Bohuslav Růžek

Crustal anisotropy in the Bohemian Massif, Czech Republic: Observations based on Central European Lithospheric Experiment Based on Refraction (CELEBRATION) 2000

[1]xa0We study the azimuthal velocity variation of Pg waves in the Bohemian Massif using data collected during Central European Lithospheric Experiment Based on Refraction (CELEBRATION) 2000. We analyze travel times of waves generated by 28 shots and recorded by 256 portable and 19 permanent seismic stations deployed on the territory of the Czech Republic and in adjacent areas. We use recording offset ranging from 30 to 190 km with azimuths covering the whole interval of angles. The observed travel times are inverted for parameters of a velocity model formed by an isotropic low-velocity subsurface layer with a varying depth lying on a homogeneous transversely isotropic half-space with a horizontal axis of symmetry. The recovered velocity displays a systematic azimuthal variation indicating a regional-scale intrinsic or effective anisotropy in the Bohemian Massif. The mean, minimum and maximum values of the velocity are vmean = 6.03 km/s, vmin = 5.98 km/s, vmax = 6.10 km/s, respectively, indicating an anisotropy of 1.5–2.5%. The direction of the maximum propagation velocity is ∼N35°E being approximately perpendicular to the present maximum compression in the Earth crust in central Europe. The observed anisotropy cannot be induced by stress-aligned cracks in the crust, because the crack models predict azimuthal velocity variations completely inconsistent with the observed one. Therefore we suggest the crustal anisotropy to be induced by a preferred orientation of rock-forming minerals and large-scale intrusion fabrics developed during a tectonic evolution of the Bohemian Massif.

Differential Evolution Algorithm in the Earthquake Hypocenter Location

Abstractu200a—u200aA novel global optimizing algorithmu200a—u200aDifferential Evolution (DE)u200a—u200ahas appeared recently. This method is easy and advantageous when used for kinematic location of the earthquake hypocenter. The motivation for implementing a robust (i.e., global and nonlinear) optimizing algorithm for the location problem is to obtain better results than those from the classical (i.e., linearized) approach (such as the FASTHYPO, HYPOELLIPSE, HYPOCENTER solutions, among others). Better solutions have lower final misfits expressed as the common L2 norm. The features of the DE algorithm are studied on a set of synthetic location problems. The DE procedure is controlled by 3 internal parameters, which are easy to adjust, and the convergence properties are very good. Location results using DE are compared with the HYPO71 solutions for real earthquake data from the Gulf of Corinth region, Greece. The DE results are significantly better. The DE optimizing algorithm seems to be very promising both for the location problem as well as for other problems in geophysics.

Statistical Interpretation of Webnet Seismograms By Artificial Neural Nets

We employed multilayer perceptrons (MLP), self organizing feature maps (SOFM), and learning vector quantization (LVQ) to reveal and interpret statistically significant features of different categories of waveform parameter vectors extracted from three-component WEBNET velocigrams. In this contribution we present and discuss in a summarizing manner the results of (i) SOFM classification and MLP discrimination between microearthquakes and explosions on the basis of single-station spectral and amplitude parameter vectors, (ii) SOFM/LVQ recognition of initial onset polarities from PV-waveforms, and (iii) a source mechanism study of the January 1997 microearthquake swarm based on SOFM classification of combined multi-station PV-onset polarity and SH/PVamplitude ratio (CPA) data.Unsupervised SOFM classification of 497 NKC seismograms revealed that the best discriminants are pure spectral parameter vectors for the recognition of microearthquakes (reliability 95% with 30 spectral parameters), and mixed amplitude and spectral parameter vectors for the recognition of explosions (reliability 98% with 41 amplitude and 30 spectral parameters). The optimal MLP, trained with the standard backpropagation error method by one randomly selected half of a set of 312 mixed (7 amplitude and 7 spectral) single-station (NKC) microearthquake and explosion parameter vectors and tested by the other half-set, and vice versa, correctly classified, on average, 99% of all events.From a set of NKC PV-waveform vectors for 375 events, the optimal LVQ net correctly classified, on average, 98% of all up and 97% of all down onsets, and assigned the likely correct polarity to 85% of the onsets that were visually classified as uncertain.Optimal SOFM architectures categorized the CPA parameter vector sets for 145 January 97 events individually for each of five stations (KOC, KRC, SKC, NKC, LAC) quite unambiguously and stable into three statistically significant classes. The nature of the coincidence of these classes among the stations that provided most reliable mechanism-relevant information (KOC, KRC, SKC) points at the occurence of further seven statistically significant subclassses of mechanisms during the swarm. The ten ‘neural’ classes of focal mechanisms coincide fairly well with those obtained by moment tensor inversion of P and SH polarities and amplitudes extracted from the seismograms interactively. The obtained results, together with those of refined hypocenter location, imply that the focal area consisted of three dominant faults and at least seven subfaults within a volume of not more than 1 km in diameter that likely were seismically activated by vertical stress from underneath.

