Jan Kozák

Seismic events with non-shear component: I. Shallow earthquakes with a possible tensile source component

The concept of seismicity of fast tensile fracturing is introduced and supported by the results of shear and of combined shear and tensile displacements along a loaded stress concentrator. The seismicity of tensile fracturing is demonstrated by means of acoustic (elastic) signals obtained during tensile fracturing in physical models under load; the basic physical relations between the parameters characterizing the loaded medium, load conditions, shear and tensile displacements, and release of acoustic energy are presented. For determining the tensile-source component in earthquakes a procedure based on the construction of radiation patterns is suggested and submitted for discussion. The criteria for selecting earthquakes with possible shear-tensile source mechanisms are listed and discussed. The existence of such a combined seismic source is sought in two shallow earthquakes which occurred in southern Iran in March 1977. In general, the paper should be regarded as a proposal to utilize the radiation characteristics of a seismic source—with all their insufficiencies—as a quick and simple tool for seeking combined shear and tensile mechanisms of seismic energy release.

The illustrated history of natural disasters

Introduction.- Natural disasters in historical pictures.- Pictorial reproduction techniques.- Volcanoes, Earthquakes and Tsunamis.- From ancient myths, through medieval perceptions to present understanding.- Volcanoes.- Earthquakes.- Tsunamis.- Plate Tectonic Theory.- Vesuvius-Somma volcano, Bay of Naples, Italy.- Etna volcano, Sicily.- Stromboli volcano, Lipari, Tyrrhenian Sea.- Phleghraean Fields.- Santorin/Thira volcano, Greece.- Ferdinandea, a new submarine volcano.- Volcanism in Iceland.- Pico de Teide, Tenerife Island, Canary archipelago.- Mt. Pelee volcano, Martinique Orizaba volcano, Mexico.- Jorullo volcano, Mexico.- Pacaya volcano, Guatemala.- Cotopaxi volcano, Ecuador.- Chimborazo Mt., Ecuador.- Antujo volcano, Chile.- Kusatsu-Shirane Mt. 1660s eruption, Japan.- Volcanoes of Indonesia.- Gamkonoro Mt., Moluccas Islands.- Avacha volcano, Kamchatka, Far East Russia.- Volcanoes of the Hawaiian Archipelago.- Kiyev Earthquake, 1230.- Aix-de-Provence (France) Earthquake, 1708.- Basel Earthquake, 1356.- Rhodes (Greece) Earthquake, 1481.- Istanbul Earthquake, 1566.- Two earthquakes at Yedo (Tokyo), around 1650 (?) and 1855.- Earthquake at Port Royal (Jamaica) in 1692.- Great Lisbon Earthquake of 1755.- Calabrian earthquakes of 1783 and later on.- Valona (Albania) earthquake, 1851.- Zagreb (=Agram) earthquake, 1880.- Two earthquakes In Italy: 1881 (Ischia) and 1883 (Casamicciola).- Imperia earthquake (French/Italian Riviera), 1887.- Earthquake at Shemakha, Azerbaijan, 1902.- Submarine explosion near the Juan Fernadez Island, 1835 (OR 1837).- Guadeloupe (Antilles) earthquake 1843.- Earthquake at San Jose (Costa Rica), 1868.- Mendoza (Argentina) earthquake, 1861.- Arequipa/Arica earthquake, 1868.- San Salvador earthquake, 1873.- Charleston earthquake (South Carolina, USA), 1886.- Hayward (California, USA) earthquake, 1868.- Great San Francisco Earthquake, 1906.- Two Calabrian earthquakes, 1905 and 1908.- Milestones of seismology.- References.- Index.

Laboratory investigations on fault plane induced tensile cracks

SummaryTensile cracks induced by stress concentration around a fault plane in physical models under uniaxial load were analyzed. The results characterizing the decisive role of the fault plane inclination to the the stress direction are presented. Two stages of tensile crack life were determined, i.e. the first, seismoactive stage, accompanied by seismic energy release and the second aseismic one characterized by low velocity of crack propagation. Results obtained seem to be relevant to a better understanding of the character and regime of actual seismoactive faults.

