Zemětřesení - jeden z nejmocnějších projevů planety Země, který pozorujeme na jejím povrchu, je většinou způsobeno křehkým porušením (praskáním, trháním) horninového masivu podél existujících zlomů v důsledku tektonického napětí v zemské kůře a ve svrchním plášti. Náhlé prasknutí horniny budí rychlé pohyby v ohnisku zemětřesení, které se dále šíří formou seismických vln do celého zemského tělesa. Vysoké tlaky panující v nitru nepřejí vzniku dutin, nýbrž upřednostňují střižný (smykový) skluz podél zlomové plochy. Vystupuje-li zlom na povrch, můžeme střižné posuvy horninových bloků přímo pozorovat (obr. 1). Z fyzikálního hlediska je zemětřesení komplikovaný nelineární proces křehkého porušení materiálu a v určité omezené oblasti (na zlomech v délce od několika metrů až do stovek kilometrů), odehrávající se v poměrně krátkém časovém úseku (od milisekund do desítek sekund). Velikost skluzu mezi sousedními bloky činí od několika milimetrů až po několik metrů. Čím je skluz rychlejší, tím intenzivněji vyzařuje seismické vlny. U některých zemětřesení se však uplatní i nesmyková složka deformace, a právě těm se v článku věnujeme., Jan Šílený, Václav Vavryčuk, Jiří Zahradník., and Obsahuje seznam literatury
The NKCA and KVCA small-aperture (~ 90 m) seismic arrays in western Bohemia represent the second deployment of this type in the Czech Republic. The arrays have a regular triangular geometric configuration with high gain three-component seismographs in each corner of the triangle. This allows very weak local earthquakes to be detected with a high degree of precision and, thereby, substantially enhances the results of the pre-existing local seismic network (WEBNET). This paper reports on the pilot measurement period. It summarises the final configuration of the arrays and compares the derived results. The measured data have been analysed using the DP/EP system developed by NORSAR. This incorporates several array techniques such as beamforming, f-k analysis, and the cross-correlation method. It has been shown that during the study period, the levels of seismic activity recorded by the seismic arrays were up to fifteen times greater than the levels recorded by the pre-existing seismic network., Jaroslav Štrunc and Milan Brož., and Obsahuje bibliografii
The West Bohemia earthquake swarm foci are approximated by a circular seismic source model, which radius is assumed to depend only on magnitude of the event. We consider two different models of average slip (i) a constant slip and (ii) a slip exponentially scaled to the magnitude of the event. Based on these assumptions, we stacked the contributions of individual events into representative final fault slip. We processed in such a way four significant swarms recorded during the last three decades in 1986, 1997, 2000 and 2008. Constant slip model indicates final slip was composed of 2 or 3 principal asperities located on one or two different planes. On the contrary, scaled slip model indicates that one big asperity prevails. It is not possible yet to select the preferred slip model. Analysis of the temporal activity of all swarms generally shows three principal phases: starting phase, main phase and fading phase; the upwards trend of activity spreading was observed (slip animation is presented in www supplement http://www.ig.cas.cz/kolar/StopPhase/Asperity). The maximal possible cumulative slip value may have reached the order of meters., Petr Kolář, Bohuslav Růžek, Alena Boušková and Josef Horálek., and Obsahuje bibliografii
This paper presents the results of the application of wavelet decomposition to processing data from the GGP sites (The Global Geodynamics Project). The GGP is an international project within which the Earth's gravity field changes are recorded with high accuracy at a number of stations worldwide using superconducting gravimeters. Data with a 5-second sampling interval from Wettzell and Bad Homburg were used for the research. The wavelet transform enables the investigation of the temporal changes of the oscillation amplitudes or the decomposition of the time series for the analysis of the required frequencies. The wavelet decomposition was performed using the regular orthogonal symmetric Meyer wavelet. The research concerned data from an earthquake period recorded at various locations and a quiet period when the gravimeters worked without any disturbances. The decomposition was followed by the Fast Fourier Transform for signal frequency components and then by correlation analyses of corresponding frequency components (for periods from 10 to 60 000 seconds) for all sensor combinations, for the quiet and the earthquake periods separately. Frequency components defining long term changes for all sensor combinations, as well as combinations between two sensors at the same site for the quiet days are characterised by high correlation coefficients. For the time of the earthquake, the Wettzell site data proved strong correlation for all frequency components, while the Bad Homburg site data showed an unexpected decrease of correlation for the majority of frequency components. The authors also showed that wavelet decomposition can be a good method of data interpolation, especially from the time of earthquakes. Moreover, it is a very useful tool for filtering the data and removing the noises., Janusz Bogusz, Anna Klos and Wieslaw Kosek., and Obsahuje bibliografii