Institute of Geophysics of the CAS, v. v. i.

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CzechGeo workshop(EN)

 

 

The Board of the large infrastucture CzechGeo/EPOS invites you to

 

 

Workshop CzechGeo/EPOS

 

 

help on Wednesday 16th, November, 2016

 

in meeting room of Institute of Geophysics CAS, Boční II/1401, Praha 4 - Spořilov

 

Program

9:30 – 12:30

CzechGeo/EPOS – data and services for research and applications


Presented by CzechGeo/EPOS team members

12:30 – 14:00 Lunch break
14:00 – 17:00 Scientific results based on CzechGeo/EPOS data and services

 


 

Oral presentations

  

14:00 – 14:15

Helena Munzarová and Jaroslava Plomerová

Tomographic studies of the BM upper mantle

14:15 – 14:30

Luděk Vecsey

Seismic data quality assurance and control

14:30 – 14:45

Jaroslava Plomerová and Vladislav Babuška

Continental lithosphere - mosaic of microplates with a rigid mantle lithosphere

14:45 – 15:00

Jan Douša and Pavel Václavovic

Precise GNSS analyses for troposphere monitoring by Geodetic Observatory Pecny analysis center

15:00 – 15:15

Josef Stemberk and Miloš Briestenský

Fault slips development RECORDED on MAJOR lugicum faults

15:15 – 15:30

Michal Janošek

Results of precise magnetic calibrations at the facilities of IG, CAS

15:30 – 15:45

Jana Doubravová, Jan Wiszniowski and Josef Horálek

Local event detection with neural networks (application to Webnet data)

15:45 – 16:00

Hana Jakoubková, Josef Horálek and Tomáš Fischer

Earthquake swarms in West Bohemia and South-West Iceland

16:00 – 16:15

Pavla Hrubcová, Václav Vavryčuk, Alena Boušková and Josef Horálek

Determination of prominent crustal discontinuities from waveforms of local earthquakes

16:15 – 16:30

Vladislav Plicka

19 years of Research Infrastructure PSLNET - significant influence to study the source parameters of Greece earthquakes

16:30 – 16:45

Vladimír Čermák, Petr Dědeček, Milan Krešl, Jan Šafanda and Tomáš Uxa

Heat transfer in shallow subsurface under different climate conditions in Europe (Czechia, Slovenia, Portugal)

16:45 – 17:00

Petr Špaček

Monitoring and research into the local microseismicity in the NE Czech Republic

 

 


 

 

Poster presentations

 

Jan Douša, Pavel Václavovic and Petr Bezděka

G-Nut/Anubis - recent developments for GNSS data quality control tool

Josef Horálek and H. Jakoubková

Earthquake statistics, spatiotemporal distribution of foci and source mechanisms - a key to understanding of the West Bohemia/Vogtland earthquake swarms

Radek Klanica

Magnetotelluric sounding - detailed and large scale experiments

Bohuslav Růžek, Lubica Valentová and František Gallovič

Significance of geological units of the Bohemian Massif, Czech Republic, as seen by ambient noise interferometry

Michal Vlk

Reconstruction of the Torsion Photoelectric Magnetometer at Budkov Observatory

 

 


Abstracts

 

Vladimír Čermák, Petr Dědeček, Milan Krešl, Jan Šafanda and Tomáš Uxa
Heat transfer in shallow subsurface under different climate conditions in Europe (Czechia, Slovenia, Portugal)

The long-term series (up to 20 years) of soil and air temperatures and other meteorological variables recorded at observatories in Czechia (Prague, Kocelovice, Svojšice), Portugal (Evora) and Slovenia (Malence) are used for analysis of the air - ground surface temperature coupling and the heat transfer in different climatic, pedological and environmental conditions. The time series obtained up to now show that solar radiation, precipitation, insulation caused by vegetation or snow cover and albedo of the surface strongly affect the air – ground temperature offset on all time scales (including the annual means and their inter-annual variations) and control the heat transfer to the shallow subsurface. Some findings are unexpected (decreasing mean annual temperature with depth in the uppermost decimetres to metres in soil; relatively strong seasonal variations of thermal diffusivity of soils) and we have been working on their explanations. The observed data have applications in several other disciplines like meteorology, climatology, plant biology or exploitation and storage of shallow geothermal energy.