Joint inversion of teleseismic P waveforms and surface-wave group velocities from ambient seismic noise in the Bohemian Massif

Joint inversion of teleseismic P-waveforms and local group velocities of surface waves retrieved from ambient seismic noise has been performed to model velocity structure of the crust and uppermost mantle of the Bohemian Massif. We analysed P-waveforms of 381 teleseismic earthquakes recorded at 54 broadband seismic stations located on the territory of the Czech Republic and in its close surroundings. Group velocities of Rayleigh and Love surface waves were obtained by cross-correlating long-term recordings of seismic noise. The basis for waveform inversion is the well-known methodology of P-to-S receiver functions constructed from converted phases. Due to instabilities in direct inversion of receiver functions caused by the necessity of applying deconvolution, we propose an alternative formulation to fit observed and calculated radial components of P waveforms. The joint inversion is transformed into a search for the minimum of the cost function defined as a weighted sum of waveform and group velocity misfits. With the use of the robust stochastic optimizer (Differential Evolution Algorithm), neither derivatives nor a starting model are needed. The task was solved for 1D layered isotropic models of the crust and the uppermost mantle. We have performed a sequence of inversions with models containing one, two, three and four layers above a half-space. By using statistical criteria (F-test) we were able to select the simplest velocity models satisfying data and representing local geological structures. Complex crustal models are typical for stations located close to boundaries of major tectonic units. The relatively low average P to S wave-velocity ratio is in agreement with the generally accepted view that the BM crust is predominantly felsic.

EARTHQUAKES IN THE CZECH REPUBLIC AND SURROUNDING REGIONS IN 1995-1999

In the time span from January 1995 to December 1999 the Czech National Seismological Network (CNSN), consisting of ten permanent digital broadband stations, several local networks and two data centers, detected and recorded 9530 regional natural seismic events, 27 greater than magnitude 2. Most of these events were located by the Czech Seismological Service (CSS), and the most prominent of them were analyzed in detail. A large number of quarry blasts were recorded as well but were not included in the analysis. We provide basic information on the configuration of the CNSN and on the way of routine data processing employed by the CSS in this paper. The over-all regional seismicity monitored by the CNSN in 1995–1999 is briefly reviewed. The main results of observations and evaluation of the local (NW-Bohemia/Vogtland, South Bohemia, Sudeten) and induced (Kladno, Příbram, Upper Silesia, Lubin/Poland) seismic activity within this period are presented in a condensed form. Finally, a summary on macroseismic observations on the territory of the Czech Republic in 1995–1999 is also presented.

Contribution of Local Seismic Networks To The Regional Velocity Model of The Bohemian Massif

The routine location of regional seismic events using data from the Czech National Seismological Network (CNSN) is based on Pn, Pg, Sn, Sg phases. A simple velocity model derived from Kárníks (1953) interpretation of an earthquake in Northern Hungary in 1951 has hitherto been used. At present, numerous local seismic networks record and locate local events, which are occasionally recorded at regional distances as well. Due to the relatively small dimensions of local networks, hypocenters (and origin times) determined by a local network might be considered as nearly exact from the point of view of regional-scale CNSN. The comparison of common locations performed by CNSN and by a local network enables us to estimate the accuracy of CNSN locations, as well as to optimize a simple velocity model. The joint interpretation of the CNSN bulletin and the catalogues of four local seismic networks WEBNET, OSTRAVA, KLADNO and LUBIN produced a new ID velocity model. The most frequent epicentral error in this model is less than 5 km, and most foci lie up to 15 km from the true position. The performed analysis indicates bimodal distribution of Sn residuals.

