A novel method called Generalized Average of Signals (GAS) for detection of very weak waves in seismograms is describe d and tested. The general principle of the GAS method is to take advantage of the cohe rency of the signal, which is extracted. The signals are shaped with moving window and converted to the frequency domain. Then they are non-linearly summed considering their complex representation (a mplitudes and phases). The method impr oves signal-to-noise ratio of coherent seismograms considerably. The GAS method is tested on sy nthetic seismograms and comp ared with the PWS method., Jiří Málek, Petr Kolínský, Jaroslav Štrunc and Jan Valenta., and Obsahuje bibliografické odkazy
The western part of the Bohemian Massif (Vogtland/West-Bohemia region at the Czech-German border) is characterized by relatively frequent intraplate earthquake swarms and by other manifestations of current geodynamic activity, such as mofettes, mineral and thermal springs. In this study we analyze variations of groundwater level in four hydrological wells in the region during the years 2005-2010. Monitoring during the previous time interval of 2000-2004 is also mentioned and used for comparison. Two of the wells are located in the epicentral region of Nový Kostel, and the other wells are more distant. The time interval includes the 2008 earthquake swarm when all the wells displayed a noticeable drop in the water level. This effect was observed up to epicentral distances of nearly 30 km, which exceeds the distances of hydrological changes observed during previous earthquake swarms. Moreover, it seems that a small rise in the water level preceded the intervals of increased seismic activity, which could represent a certain precursory phenomenon. On the other hand, the hydrological changes in the Nový Kostel area were relatively small, indicating that this epicentral area is not hydrologically linked with the seismically active fault at depth. Consequently, more suitable localities for hydrological monitoring should be sought in a broader vicinity of Nový Kostel., Renata Gaždová, Oldřich Novotný, Jiří Málek, Jan Valenta, Milan Brož and Petr Kolínský., and Obsahuje bibliografii
In October 2005 local seismic monitoring started in the area of the Hronov-Poříčí Fault Zone. In the present paper we summarize seismic measurements using a small-aperture seismic array Ostaš. Parameters of the array and methodology of the data registration and processing are described. The list of local microearthquakes during two years of registration is presented., Jiří Málek, Milan Brož, Vladimír Stejskal and Jaroslav Štrunc., and Obsahuje bibliografické odkazy
The present paper describes a method for identification of reflected waves on the seismogr ams from a cluster of quarry blasts. These reflections are used for determination of the Moho depth. Only one seismic station is sufficient, but a cluster of seismic sources is needed. To increase the signal-to-noise ratio of reflected waves, several techniques are applied, such as filtering, polarisation analysis and stacking of seismograms. Th e method was tested on seismic data from the central part of the Ore Mountains region, Czech Republic. Seismic waves were generated by strong quarry blasts at the Tušimice open-pit coal mine, and recorded at the Přísečnice (PRI) temporal seismic station at an epicentral distance of about 16 km. As the station was equipped with a three-component seismograph, also S-wave onsets could be determined. Although Pg, Sg and surface waves dominate the seismograms, weak Moho reflections of P and S waves could also be recognized at travel times of 9.7 s and 17.3 s, respectively. From these times we found the mean ratio of the P- to S-wave ve locities in the crust to be 1.78. Considering P-wave velocity model of Beránek (1971), the observed travel times of the reflections yield a crustal thickness of 29.5 -n31.5 km, which agrees with recent receiver function studies. These agreements indicate that the proposed method of reflected seismic waves, generated by quarry blasts, could represent a simple way for mapping the Moho discontinuity., Hana Kampfová, Jiří Málek and Oldřich Novotný., and Obsahuje bibliografické odkazy
Since 1998, a seismic network has been monitoring the underground gas storage located near the town of Příbram in the Central Bohemian Pluton, Czech Republic. Hundreds of weak induced seismic events have been recorded there. Moreover, several weak earthquakes have also been recorded from the vicinity of the nearby Orlík water reservoir. To improve location of both types of seismic events, shallow crustal structure of the region is studied in the present paper. Refraction measurements to distances of about 20 km were carried out using quarry blasts as seismic sources. Smoothed P-wave travel times were interpreted using the Wiechert-Herglotz method, which yielded a 1-D velocity model of shallow crustal structure down to a depth of about 1.7 km. The P-wave velocity of the model increases from about 5.0 km/s at the surface to about 6.15 km/s at the 1.7 km depth., Jiří Málek, Oldřich Novotný and Libor Žanda., and Obsahuje bibliografii
Rayleigh waves in the period range 0.2 - 3.0 s from eight quarry blasts are analyzed to obtain S-wave velocity model beneath the Příbram seven-station array in the Czech Republic. Locations and origin times of blasts are estimated using P- and S-wave onsets and then verified at the quarry in the vicinity of the location. This blind test confirms a sufficient precision of the location procedure for identification of quarries. Epicentral distances are in the range from 16 to 52 km. Group velocity dispersion curves of Rayleigh waves are determined by the frequency-time analysis. An average group velocity beneath the array for each period is computed with the help of mean travel-time curve for all blasts and stations. The resultant group velocity dispersion curve is inverted to obtain a 1-D S-wave velocity model using the Isometric method. The results are compared with known geological structure in the area of interest., Renata Gaždová, Petr Kolínský, Jiří Málek and Jan Vilhelm., and Obsahuje bibliografii