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Tomographic P-wave velocity inversion has been performed for the Pannonian region using first arrival times of local and regional events from the bulletins of the International Seismological Center and the Hungarian Earthquake Bulletins. The input data consist of 4071 arrival times of 570 events. A three-dimensional velocity model has been gained by joint hypocenter-velocity inversion on a coarse and a fine non-uniform cartesian grid of nodes. The initial one-dimensional model was determined by genetic algorithm. A bending ray tracer has been used to calculate the theoretical travel times and ray paths. The reliability of the inverted velocity parameters were checked by the checkerboard method and by the analysis of the model resolution matrix. The results are generally in agreement with the known structural characteristics of the Pannonian Basin. An interesting high-velocity anomaly has been found in the uppermost mantle beneath the southern part of the Great Hungarian Plain.

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The crustal and uppermost mantle structure beneath the Mátra Mountains has been investigated by means of the inversion of teleseismic P-wave receiver functions.  Earthquakes recorded at the PSZ station in the period 1995-2000 with epicentral distances of between 20° and 100° and with magnitudes greater than 5.5 have been selected for analysis if their signal-to-noise ratio was sufficiently high. Source equalisation has been carried out to gain the radial and tangential receiver functions.  The traces from similar backazimuths and distances were stacked to improve the data quality. The inversion of teleseismic waves coming from the north-east and the west shows that in the upper part of the crust the S-wave velocity has strong positive gradient, the middle crust exhibit almost uniform velocities, while in the lower crust and uppermost mantle the velocity increases monotonically. Velocity profiles for the eastern backazimuths are similar to the previous ones in the case of the upper and middle crust and uppermost mantle, however in the lower crust a low-velocity zone can be observed. The properties of polarity distribution of tangential receiver functions can be associated with WNW-dipping lower crustal interfaces.

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In this paper, a theory describing the possible origin of shallow depth earthquakes occurring within the sedimentary depressions of the Pannonian Basin is presented. In addition, the seismicity of Budapest is re-evaluated by synthetic seismogram based deterministic seismic hazard estimation.

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The spatio-temporal patterns of the seismicity are shown. The earthquake occurrence is connected to the strain rate on the basis of Kostrov (1974) equation. It is shown that in the Pannonian Basin the seismic rates are 10-7-10-6. This values were derived from the local earthquake catalogues of the area of Komárom, Dunaharaszti (near Budapest) and Kecskemét.  It is shown that the earthquake occurrence in Hungary  has a well expressed diurnal periodicity.  Similar phenomenon was detected in the case of earthquakes of all Europe and in Turkey in case of weak (M L = 3.0) seismic events.

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Analysing the areal distribution of earthquakes produced in the Carpathian Basin the conclusion can be drawn that only certain parts can be considered active along fracture lines, namely those parts which separate individually moving blocks. Generally accepted working theory states that if one fracture line ever generated an M-magnitude earthquake then at any point of the same line a similar or larger tremor may happen again. However, this principle is not supported by domestic experiences. In accordance with focal depths analysis we are going to verify that earthquakes were produced within more or less consolidated layers inside subsiding basins. Our analysis is aimed to explain the possible origin of earthquakes within small depth range and to point out the practical benefit of these investigations. We present also an analysis on the possible origin of „basin tremors” not taken so far into consideration and we offer a plausible explanation. Uncertainties of focal depth (hypocenter) determinations will also be given and we define relations between the focal depth (h), the magnitude (M), and the epicentral intensity (Io). The result will be presented in tables and comparison will be given between the focal depth data determined by us and by others. Finally viewpoints will be presented to help the recognition of earthquake foci and to set earthquake hazard determination on a real basis.

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Acta Geodaetica et Geophysica Hungarica
Authors: Z. Wéber, Z. Bus, K. Gribovszki, B. Süle, Gy Szeidovitz, and P. Varga

In this paper, research activities at the Theoretical Division of the Seismology Department of GGRI (in short: Seismological Observatory) are summarized. The reported investigations have been carried out since 1999, when the division was founded with three members. Detailed discussions of the presented results can be found in various national and international scientific journals.

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Acta Geodaetica et Geophysica Hungarica
Authors: P. Varga, Z. Bus, B. Süle, A. Schreider, C. Bizouard, D. Gambis, and C. Denis

In its first part this work focuses on connection of length of day (LOD) with centered and eccentric geomagnetic dipole fields described with the use of Gaussian coefficients derived from global geomagnetic observations is discussed for the epoch 1900–2000. The statistical comparison of temporal variation of earth magnetic and astronomical data shows close correlation of geomagnetic dipole momentum M 0 and ΔLOD. It should be mentioned that the time-correlation is closer when the centred geomagnetic dipole is used for statistical modelling. In the same time no relation was found between ΔLOD and the orientation variations of the geomagnetic dipole.In the second part of present study the connection of geomagnetic field and the LOD is investigated in geological time-scale. A significant ΔLOD was found which coincides in time with the geomagnetic Mesozoic low. The reason of this coincidence is enigmatic.

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