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Jana Doubravová, Jan Wiszniowski and Josef Horálek
Local event detection with neural networks (application to Webnet data)

We present a new method of local event detection based on neural networks. The proposed algorithm uses a unique neural network architecture. It combines features used in other neural network concepts like Real Time Recurrent Network and Nonlinear Autoregressive Neural Network to achieve a good detection performance. We use the recurrence combined with various delays applied to recurrent inputs to make the network remember history of many samples - the Single Layer Recurrent Neural Network (SLRNN). This method has been tested on data from local seismic network in West Bohemia (Webnet).

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Josef Horálek and Hana Jakoubková
Earthquake statistics, spatiotemporal distribution of foci and source mechanisms - a key to understanding of the West Bohemia/Vogtland earthquake swarm

West Bohemia-Vogtland is one of the most active intraplate earthquake-swarm areas in Europe which also exhibits high activity of crustal fluids. The Nový Kostel focal zone (NK) dominates the recent seismicity, there were swarms in 1997, 2000, 2008 and 20011, and a striking non-swarm activity (mainshock-aftershock sequences) up to magnitude ML = 4.5 in May to August 2014. The swarms and the 2014 mainshock-aftershock sequences are located close to each other at depths between 6 and 13 km. The frequency-magnitude distributions of all the swarms show bimodal-like character: the most events obey the b-value = 1.0 distribution, but a group of the largest events depart significantly from it. All the ML > 2.8 swarm events are located in a few dense clusters which implies step by step rupturing of one or a few asperities during the individual swarms. The source mechanism patters (moment-tensor description, MT) of the individual swarms indicate several families of the mechanisms, which fit well geometry of respective fault segments. MTs of the most events signify pure shears except for the 1997-swarm events the MTs of which indicates a combine sources including both shear and tensile components. The origin of earthquake swarms is still unclear. Nevertheless, we infer that the individual earthquake swarms in West Bohemia-Vogtland are mixture of the mainshock-aftershock sequences which correspond to step by step rupturing of one or a few asperities. The swarms occur on short fault segments with heterogeneous stress and strength, which may be affected by pressurized crustal fluids reducing normal component of the tectonic stress and lower friction. This way critically loaded faults are brought to failure and the swarm activity is driven by the differential local stress.

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Pavla Hrubcová, Václav Vavryčuk, Alena Boušková and Josef Horálek
Determination of prominent crustal discontinuities from waveforms of local earthquakes

The West Bohemian area is known for increased geodynamic activity with reoccurrence of intraplate earthquake swarms. The seismic activity is monitored by seismic stations of the West Bohemian Network (WEBNET) with uniform and dense station coverage. These stations provide high-quality recordings representing waveforms of local earthquakes. High-frequency seismic waves generated by local sources are sensitive to sharp changes in velocities or density and serve for detecting prominent discontinuities within the crust. With a dense network of sensitive seismic stations and their good azimuthal distribution, we retrieve a depth of these discontinuities and map their lateral variations. Clustered seismicity in West Bohemia indicates a strong-contrast interface at depths of 3.5-6.0 km, which is in agreement with previous profiling and might be related to trapping of fluids ascending from the mantle. At the crust/mantle Moho depths, details in the velocity structure are inferred and resultant reflective zone shows one or two strongly reflective interfaces, which is in agreement with the zone interpreted by previous investigations.

