Z. Schenkova

Isoseismal map of the Vrancea earthquake of March 4, 1977

Abstract The iaoseismal map of the intermediate Vrancea earthquake of March 4, 1977 was compiled using the contributions from many European countries within the shaken area. Characteristic features of the macroseiamic field are discussed. Mean radii of isoseismals and intensity attenuation are determined.

Seismic hazard computations for regions with low earthquake activity — A case study for the Belgium, The Netherlands and NW Germany area

The algorithms to evaluate seismic hazard, used and/or developed by five teams participating in the TERESA project, applied to the low seismicity area ‘Belgium, The Netherlands and NW Germany’ are compared. The main differences in the results can be explained by the majority criterion of Egozcue et al. (1989), the differences in the upper bound and zonification and, in some cases, by a higher attenuation.

Categorisation and harmonisation of probabilistic earthquake hazard assessments with respect to statistic representation of input data

Probabilistic hazard calculations were tested with respect to input data uncertainties. Statistical approximations of input data allow the regression coefficients and their standard deviations to be determined. Since inaccuracies of the regressions closely depend on the input data uncertainties, the standard deviations of the individual coefficients were used as a measure of input data parametrisation. Differences in the hazard outputs, caused only by various combinations of the regression coefficients and their standard deviations determined for the cumulative recurrence graph and the attenuation law, were studied. It was concluded that the hazard calculation conditions should be (a) strictly defined and always quoted with every hazard assessment, and (b) standardized in order to establish a common liability of earthquake hazard calculations. The established categorisation of the earthquake hazard assessments divides the probabilistic hazard calculations according to their reliability and helps users to apply them in earthquake engineering practice. The credible hazard assessments corresponding to 90% probability can be recommended for common structures whereas the maximum credible hazard assessments coinciding with 95% probability yield values close to hazard assessments of important structures, hospitals, bridges, etc.

Seismic hazard assessments for two test areas -- 'Southern Italy' and 'Belgium--The Netherlands--Germany'

Within the activity of the ESC Subcommission 8 ‘Engineering Seismology’, Project TERESA, seismic hazard calculations have been performed for two areas of a different earthquake activity. Fundamental seismological data (earthquake catalogues, macroseismic observations and maps) and some additional geological information were analyzed and processed to prepare inputs for hazard computations. Great attention has been paid to verifying the reliability of the input data. Seismic hazards obtained for five sites of the region of high seismic activity (Sannio-Matese, southern Italy) and six sites of the region of low seismic activity (Brabant Massif and northern Rhine, Belgium—The Netherlands—Germany) are presented and discussed.

Seismic zoning map of Czechoslovakia — version 1987

РезюмеПосmроенa новaя кaрma сеŭсмuческого рaŭонuровaнuя Чехословaкuu, в nервую очере¶rt;ь nо¶rt;гоmовленноŭ ¶rt;ля уmверж¶rt;енuя нормыČSN 730 036 „Нaгрузкa nосmроек сеŭсмuческuмu воз¶rt;еŭсmвuямu“. Кaрma со¶rt;ержum све¶rt;енuя об ожu¶rt;aемых мaксuмaльных велuчuнaх uнmенсuвносmu nо шкaлеMSK-64. Кaрma основaнa нa всех ¶rt;о сuх nор uзвесmных мaкросеŭсмuческuх нaблю¶rt;енuях нa mеррumорuu Чехословaкuu u нa мо¶rt;елях мaкросеŭцмuческuх nолеŭ. Былa с¶rt;елaнa оценкa ожu¶rt;aемоŭ uнmенсuвносmu с учеmом сеŭсмоmекmонuческого aнaлuзa. Нuзкaя nовmоряемосmь сuльных землеmрясенuŭ нa mеррumорuu ЧССР не nозволяеm оmнесmu новую кaрmу сеŭсмuческого рaŭонuровaнuя к оnре¶rt;еленному временному uнmервaлу, m. е. оnре¶rt;елumь с ¶rt;осmamочноŭ mочносmью верояmносmь ожu¶rt;aемых мaксuмaльных велuчuн uнmенсuвносmu.SummaryA new version of the seismic zoning map of Czechoslovakia is presented mainly for use with the revised Building Code ČSN 730 036 (Seismic Loads on Buildings). In principle, the map contains the expected maximum intensities according to the macroseismic scale MSK-64. The map is based on all historical macroseismic observations in Czechoslovakia, as well as on models of macroseismic fields. The assessment of expected intensities has also been completed by a seismotectonic analysis. It is not feasible to relate the map to a particular time interval, i.e. to estimate the probability of expected maximum intensities, because of the unknown recurrence of extreme events on the territory of Czechoslovakia.

Macroseismic fields of the main Friuli shocks of 1976

Abstract Isosieismal maps of the main Friuli shock and of three large aftershocks were compiled on the scale 1 : 1 million as a joint effort of seismologists from countries shaken by the earthquakes. All macroseismic fields are irregular and display different damping in different directions. Depths of foci and attenuation coefficients are calculated.

