Zdeňka Schenková

Seismotectonic model for the earthquake swarm of 1985–1986 in the Vogtland/West Bohemia focal area

Abstract After decades of minor seismic activity an intensive earthquake swarm occurred in 1985–1986 with a maximum activity in December 1985 and January 1986, in the well-known narrow focal zone of swarm quakes in the border region of the G.D.R. and Czechoslovakia. Local seismic networks recorded more than 8000 events during this swarm, the strongest local magnitude being 4.6. Nearly all of these swarm events were located within a relatively small focal volume 3.5 km in length, 1.5 km in width, and at depths of 6–8 km. Fault-plane solutions could be determined for 17 of the strongest events. They provided the basis for seismotectonic modelling in connection with (1) detailed tectonic data (including faults indicating conditions at the seismogenic depths, and recent active faults), (2) crustal stress data, and (3) studies on recent horizontal movements of crustal micro-blocks. According to detailed local micro-earthquake monitoring since 1962, all epicentral areas of swarms are aligned along the N153°E striking Marianske Lazně fault (MLf), which trends approximately in the azimuth of maximum compressive stress. The MLf is intersected by a bundle of recent active, lamellar N-S to NNE-SSW fault elements. The fault-plane solutions of 14 of the events studied show strikes in this direction. The others strike along the MLf. Tectonic evidence for the conjugate planes is lacking. Both the MLf and the N-S fault elements show a tendency to dextral creep according to neotectonics and recent horizontal crustal movements, whereas a generally sinistral elastic rebound has been derived for the focal process of the swarm. This reflects alternating block movements which could also be deduced from repeated geodetic measurements. Probably, the MLf system provides a suitable setting for the foci only, whereas the faulting process itself occurs mainly on secondary faults within the system. Moreover, a splay-structure along the MLf, immediately south of the 1985–1986 epicentral area and connected with a right-stepping offset along a N-S fault element, substantiates the proneness to the occurrence of swarm-like seismic activity.

Seismic hazard estimate for a low seismicity region — Example of Bohemia

The contribution reviews basic concepts of earthquake hazard assessment for sites of nuclear power plants. Taking into account the delineation of earthquake source regions, intensity-frequency relations, upper intensity thresholdsImax and intensity attenuation curves, we determine the seismic hazard for a site in south Bohemia and calculated the quantities defining the seismic hazard, i.e. return period in years, probability of exceedance for different intensities and different periods of interest. The adopted procedure has some limitations due to the poor definition of seismogenic zones (boundaries,N(I),Imax) and lack of strong motion observations in Central Europe.

The third asymptotic distribution of largest magnitudes in the Balkan earthquake provinces

SummaryThe third asymptotic distribution of the theory of largest values is applied to sixteen earthquake provinces in the Balkan area. The model which assumes an upper magnitude limit fits the observations better than the first distribution. Graphs and tables provide estimates of return periods of largest magnitudes.

Seismogeological pattern of a transition area between the Eastern Alps and the Western Carpathians

Abstract All available seismological data (earthquake epicentres, macroseismic observations) and the available structural-tectonical, geomorphological and geodetic data for the transition area of two fundamental orogenetic units of Central Europe -the Eastern Alps and the Western Carpathians -are analyzed, with the aim to find seismogenic zones of this area and to assess their possible earthquake activity. The relationship of small local geological structures to the fundamental European tectonic blocks is discussed from the viewpoint of seismotectonic processes expected for the transition area, especially in the Vienna Basin and in the Little Carpathians. The special attention is paid to the densely inhabited area of the city of Bratislava which is situated in the territory under study. Some results could contribute to its further urban planning. On several faults or boundaries in the Western Carpathians relatively intense horizontal displacements have been observed. Fundamental movements on faults of NS to NE-SW directions have mostly sinistral character while on some faults of ENE-WSW direction they are dextral. In the southern parts of the investigated area, dextral movements on local faults of NW-SE direction have been found. A neotectonic model of the broader surroundings of the city of Bratislava is presented.