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Hana Jakoubková, Josef Horálek and Tomáš Fischer
Earthquake swarms in West Bohemia and South-West Iceland

West Bohemia/Vogtland and South-West Iceland are two regions with diametrically opposite tectonic basis. Whereas West Bohemia/Vogtland is located inside a tectonic plate being characterized by an intraplate seismicity, South-West Iceland lies directly on the Mid-Atlantic Ridge hosting seismicity of an interplate character. Despite the different tectonic features, in both areas typically earthquake swarms occur. To see if seismic activities appearing in the two regions show similar or vice versa dissimilar behavior we analyzed some basic statistical features including distribution of events in space and time, magnitude-frequency distribution and distribution of interevent times. In South-West Iceland we concentrated on three areas where earthquake swarms are of different origin: pure tectonic swarms (Krísuvík), volcanic swarms (Hengill) and swarms occurring on the edge of a fault where typically mainshock-aftershock sequences occur (Ölfus). We found that events from all the activities show migration in space and time. Then, it was proved that in case of earthquake swarms larger events have higher contribution to the total seismic energy release than it has been supposed. Our results also suggest that events do not occur randomly but they are driven by redistribution of stress.seismicity, South-West Iceland lies directly on the Mid-Atlantic Ridge hosting seismicity of an interplate character. Despite the different tectonic features, in both areas typically earthquake swarms occur. To see if seismic activities appearing in the two regions show similar or vice versa dissimilar behavior we analyzed some basic statistical features including distribution of events in space and time, magnitude-frequency distribution and distribution of interevent times. In South-West Iceland we concentrated on three areas where earthquake swarms are of different origin: pure tectonic swarms (Krísuvík), volcanic swarms (Hengill) and swarms occurring on the edge of a fault where typically mainshock-aftershock sequences occur (Ölfus). We found that events from all the activities show migration in space and time. Then, it was proved that in case of earthquake swarms larger events have higher contribution to the total seismic energy release than it has been supposed. Our results also suggest that events do not occur randomly but they are driven by redistribution of stress.

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Michal Janošek
Results of precise magnetic calibrations at the facilities of IG, CAS

Czech Technical University, together with Czech Metrological Institute, has a long-term interest in precise magnetic measurements, magnetometer development and calibration. Such a precise task cannot be performed in the city due to magnetic noise, so the CTU and CMI extensively collaborate with Institute of Geophysics (IG) which provides us the necessary localities and infrastructure at the Budkov BDV observatory and Průhonice ex-observatory.

During calibrations of state-of-the-art triaxial fluxgate magnetometers (Earth’s field range) we have verified that even 20 km away from Prague city centre (ex-observatory Průhonice) the artifical noise due distant DC traction (tramway and subway traffic) causes tens of nT peak-peak. This somehow hindered our calibration limits, even when using partial noise compensation, to about 200-400 ppm.

Because of the above reasons, the so-called “scalar calibration” is used more extensively for the instruments developed at CTU. For scalar calibration, large homogeneity (< 1 nT/m) is required which is easily achieved at the Absolute building of the observatory. The careful selection of the location and magnetic cleanliness of the facilities at the BDV observatory allow us for year-round calibrations and instrument checks. As the scalar calibration relies on scalar magnetic field values during the calibration, we also benefit from multiple observations of scalar instruments (Overhauser magnetometers) available at Budkov.

In 2015 we also collaborated with IG on a specific metrologic task where precision of Overhauser magnetometers was compared among worldwide users in Geophysical, Metrological and Geological institutes. By developing a specific procedure and careful magnetic cleanliness during the tests in collaboration with our Budkov colleagues, we were able to determine the measured value by the travelling standard Overhauser magnetometers with an uncertainty of ± 0.5 nT (10 ppm from 50 000 nT of the Earth’s field total intensity). This result was third best fit to the estimated value, which was determined by the leading participant VNIIM (D.I. Mendeleyev Institute for Metrology, St. Petersburg).

