrlina Bedřich Mlčoch Bohuslav Růžek 《Surveys in Geophysics》2009,30(6):561-600
The refraction data from the SUDETES 2003 experiment were used for high-resolution tomography along the profile S01. The S01
profile crosses the zone Erbendorf-Vohenstrauss (ZEV) near the KTB site, then follows the SW–NE oriented Eger Rift in the
middle part and continues toward the NE across the Elbe zone and the Sudetic structures as far as the Trans-European Suture
Zone. To get the best resolution in the velocity image only the first arrivals of Pg waves with minimum picking errors were
used. The previous depth-recursive tomographic method, based on Claerbout’s imaging principle, has been adapted to perform
the linearized inversions in iterative mode. This innovative DRTG method (Depth-Recursive Tomography on Grid) uses a regular
system of refraction rays covering uniformly the mapped domain. The DRTG iterations yielded a fine-grid velocity model with
a required level of RMS travel-time fit and the model roughness. The travel-time residuals, assessed at single depth levels,
were used to derive the statistical lateral resolution of “lens-shaped” velocity anomalies. Thus, for the 95% confidence level
and 5% anomalies, one can resolve their lateral sizes from 15 to 40 km at the depths from 0 to 20 km. The DRTG tomography
succeeded in resolving a significant low-velocity zone (LVZ) bound to the Franconian lineament nearby the KTB site. It is
shown that the next optimization of the model best updated during the DRTG iterations tends to a minimum-feature model with
sweeping out any LVZs. The velocities derived by the depth-recursive tomography relate to the horizontal directions of wave
propagation rather than to the vertical. This was proved at the KTB site where pronounced anisotropic behavior of a steeply
tilted metamorphic rock complex of the ZEV unit has been previously determined. Involving a ~7% anisotropy observed for the
“slow” axis of symmetry oriented coincidentally in the horizontal SW–NE direction of the S01 profile, the DRTG velocity model
agrees fairly well with the log velocities at the KTB site. Comparison with the reflectivity map obtained on the reflection
seismic profile KTB8502 confirmed the validity of DRTG velocity model at maximum depths of ~16 km. The DRTG tomography enabled
us to follow the relationship of major geological units of Bohemian Massif as they manifested in the obtained P-wave velocity
image down to 15 km. Although the contact of Saxothuringian and the Teplá-Barrandian Unit (TBU) is collateral with the S01
profile direction, several major tectonic zones are rather perpendicular to the Variscan strike and so fairly imaged in the
S01 cross-section. They exhibit a weak velocity gradient of sub-horizontal directions within the middle crust. In particular,
the Moldanubian and TBU contact beneath the Western Krušné hory/Erzgebirge Pluton, the buried contact of the Lusatia unit
and the TBU within the Elbe fault zone were identified. The maxima on the 6,100 ms−1 isovelocity in the middle crust delimitated the known ultrabasic Erbendorf complex and implied also two next ultrabasic massifs
beneath the Doupovské hory and the České středohoří volcanic complexes. The intermediate mid-crustal P-wave velocity lows
are interpreted as granitic bodies. The presented geological model is suggested in agreement with available gravity, aeromagnetic
and petrophysical data. 相似文献
6.
J.?Miku?kaEmail author R.?Pa?teka J.?Mrlina I.?Maru?iak 《Studia Geophysica et Geodaetica》2008,52(3):381-396
We analyzed the gravitational effect of topography and bathymetry beyond the angular distance of approximately 1.5 degrees
(referred to as the distant relief effect or DRE), and its impact on measured gravity values in the region of the former Czechoslovakia.
Our work was strongly motivated by the contents of the pioneering contribution of outstanding Czech geophysicists Miloš Pick,
Jan Pícha and Vincenc Vyskočil, which appeared at the turn of the 1950’s and 1960’s. Our numerical calculations were based
upon the direct evaluation of the gravitational effects of compartments of a spherical layer, while the respective heights
and depths were obtained from the 2 × 2 minutes digital elevation model (DEM) ETOPO2, taking into consideration also the influence
of distant bathymetry. Our results are in close agreement with, but not identical to, those of the above cited authors. We
also analyzed the influence of the grid cell size of the involved DEM upon the calculation results. We introduced an approximation
of the analyzed effect, based on a simple linear relationship between the calculation point height, the DRE and its vertical
gradient (VGDRE). Since when calculated at zero elevation the involved quantities DRE and VGDRE are smooth functions of latitude
and longitude and can be easily interpolated, the approximation gives acceptable results in terms of desired accuracy of several
μGal (1 μGal = 10−8 m/s2). In general, we can state that within the territories of the Czech and Slovak Republics the studied distant relief effect
has negligible impact upon local gravity survey data. However, when applied to regional gravity studies, there could be a
question of its possible influence in the form of a quasilinear W-E trend ranging approximately from −106.6 to −102.5 mGal
within the territory of former Czechoslovakia. If we wanted to correct for this phenomenon, we should subtract this negative
quantity from the standard Bouguer anomalies as they have been defined in the recent geophysical literature, thereby considerably
increasing their values. But, instead of straightforward correcting the Bouguer anomalies for DRE only, we would rather recommend
to wait until after the crustal and even lithospheric effects have been studied more carefully based upon the present day
independent knowledge about the deep seated sources of those effects. 相似文献
7.