Earthquake Belts in Central and Eastern Europe. Some Seismotectonic Aspects

Abstract The distribution of earthquake foci in the crust and the upper mantle indicates the position of mobile zones whereas the distribution of the released seismic energy in time gives evidence about the intensity of movements or in other terms about the released stress. We attempted to define some general seismotectonic conclusions about the region of Central and Eastern Europe which is marked by very differing levels of earthquake activity. For this purpose the available seisraological and tectonic data bases were used. Epicentre maps demonstrating the general pattern of earthquake distribution on this territory Schenkove et al. 1979) help to define earthquake belts and the level of their activity. Some belts are very conspicuous, some are uncertain. Combining the epicentre maps (a) Lineaments of the first order and regional faults separating large units, proved by seismic or gravimetric surveys, were plotted into the seismotectonic scheme (Fig. 1.) together with the largest observed events (M≥5 1/2 or I o ≥8 1/2° MSK). Analyzing all available information some conclusions can be made : 1 There is a different degree of correlation between earthquake foci and fault zones. For instance the Peripienian lineament shows a large earthquake activity along its western branch, whereas the crustal activity is vanishing in the Eastern Carpathians. An example of a good correlation are the Struma, Marica, Yambol and Central-Rhodope faults. The lineament of the first order, the Vardar fault zone, has been active only in some sections. An interesting problem is posed by the assumed connection between the Vardar and Elbe lineaments (Boncev 1965). The Tornquist—Teisseyre lineament is without any recent earthquake events. 2 There are regions where the earthquake pattern indicates lines which are transversally aligned as compared with the general trend of geological formations, one extending from the Skadar Lake in the NE direction to Bulgaria or the Raab line, which seems to extend farther to the SW forming a transversal earthquake belt. 3 The most difficult task is to find relations in the Pannonian basin, where also large events originated in the past. One possible interpretation of this activity can be explained by the crossing of the Vardar-Elbe lineaments with the Periadriatic line. The above seismotectonic correlations are also made for estimating zones which have not been recently active but may have a long-term earthquake generating potential. The study continues.

Properties of Seismic Regime and Recent Earthquake Sequences in Europe

Abstract The development in time of three large sequences, Friuli 1976, Thessaloniki 1978 and Monte Negro — Albania 1979, was investigated by analysing the characteristics of the time distribution of magnitudes of the events, M(t), of average magnitudes, M ¯ (t), and of the number of events, n(t), as well as the strain release curves and the magnitude-frequency relations, N(M). The existence of time gaps and of an anomalous decay of the numbers of shocks before major events, as well as the oscillation of M ¯ (≈ const / b) may be considered as precursory phenomena. The clustering of events has been tested by the Poisson and negative binomial distributions. The clustering of events in time is not surprising, however, the unusual behaviour of numbers of shocks and existence of time gaps before some major shocks seem to be promising in detecting the approaching large event. For this purpose it is important to record and locate weak events down to M = 1 − 2 1/2. The b-values vary systematically during the stress build-up until the point of rock failure, therefore the method based on the formula M ¯ ≈ const / b seems to be a significant simple precursory phenomenon. Other possible methods (changes in the orientation of fault planes, in the spectral content, in the stress drop, etc.) were not investigated for lack of sufficiently numerous data sets. Remark : Full text will be published under the title “Time Pattern of Large Earthquakes Sequences in Europe in 1976 — 1979” in Studia geophysica et geodaetica 25 (1981).

Seismic Hazard Estimate for a Low Seismicity Region. Example of Central Europe

ABSTRACT The contribution reviews basic concepts of earthquake hazard assessment applied to sites of nuclear power plants. The procedure involves the delineation of earthquake source regions, determination of intensity — frequency relations, upper intensity threshold I max and intensity attenuation curves. This is demonstrated on the example of one ite in South Bohemia, and the quantities defining the seismic hazard, i.e. return period in years, and probability of exceedance for different intensities and different periods of interest are calculated. The possibilities to solve the tasks of a realistic determination of earthquake hazard are very limited in all aspects because of the lack of sufficient observations, of a proper theory and of strong motion observations in Central Europe. The application of empirical relations between magnitude, fault dimensions, frequency of earthquakes, amplitude of motion, distance, and simple statistical laws of earthquake occurrence help to approach the problem only in a simplified way. The existing deviations of simple assumptions from the complicated reality introduce uncertainties in results, so that the final values on predicted hazard figures must be considered only as rough estimates, which sometimes, particularly for long periods of interest, become unrealistic. Macroseismic intensity as the quantity characterizing earthquake effects is not convenient for design purposes, i.e. some historical macroseismic reports are exaggerated and most macroseismic observations do not relate to average soil conditions, but to poor ground and building conditions. Therefore, the intensity calculated on the basis of historical macroseismic observations can be decreased in case of good ground conditions at a construction site. Remark: Full text was submitted for publication in Pure and Applied Geophysics 1981.