Recently, we used the BDV facility for calibration, alignment and long-term estimation of parameters of the low-noise variometer newly developed at CTU (< 5 pT/√Hz @ 1Hz), which is now in use at the National Observatory Athens and is being also installed at the IG.

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Radek Klanica
Magnetotelluric sounding - detailed and large scale experiments

Magnetotelluric method is an electromagnetic induction method used for probing the electrical resistivity at depth in the Earth by recording and analysing natural geomagnetic variations on the Earth’s surface. Magnetotellurics has been employed to solve a wide range of problems on depth scales from first hundreds of meters down to several tens of kilometres.

We present results of two different recent MT experiments, one for a detailed near-surface geophysical investigation and another one for a large-scale regional study. The small-scale measurements were carried out in September 2016 in the West Bohemian seismoactive region and aimed at investigating the dip of the Mariánské Lázně Fault at the eastern margin of the Cheb Basin. Eight MT stations were deployed along a 1050 m long profile, with a regular inter-site spacing of 150 m. Only higher frequencies of the audio-MT range, from 100 Hz to 10 kHz, were recorded and evaluated. The interpreted resistivity section down to 1 km beneath the profile indicates a rather steep westward dip of the Mariáanské Lázně Fault.

The regional MT study was carried out on the eastern margin of the Bohemian Massif and aimed at investigating deep geological structures and the eastern termination of the Bohemian Massif at the contact with the Cadomian Brunovistulicum basement. Seventeen broadband MT stations (period range from 0.001 to 1000 s) were arranged along a 130 km long W-E profile, which crossed the Moldanubian zone, the Carpathian Foredeep and the Outer Western Carpathians. The geoelectrical model compares well with known near-surface geological units and brings new informaion about geological structures down to a depth of about 30 km, especially as regards the contact between the Moldanubian zone and the Brunovistulicum in the deeper parts of the Earth’s crust.

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Vladislav Plicka
19 years of Research Infrastructure PSLNET - significant influence to study the source parameters of Greece earthquakes

A mutual long-term cooperation between Charles University in Prague and Patras University was to contribute to better understanding of source parameters of the significant Greece earthquakes. This includeded not only the fault process but also the strong-ground motions and their damaging effects. Towards this goal, research on several parallel lines has been carried out by our group (http://geo.mff.cuni.cz) during the last 19 years.

Several methods and codes were developed for the forward and inverse seismic source solution (PEXT, ASPO, ISOLA, MUFEX, EMPIRE,...) and several moderate and large Greece earthquakes were studied based on the PSLNET data (M5.9 Athens 1999, M6.5 Skyros 2001, M6.3 Lefkada 2003, M6.7 Cythera 2006, Mw6.2 Leonidio 2008, Mw7.2 Van 2011, M5 Efpalio 2010, Mw6, Cephalonia 2014, Mw6.4 Lefkada 2015, ...)

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Bohuslav Růžek, Lubica Valentová and František Gallovič
Significance of geological units of the Bohemian Massif, Czech Republic, as seen by ambient noise interferometry

Broadband recordings of 88 seismic stations distributed in the Bohemian Massif, Czech Republic, and covering the time period of up to 12 years were processed by a cross‑correlation technique. All correlograms were analyzed by a novel approach to get both group and phase dispersion of Rayleigh and Love waves. Individual dispersion curves were averaged in five distinct geological units which constitute the Bohemian Massif (Saxoturingian, Teplá‑Barrandean, Sudetes, Moravo‑Silesian, Moldanubian). Estimated error of the averaged dispersion curves are by an order smaller than the inherent variability due to the 3D distribution of seismic velocities within the units. The averaged dispersion data were inverted for 1D layered velocity models including their uncertainty, which are characteristic for each of the geological unit. We found that, overall, the differences between the inverted velocity models are of similar order as the variability inside the geological units, suggesting that the geological specification of the units is not fully reflected into the S‑wave propagation velocities on a regional scale. Nevertheless, careful treatment of the dispersion data allowed us to identify some robust characteristics of the area. The vp‑to‑vs ratio is anomalously low (~1.6) for all the units. The Moldanubian is the most rigid and most homogeneous part of the Bohemian Massif. Middle-crust in the depth range of ~3‑15 km is relatively homogeneous across the investigated region, while both uppermost horizon (0‑3 km) and lower-crust (>15 km) exhibit lower degree of homogeneity.