Jiří Žák Zuzana Kratinová Jakub Trubač Vojtěch Janoušek Jiří Sláma Jan Mrlina 《International Journal of Earth Sciences》2011,100(7):1477-1495
The Štěnovice and Čistá granodiorite–tonalite plutons are small (~27 and ~38 km2, respectively) intrusions that are largely discordant to regional ductile structures in the center of the upper-crustal Teplá–Barrandian
unit, Bohemian Massif. Their whole-rock and trace-element compositions are consistent with medium-K calc-alkaline magma, generated
above a subducted slab in a continental margin arc setting. The U–Pb zircon age of the Štěnovice pluton, newly determined
at 375 ± 2 Ma using the laser ablation ICP-MS technique, is within the error of the previously published Pb–Pb age of 373 ± 1 Ma
for the Čistá pluton. The two plutons also share other characteristics that are typical of concentrically expanded plutons
(CEPs), such as elliptical cross-section in plan view, steep contacts, inferred downward-narrowing conical shape, faint normal
zoning, and margin-parallel magmatic foliation decoupled from the regional host-rock structures. We interpret the Štěnovice
and Čistá plutons as representing the initial Late Devonian stage of much more voluminous early Carboniferous arc-related
plutonism (represented most typically by the Central Bohemian Plutonic Complex) in the upper crust of the central Bohemian
Massif. These two plutons are important tectonic elements in that they indicate an overall shift of the arc-related plutonic
activity from the ~NW to the ~SE, accompanied with a general compositional trend of the magmas from medium-K calc-alkaline
to shoshonitic/ultrapotassic. Such a pattern is compatible with SE-directed subduction of the Saxothuringian Ocean beneath
the Teplá–Barrandian overriding plate as a cause of arc-related magmatism in this part of the Bohemian Massif. 相似文献
8.
Nickschick Tobias Flechsig Christina Meinel Cornelia Mrlina Jan Kämpf Horst 《International Journal of Earth Sciences》2017,106(8):2915-2926
International Journal of Earth Sciences - The Hartoušov mofette field in NW Bohemia, Czech Republic, is characterized by strong CO2 degassing from the Lithospheric Mantle. In a test survey... 相似文献
9.
Since 1994 annual campaigns of precise levelling have been performed on the network established in the surroundings of Nový Kostel. The network covers the most dynamic part of the West Bohemia seismoactive region with a total of 70 points in 1999. The measurements are connected to a reference point of the national levelling system and to our GPS-gravity network. It was proved by the error and confidence level of the data that small displacements of three and more millimetres can be recognised.
The data analysis showed that all points could be divided into several groups according to temporal changes of height. Mutual comparison of these changes enabled to detect both general and very local short-term movements. It appeared that, in general, the involved part of the mountain block is subsiding relative to the Cheb basin. This is in contradiction to the recent uplift of the Kruné hory Mts. The correlation between vertical displacements and earthquake swarms was examined with the conclusion that during a swarm period the movements have special homogeneous pattern, contrary to inter-periods. The division line of different displacements (a fault zone) for the swarms 1994 and 1997 was determined. 相似文献
10.
The gravimetric investigation of the seismoactive area in Western Bohemia started in 1991 with local experimental measurements in a small area around the principal Mariánské Lázn fault close to Nový Kostel. With respect to the aims of the study, the method was designed for complex investigations in combination with GPS measurements, hydrological and geological research starting in 1993. Precise levelling was included into the project to specify vertical displacements with high accuracy in order to eliminate their influence on gravity. Temporal changes of gravity were observed during the initial stage in 1993 - 1995. 相似文献