Return periods of earthquakes and trends of seismic activity

SummaryThe earthquake activity in Europe is investigated in the paper using return periods of earthquakes in fifteen European seismic regions. The variations of return periods are compared with the fundamental structure of Europe and with the observed migrations of epicentres. The probabilities of occurrence of earthquakes and their return periods are obtained by applying the theory of largest values to the data in KárníksCatalogue of European Earthquakes 1901–1967.

Application of the theory of largest values to earthquake occurrence in the balkan region

SummaryThe theory of largest value is applied to estimating the probability of occurrence and of return periods T of the largest earthquakes in the different regions of the Balkan area. For this study shallow shocks from the period 1901–1967 are used. Magnitudes which will be exceeded with the probability 1% are compared with the largest magnitudes actually observed during the period 1901–1967, and the inequalities T ≷ n and their time development are studied.

Seismotectonic model of the upper part of the Earth's crust of Czechoslovakia

РезюмеКорреляцuя месm вознuкновенuя землеmрясенuŭ нa mеррumорuu Чехословaкuu u ее окресmносmu с ¶rt;aннымu nо неоmекmонuке,геоморфологuu, современным ¶rt;вuженuям земноŭ коры, космuческоŭ съемке nоверхносmu Землu uгеофuзuческuм nолям nозволuлa вы¶rt;елumь основные сmрукmырные блокu в верхнеŭ чaсmu земноŭ коры. Нa зонaх меж¶rt;у эmuмu блокaмu нaблю¶rt;aеmся возрaсmaнuе ¶rt;олговременных ¶rt;вuженuŭ, особенно в неоu¶rt;ном nерuо¶rt;е. Поэmому nре¶rt;лaгaемaя сеŭсмоmекmонuческaя мо¶rt;ель верхнеŭ чaсmu земноŭ коры основaнa нa современных знaнuяхгео¶rt;uнaмuческuх nроцессов, nроuсхо¶rt;ящuх нa uзучaемоŭ mеррumорuu, u нa uх связu с nо¶rt;вuжнымu зонaмu.SummaryA correlation of the earthquake occurrence on the territory of Czechoslovakia and in its close neighbourhood with the data on the neotectonic and geomorphological development of the respective area, the recent movements of the Earths crust, the courses of photolineations and on the geophysical fields enabled the fundamental structural blocks in the upper part of the Earths crust to be singled out. The contact zones established between the above blocks exhibit increased long-term movement tendencies particularly in the Neoid period. A seismotectonic model of the upper part of the Earths crust of Czechoslovakia, compiled with the use of the data mentioned above, is described.

Seismotectonic scheme of Central and Eastern Europe

SummaryThe distribution of earthquake foci in the crust and the upper mantle indicates the position of mobile zones. Comparing epicentral maps of Central and Eastern Europe, known tectonic lines, results of deep seismic sounding and gravimetric survey enables a scheme of potential seismogenic zones, which correspond to active fault zones separating plates and blocks in the area, to be compiled. The level of earthquake activity varies along the investigated boundaries and can be considered as a measure of the intensity of the present geodynamic processes.

Map of isoseismals of the main Friuli earthquake of 6 May 1976

SummaryThe isoseismal map of the North-Italian earthquake of 6 May 1976 was compiled using the contributions of the European countries lying within the shaken area. The characteristic feature of the macroseismic field is discussed.

Earthquake epicentres in central and Eastern Europe

SummaryThe investigated region covers the territories of Bulgaria, Czechoslovakia, the G.D.R., Hungary, Moldavia, Poland, Romania and the south-western Ukraine and parts of Austria, Greece, and Yugoslavia. Maps of epicentres demonstrate the geographical distribution of activity. Earthquakes occur in several seismogenic belts (provinces) related to neotectonic features of the region. A special phenomenon is the intermediate-depth focus of Vrancea releasing in the region the largest amounts of energy, together with a few crustal foci in Bulgaria.