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Petr Špaček
Monitoring and research into the local microseismicity in the NE Czech Republic

Increased seismicity in the NE Bohemian Massif, close to its contact with the Western Carpathians´ orogenic front, is studied in detail using CzechGeo/EPOS infrastructure with 8 SP stations of the MONET Network and 9 BB stations of the CRSN Network.

While the present-day seismicty is low-magnitude and maximum intensity of historical earthquakes does not exceed I≈7.5° (estimated ML≈4.4-4.9), strikingly fast faulting with minimum slip rate of 0.1-0.3 mm/year was inferred for Late Pleistocene based on paleoseismic studies at some NW-SE striking faults in the region.

Since 2008 (after major upgrade of the network) approximately 200-300 local microearthquakes per year are recorded, of which 100-180 are routinely located. The magnitude range of nearly 1400 natural earthquakes located by MONET network in the period 1998-2015 is -1.2<ML<2.5, and the typical hypocentral depths are 9-18 km. The cross-correlation analyses show that majority (75%) of the weak, non-located microearthquakes are multiplets of stronger, located events and the repeated occurrence of events in nearly identical foci seems to be characteristic for the region. Fault plane solutions so far available indicate a combination of dextral horizontal shears with normal dip-slips on steeply dipping faults.

The ongoing improvements and continuing operation of the monitoring infrastructure will allow to increase the number of well-constrained focal mechanisms and improve our knowledge on the extent and internal structure of the main seismogenic zones.

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Josef Stemberk and Miloš Briestenský
Fault slips development RECORDED on MAJOR lugicum faults

Based on results of 3D monitoring of fault slip within the Rasovna and Západní caves developed in zones of major Lugicum faults as Lusatian Fault and Sudetic Marginal Fault a comparison of fault movement development along Lausatian Fault and Sudetic Marginal Fault is presented. Fault slips are recorded in three dimensions using specially designed optical-mechanical crack gauges known as TM-71s as a part of the EU-TecNet monitoring network. Network was established about fifteen years ago to record fault displacements across selected tectonic structures in the shallow crust. The network comprises more than one hundred fifty sites, most of which are situated underground, spread across the globe (www.tecnet.cz). Regular monitoring is supported by research infrastructure programme CzechGeo.

The Rasovna Cave is located within the Sudetic Marginal Fault zone approximately 5 km west of Jeseník in north-eastern Bohemia. Extensometer TM-71 was installed across fault 27°/38° in February 2006.

The Západní Cave is located within the Lusatian Fault zone approximately 15 km west of Liberec, close to the village of Jitrava, in northern Bohemia. It is the largest cave found within the karstic part of Ještěd Ridge. Extensometer TM-71 was installed across fault 148°/90° in April 2007.

Data obtained during the past decade demonstrate that periods of tectonic quiescence alternate with shorter periods of increased fault activity. The fault displacement monitoring has also shown notable periods of increased geodynamic activity affected both faults, referred to as pressure pulses, in 2008, 2010/2011, and 2013/2014. It has been found that these periods of pressure pulses occur contemporaneously along both major faults. Based on comparison with fault slips recorded on other major faults across the Bohemian Massif these periods are interpreted to reflect the widespread redistribution of stress and strain through the shallow crust. It is possible to correlate the identified displacement cycles with the strongest earthquakes - swarms which have occurred during the past decade in the Bohemian Massif.