Periodicity of Mining and Induced Seismicity in the Mayrau Mine, Czech Republic

Mine excavation in deep black-coal Mayrau Mine, Czech Rep., has a pronounced 7-day periodicity due to the absence of mining during weekend days. Also seismicity induced by mining exhibits some features of the 7-day period. It was found that the number of events generally depends on the intensity of mining, while the analogous dependences of mean energy per event and the total energy per day are weaker. Selective analysis showed that total extent of exploitation has more influences on the seismicity than coining of drifts. The statistical parameters of seismicity (b-value) do not change in accordance with the day of the week.

Orbital Signals in Carbon Isotopes: Phase Distortion as a Signature of the Carbon Cycle

Isotopic mass balance models are employed here to study the response of carbon isotope composition (δ13C) of the ocean-atmosphere system to amplitude-modulated perturbations on Milankovitch time scales. We identify a systematic phase distortion, which is inherent to a leakage of power from the carrier precessional signal to the modulating eccentricity terms in the global carbon cycle. The origin is partly analogous to the simple cumulative effect in sinusoidal signals, reflecting the residence time of carbon in the ocean-atmosphere reservoir. The details of origin and practical implications are, however, different. In amplitude-modulated signals, the deformation is manifested as a lag of the 405xa0kyr eccentricity cycle behind amplitude modulation (AM) of the short (~100xa0kyr) eccentricity cycle. Importantly, the phase of AM remains stable during the carbon cycle transfer, thus providing a reference framework against which to evaluate distortion of the 405xa0kyr term. The phase relationships can help to (1) identify depositional and diagenetic signatures in δ13C and (2) interpret the pathways of astronomical signal through the climate system. The approach is illustrated by case studies of Albian and Oligocene records using a new computational tool EPNOSE (Evaluation of Phase in uNcertain and nOisy SEries). Analogous phase distortions occur in other components of the carbon cycle including atmospheric CO2 levels; hence, to fully understand the causal relationships on astronomical time scales, paleoclimate models may need to incorporate realistic, amplitude-modulated insolation instead of monochromatic sinusoidal approximations. Finally, detection of the lagged δ13C response can help to reduce uncertainties in astrochronological age models that are tuned to the 405xa0kyr cycle.

Significance of Geological Units of the Bohemian Massif, Czech Republic, as Seen by Ambient Noise Interferometry

A bstractBroadband recordings of 88 seismic stations distributed in the Bohemian Massif, Czech Republic, and covering the time period of up to 12xa0years 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 (Saxothuringian, Teplá-Barrandean, Sudetes, Moravo-Silesian, and 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 ~3–15xa0km is relatively homogeneous across the investigated region, while both uppermost horizon (0–3xa0km) and lower crust (>15xa0km) exhibit lower degree of homogeneity.

Estimation of finite seismic source parameters for selected events of the West Bohemia year 2008 seismic swarm

Finite seismic source parameters were determined for a set of 91 selected events of the West Bohemia year 2008 earthquake swarm (ML from 0.6 to 3.7) using the stopping phases method. According to the theory, two stopping phases are generated along the source border where the rupture process terminates. These two phases form a Hilbert transform pair; it is also a criterion for their identification. Circular and elliptical source models were considered and their parameters were recovered using the differences in arrival times between the identified stopping phases. Generalization of the circular to elliptical model was found to be statistically significant only for a minority of the events; consequently, only circular source models were investigated in detail. Individual source parameter errors were estimated with the use of the jackknife method. Our results are in good agreement with a previously published theoretical formula relating source radius and magnitude and also with the relation derived in the year 2000 swarm. Our results also confirm rather well the general theoretical assumption about the constant stress drop (with median value of 2.4xa0MPa and with the majority of values ranging from 1 to 10xa0MPa).