Possible mechanism of rockbursts in coal mines

One of the important questions of rockburst prevention is the understanding of the mechanism of rockburst source. This question can be effectively studied by direct seismic observations in the rockburst regions. For this purpose, the distribution ofP-wave onset signs and the inversion of first motion amplitudes were utilized as the basic method. In such a way the coal mine regions in Poland (Upper Silesia) and in Czechoslovakia (Kladno coal mine district) were studied as part of the Polish-Czechoslovak rockburst investigation project. More than 250 rockburst events were recorded here in the decade 1977–1983, and analyzed. The results of the statistical analysis of these data allow us to formulate and introduce a model of the rockburst source with an implosion component. The suitability of this conception was verified by laboratory simulation conditions; it was confirmed that the seismoactive displacements with a clear implosive component were recorded in the neighbourhood of a stress concentrator weakened by holes. The results of both the field observations and laboratory tests were in good agreement with the theoretically derived radiation patterns for a combined shear-implosive source and also with the theoretical conception of such a source based on real geometrical configurations of mine excavations and tectonic dislocations.The rockbursts treated exhibit a dominant shear component, the magnitude of the additional implosive component not exceeding 10 percent of the shear component.

Refraction of elastic waves into a medium of lower velocity — Pseudospherical waves

SummaryAn irregular wave group (here called pseudospherical), the existence of which is connected with the velocity boundary at which the velocity decreases discontinuously, is investigated. A schlieren modelling device was chosen for a model investigation of this wave since it permits the investigation of wave fields inside the measured models. The model consisted of two layers of transparent gels, the source lay in the layer of higher velocity. The measurements have shown that an irregular wave in the layer of lower velocity exists only in a certain region along the boundary; its wave front has a spherical form and its intensity decreases rapidly with increasing distance of the source from the boundary. The wave always comes only after the regular refracted wave which conforms with the ray theory. These properties correspond to the properties of a wave first described byOtt [1]3) andBrekhovskikh [2]. In the conclusion of the present paper the possibilities of recording pseudospherical waves in seismology are outlined.

Do some shallow earthquakes have a tensile source component

The laboratory tests carried out in studying shear and tensile seismogenic displacements occurring in compressed samples, led us to search for earthquakes with a tensile source component. To determine this component in the seismic focus, a special procedure based on the construction of radiation patterns of the combined shear-tensile type is introduced.The criteria for selecting the events produced by the combined source mechanism are listed, and their limitations are mentioned. From the seismic zones with good azimuthal distribution of stations in the world seismic network nine earthquakes which occurred in the 6-year period 1976–1981 were analyzed; for these events better agreement of the observed and theoretical patterns was found for the combined shear-tensile source mechanism than for the pure double-couple mechanism. However, the share of the tensile component was always, found to be relatively small, ranging from 1 to 13 percent of the shear component. The comparison of the two solutions (double-couple vs. combined shear/tensile) is based on the first onset signs statistics.The results obtained indicate that tensile fracturing does not play a substantial role in the total amount of released seismic energy; on the orther hand, it is expected to be more important in the creation and development of focal zone morphology from both the instantaneous and long-term point of view.

Contact conditions on seismoactive faults

Square plate models with a diagonally located slit in compression were studied photoelastically. The resulting stress field surrounding the discontinuity (slit) is evaluated before and after seismoactive (seismic-energy-releasing) shear displacement. The mechanism of the fast shear movement (stick-slip), including its radiation properties, in interpreted. The results confirm the existence of a central locked zone in the loaded slit, the limits of which coincide with the initiation points of stick-slip movements. The zone is interpreted as the source of the seismic energy release. The complementary measurements (direct optical and ultrasonic) are presented to verify the results of the photoelastic analysis. The results obtained are discussed in regard to the conclusions that follow from the theory of elasticity.