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Michal Vlk
Reconstruction of the Torsion Photoelectric Magneometer at Budkov Observatory

Budkov Observatory uses Quartz variometers of Bobrov type for photoregistration since late 60s. In the 70-s several sets were modified with photosensors as mobile triaxial variometer to accompany mobile electrotelluric station. Photosensor used chopped incadescent light source at cca 30 Hz, two antiserialy connected photodiodes and OA based envelope detectors. Due to using of modulation, distance between Bobrov element and photosensor was only several centimeters and the unit was designed to work in linear regime without feedback. One unit was installed at the Budkov observatory in the early 1990s and was operating till failure in 2010. Due to high level of spurious signals and complicated PSU the system was modified rather than repaired. Light source was replaced by red laser diode chopped at 14 kHz. Amplifier after antiparallel photodiodes uses LC tuned input circuit and capacitor - type noiseless feedback for its damping. Distance between Bobrov unit was increased up to one meter to use former LaCour photoregistrator pillar. Feedback regulator uses two blocks in parallel: integrator and proportional (leaky) derivator. Voltage output of integrator part is digitised by ADAM4017 converter. Feedback regulator can be switch-off to achieve initial set-up via Bobrov unit magnets. System is powered by 12 V VRLA battery continuously charged from AC mains. Negative supply for OAs is provided by flyback converter using constant duty MOSFET switch, MAGAMP for regulation and SiC schottky diode as output rectifier. System is running since May 2016 and outperforms common fluxgate variometer in terms of noise background and EMI resistance.

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Luděk Vecsey
Seismic data quality assurance and control

Upgrading isotropic seismic methods to anisotropic ones, number of searched seismic parameters (e.g., elastic coefficients, full 3D tasks) and volume of seismic data increase substantially. Good back-azimuth coverage of source regions and high-quality data are crucial for obtaining reliable 3D anisotropic models. We present several quality assurance procedures focusing on questions related to potential difficulties occurring during data analyses, on how they can be identified and how they can influence results.

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Helena Munzarová and Jaroslava Plomerová
Tomographic studies of the BM upper mantle

New high-resolution tomographic models of P- and S-wave isotropic-velocity perturbations for the Bohemian upper mantle are estimated from carefully pre-processed travel-time residuals of teleseismic P, PKP and S waves recorded during the BOHEMA passive seismic experiment. The new data resolve anomalies with scale lengths 30-50 km. The models address whether a small mantle plume in the western Bohemian Massif is responsible for this geodynamically active region in central Europe, as expressed in recurrent earthquake swarms. Velocity perturbations of the P- and S-wave models show similar features, though their resolutions are different. No model resolves a narrow sub-vertical low-velocity anomaly, which would validate the hypothetical ‘baby-plume’ concept. The new tomographic inferences complement previous studies of the upper mantle beneath the Bohemian Massif, in a broader context of the European Cenozoic Rift System (ECRIS) and of other Variscan massifs in Europe.

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Jaroslava Plomerová and Vladislav Babuška
Continental lithosphere - mosaic of microplates with a rigid mantle lithosphere

Several domains (microplates) have been recognized by seismic anisotropy in the European continental mantle lithosphere. The mantle domains often correspond to major crustal units and each of the domains bears a consistent fossil olivine fabric formed most probably during their origin. From characteristic features of changes of anisotropic parameters of body waves, we infer geometry of boundaries between the domains, on a smaller-scale, e.g. within the Bohemian Massif, or, on a larger-scale, on contacts of cratons with younger tectonic provinces. The south-western edge of the Baltic Shield is characterised by sharp changes of the lithosphere thickness, as well as of the anisotropy pattern, mantle velocities and other physical parameters. The Shield is bounded by the steep and narrow Sorgenfrei-Tornquist Zone. On the other hand, deep contacts of the Karelian and East European Cratons with Proterozoic or Phanerozoic mantle lithosphere appear as transition zones, where seismic velocity and anisotropy change gradually indicating a wedge-like penetration of the cratonic mantle lithosphere into or beneath neighbouring younger mantle domains.

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