[1] A train of large amplitude infrasound wave packets was observed by multipoint Continuous Doppler sounding system in the ionosphere over the Czech Republic on 11 March 2011. It is shown that these infrasound wave packets originated from vertical motion of the ground surface that was caused by arrival of seismic waves generated by the strong Tohoku earthquake. The infrasound wave packets were observed in the ionosphere at heights of $210-220 km about 9 min after the detection of corresponding wave packets on the ground, which is consistent with the calculated time for vertically propagating infrasound waves. Absolute values of cross-correlation coefficients between ionospheric and ground measurements are typically higher than 0.9 (for two wave packets $0.98). The individual wave packets recorded on the ground have different observed horizontal velocities and correspond to different types of seismic waves. A comparison of vertical velocities of ground motion with oscillation velocities of air particles in the ionosphere indicates that almost 1/10 of the infrasound energy flux excited at the ground reached the altitudes of $210-220 km for wave periods longer than $30 s. Estimates of sound attenuation are performed. It is also shown that it is necessary to consider the value of electron density gradient at the reflection height of the sounding radio wave, and air (plasma) compression owing to the infrasound wave to get reasonable estimates of oscillation velocities of air particles from Doppler shift frequencies.
P‐SV conversions provide new insights into the lithosphere of the western Eger (Ohře) Rift, a presently active CO2 emanation area, Quaternary volcanic field, and earthquake swarm region in central Europe. Gas and isotope (He and C) mapping of free gas phases in mineral springs and mofettes proved the origin of CO2‐ dominated gases from a subcrustal magmatic fluid reservoir. Analyzing teleseismic data from several seismic networks in the western Bohemian Massif, the source region of these gases was investigated. Moho Ps conversions have 3 to 4.5 s delay. Crustal thicknesses vary between 27 and 38 km; vp/vs ratios vary between 1.63 and 1.81. Beneath the western Eger Rift an approximately 40 km wide Moho updoming up to 27 km exists. Locally observed weak conversions indicate a complex Moho transition zone in this area. A local “6 s phase” possibly originates at a discontinuity in approximately 50 to 60 km depth or may represent multiples from velocity inversions at the base of the upper crust. Moho updoming and the distribution of the “6 s phase” coincide with the CO2 degassing fields and the positions of Quaternary volcanoes at the surface. We hypothesize the release of CO2‐dominated fluid/magma from isolated melt reservoirs in the depth range of 60 to 30 km, separation of CO2 from the melt at 29 to 21 km depths, and CO2 transport through the crust. The geophysical indications may point to presently active magmatic underplating beneath the study area, supporting the results of gas geochemical and isotope investigations. This is the first attempt that combines seismic and gas geochemical data for a tectonic model. Our model may be transferable to other continental rift areas worldwide.
Abstract--The main shock of the West-Bohemian earthquake swarm, Czechoslovakia, (magnitude m = 4.5, depth h = 10 km) exhibits an irregular areal distribution of macroseismic intensities 6 ~ to 7 ~ MSK-64. Four lobes of the 6 ~ isoseismal are found and the maximum observed intensity is located at a distance of 8 km from the instrumentally determined epicentre. This distribution can be explained by the energy flux of the direct S wave generated by a circular source, the hypocentral location and focal mechanism of which are taken from independent instrumental studies. The theoretical intensity, which is assumed to be logarithmically proportional to the integrated squared ground-motion velocity (i.e., ! = const + log S v2(t) dt), fits the observed intensity with an overall root-mean-square error less than 0.5 ~ It is important that the present intensity data can also be equally well explained by the isotropic source. The fit was attained by means of a horizontally layered model though large fault zones and an extended sedimentary basin suggest a significant lateral heterogeneity of the epicentral region. The results encourage a broader application of the simple modelling technique used.
Úvod V první polovině června 2014 zaregistrovaly seismické stanice Ústavu fyziky Země PřF Masarykovy univerzity (ÚFZ) sérii čtyř zemětřesení na j. Moravě s ohniskem u obce Hostěradice. Magnitudo nejsilnějšího otřesu z 1. 6. 2014 bylo 2,0. Toto zemětřesení bylo dokonce pocítěno obyvateli v nejbližším okolí epicentra. Z emětřes ení s ohniskem na jižní Moravě se vyskytují výjimečně. O lokálních historických zemětřeseních máme jen několik zpráv. Od roku 1995 je oblast sledována seismickou stanicí Moravský Krumlov (KRUC), od roku 2013 další stanice monitorují případnou seismickou aktivitu v okolí jaderné elektrárny Dukovany. I citlivé přístroje však dosud zaregistrovaly pouze několik velmi slabých otřesů. Série otřesů z června 2014 V neděli 1. 6. 2014 v 00.43 hod. UTC (Coordinated Universal Time, tj. 02.43 hod. středoevropského letního času-SELČ) zaregistrovaly seismologické stanice ve střední Evropě zemětřesení s ohniskem západně od obce Hostěradice (obr. 1). Souřadnice epicentra jsou 48,95° severní zeměpisné šířky
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