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For a given earth model and hypocenter location, short-period local waveforms are inverted for the six unknown moment tensor rate functions (MTRFs) by a linear procedure. Subsequently, the MTRFs are decomposed into the source time function (STF) and the time independent moment tensor, fully describing the focal mechanism of the event. In case of short epicentral distances, routinely determined hypocenters are usually not accurate enough to be used in focal mechanism inversion. However, introducing hypocenter co-ordinates as unknown parameters leads to a nonlinear inversion problem. In order to solve this nonlinear problem, a probabilistic approach is applied in this study. The a priori probability density function (PDF) for the hypocenter location is given by a routinely used location algorithm. Assuming that all uncertainties can be described by Gaussian PDFs, measurement errors and theoretical errors are estimated. Then the results of the Bayesian approach are the posterior PDFs for both the hypocenter co-ordinates and the MTRFs. Decomposing the MTRFs, the PDFs for the STF and the moment tensor are also deduced. The estimated uncertainties in the moment tensor components are plotted on the focal sphere in such a way, that the significance of the double-couple, the CLVD, and the volumetric parts of the source can be assessed. The method is illustrated through the waveform inversion of a local event that occurred in the central part of the Pannonian Basin. The moment tensor solution for the selected event has negligible volumetric part, implying the tectonic nature of the event. The retrieved mechanism is in agreement with the available clear readings of first-arrival P-wave polarities. The principal axes of the resulting source mechanism also agrees well with the main stress pattern published for the epicentral region.
We determined a new one-dimensional P-wave velocity model for the territory of Hungary based on the first arrival times of local earthquakes. During the computations 910 P-wave arrival data of 86 events from the time period between 1985 and 2010 have been used. The applied methodology is a combination of a genetic algorithm based procedure and an iterative linearized joint inversion technique. The preferred velocity profile has been chosen from the best models based on the data of a series of controlled explosions.The resulting flat-layered model consists of three crustal layers and a half-space representing the uppermost mantle. The crustal compressional velocities vary in the range of 5.3-6.3 km/s, while the uppermost mantle velocity was found to be 7.9 km/s. The Moho is located at an average depth of 26 km.Additionally, the V p/V s ratio was calculated by the Wadati-method, which gave a value of 1.74±0.05.
The Carpathian Basin is situated in the territory between the Mediterranean area, which is seismically one of the active regions and the Carpathian Mountains belt. The temporal variation of seismicity is investigated on the example of three seismo-tectonically different regions: the Carpathian Basin, the Vrancea region and the Dinarides. The seismicity is analyzed since 1900 in order to investigate the existence of diurnal periodicities using hodographs. There are two different diurnal distributions opposing each other: maximum early morning dominates until the year 1963, followed by a period of time when earthquakes seem to occur more often around 13h local time mainly concerning the weak M M < 3.2 events. The midday maximum in the number of minor events may be caused by the inclusion of quarry blasts, but the diurnal geomagnetic variations correlate well with diurnal changes in earthquake activity. Duma and Rhuzin (2003) suppose that the current vortices induced by Sq variations in the lithospheric layer, flowing across the horizontal component of the geomagnetic field generate a torque which can be added to the tectonic loading stress (which have a maximum about noon) and may help trigger instability in a fault approaching the failure threshold.The spatial and temporal fractal structures of earthquakes were analyzed using the box counting method. The regions were divided into different size r of a square box and were counted the minimum number N ( r ) of boxes necessary to cover all the data. The recurrence times of earthquakes are shown to be a clustering process and are much higher in the Carpathian Basin. The earthquakes in these regions have self-similar structures. The slope of log N -log r function for Carpathian Basin breaks at about 20 km, which divides the range into two bands. This breaking at about 20 km maybe connected to the intrinsic weakness of the Carpathian Basin lithosphere.
Seismic coda Qattenuation (Q c) in the frequency range from 6 to 20 Hz of two distinct Scoda windows (early and later portions) are compared to analyse the effects of both coda windows on crustal seismic attenuation estimates around Samambaia fault (João CâmaraNortheastern Brazil). Q cvalues associated with the later portion are systematically higher than those related to the early portion. These values follow a frequency (f) function given by Q c(f) = Q 0 f , where Q 0= 11739 and= 1.000.06. In general, Q 0estimate is less sensitive to site effects and stabler than that obtained from the early portion of S coda waves, while its corresponding frequency dependence is similar to that obtained from the early portion of S coda waves. It suggests thatparameter does not depend on coda window's location along the seismic signal. A comparative analysis of both Q 0andvalues with those found recently shows that there is no difference in using early or later portion of S coda waves in the stations located on Pre-Cambrian basement in the João Câmara area. This comparison also shows that the major variations in Q 0values were observed at seismic stations installed on sedimentary terrain. Differences in the seismic attenuation, in both sides of the Samambaia fault, were also observed in this study, and it is in agreement with the hypothesis that Samambaia fault is a kind of boundary between two seismic attenuation zones.
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.
Systematic earthquake data collection in the Pannonian region started early in the nineteenth century. Officially organised collection and evaluation of earthquake data started in 1881 when the Permanent Earthquake Committee was founded within the Hungarian Geological Society. We can regard this Committee as the ancestor of the Seismology Department of the Geodetic and Geophysical Research Institute (GGRI). Today the Observatory Division of the Seismology Department of GGRI (in short: Seismological Observatory) gathers instrumental and macroseismic earthquake data and also deals with determination of earthquake focal parameters, among other tasks. The Seismological Observatory operates 7 stations in the country. Six of these stations are very broadband ones with near real-time accessibility of data. Based on the national seismograph station network and also integrating data from agencies of neighbouring countries our Division localises 80-100 local earthquakes annually. The relatively numerous localised earthquakes serve a base for, among others, crustal structure studies and also for more precise earthquake hazard evaluation.
Estimation of seismic wave attenuation in the shallow crust in terms of coda wave Q structure previously investigated (Sayed et al. 2002) in the vicinity of Cairo Metropolitan area was improved using seismograms of local earthquakes recorded by the Egyptian National Seismic Network (ENSN). The seismic wave attenuation was measured in the study area from the time decay of coda wave amplitudes on narrow bandpass filtered seismograms on the basis of the single scattering theory. The frequency bands of interest are from 1.5 Hz to 18 Hz. In general, the values obtained for paths and events foci and El-Fayoum (FYM) and Wadi Hagul Sations (HAG) are very similar for all frequency bands. A regional attenuation law Q c = 85.66 f 0.79 was obtained.
It is well known that surface geology may significantly affect strong ground motion and hence it should be incorporated into seismic hazard estimation at a specific site. Numerical and empirical methods are used to determine the site effect. On areas with moderate seismicity the empirical methods are based on microtremor measurements. Solely the single station methods, for example the H/V spectral ratio technique, could be used in the absence of a nearby reference station situated on rock site. The one year long noise measurements of 8 stations belonging to the Hungarian Microseismic Monitoring Network have made it possible the testing of this method. The stations are situated in the middle part of Hungary. All seismometers are triaxial LE-3D high sensitivity 1 Hz geophones. Three stations are situated on hard rocks while five ones on loose site above a deep sedimentary basin with more than 1000m thick sediments. The large amount of noise data have made it possible to examine the stability of the method. The examinations have shown that the computed spectral ratio curves are very stable in time. While all stations on rock sites show nearly at H/V spectral ratio, all stations on soft sites show a peak at very low, 0.1- 0.2Hz frequencies. Analytical computations have been carried out to verify this low frequency behaviour and have shown that these peaks are due to the fundamental frequency of the overall sediment thickness. H/V ratios have also been calculated from seismograms of weak local earthquakes, so they could be compared with ratios obtained from noise measurements.