Remarks on seismic energy release related to strike slip and tensile crack mechanisms

РезюмеОсвобож¶rt;енuе сеŭсмuческоŭ энерuu в фuзuческuх мо¶rt;ельях nо¶rt; ¶rt;aвленuе м во время бысmроŭ с¶rt;вuовоŭ uлu оmрывноŭ nо¶rt;вuжкu uзучено nuезоэлекmрuческuм меmо¶rt;ом. В сmamье nре¶rt;ложены хaрaкmерuсmuкu нanрaвленносmu uзлученных волн; оnре¶rt;елены mочкu нaчaлa сеŭсмоaкmuвноŭ с¶rt;вuовоŭ nо¶rt;вuжкu с целю лучщео nоняmuя мехaнuзмa освобож¶rt;енuя сеŭсмuческоŭ энерuu в очaе землеmрясенuя эmоо muna.

Propagation velocity and radiation properties of induced tensile cracks

РезюмеИзучены nлексuглaсные мо¶rt;елu квa¶rt;рamноŭ формы в коmорых вырезaнa ¶rt;uaгонaльнaя щель. Мо¶rt;елu нaгружaюmся о¶rt;ноосно ¶rt;о велuчuны крumuческoŭ нaгрузкu р uрu коmороŭ нa обоuх концaх щелu обрaзуюmся mрещuны оmрывa. Скоросmu рaсnросmрaненuя mрещuны оmрывa nре¶rt;ложены в зaвuсuмосmu оm углa нaклонa щелu к нanрaвленuю nрuложенноŭ нaгрузкu. Доnолнumельные ульmрaзвуковые uзмеренuя уnругuх uмnульсов uзученных во время nробега mрещuны оmрывa nре¶rt;осmaвuлu оnре¶rt;елumь временную фuнкцuю эmого uсmочнuкa. Полученные резульmamы ¶rt;uскуmuруюmся с mочкu зренuя мехaнuзмa хруnкого рaзрушенuя muna оmрывa кaк uсmочнuкa сеŭсмuческuх uмnульсов. По¶rt;робно aнaлuзuровaн учaсmок nробегa mрещuны оmрывa, в коmором uзлучaеmся сеŭсмuческaя энергuя.

Beginnings of Regular Seismic Service and Research in the Austro-Hungarian Monarchy: Part II

We closed the preceding part of our paper with the statements that a regular macroseismic service of unprecedented effectivity had been successfully established in the Austrian part of the Monarchy in 1896-1899, and that first continuous instrumental observations had been started at the seismic stations in Ljubljana, Trieste and Kremsmünster in 1897, 1898 and 1899, respectively. In the present part we report how the macroseismic service performed its task from the beginning of the 20th century until the outbreak of World War I, we briefly summarize the beginnings and development of observational seismology in the Hungarian part of the Monarchy, and we inform the reader about the state of European seismometry at the time of establishment of the first stations of the Austro-Hungarian seismographic network.Main topics of the present paper are the history of the development, the principles and properties of the instruments, and the milestones in the interpretation of instrumental observations in both parts of the Monarchy in 1897-1914. The wealth of information extracted from over seventy original papers and books of geoscientists of the time is summarized in the form of two, to a large extent self explaining tables. In Table 1 the altogether seventeen seismic stations gradually established in the Austrian as well as Hungarian parts of the Monarchy in 1897-1914 are ordered chronologically according to the date of initiation of regular measurements at them, and the instruments by which the stations were originally equipped and later successively upgraded are specified. The most important facts about progress in the instrumentation and in the analysis, interpretation and archivation of the observational material are summed up in the last column of Table 1. The principles of the altogether sixteen different types of seismic instruments that were in operation at the stations of the Austro-Hungarian network in the discussed period are explained and their basic technical parameters are specified in Table 2. Those instrumental problems, those moments in the methodology of interpretation of the instrumental observations, and the contributions of those scientists who most decisively influenced the progress of Austro-Hungarian seismology in 1897-1914 are commented in more detail in the text.At the end of the first decade of the 20th century, the instrumentation of the stations of the Austro-Hungarian seismographic network as well as the scientific erudition and publication activities of the station directors and involved geosavants, especially of A. Belar, H. Benndorf, R. Kövesligethy, V. Láska, E. Mazelle, A. and S. Mohorovičić and A. Réthly, had reached a standard comparable with that of analogous activities in Italy and Germany. The well developed Austrian macroseismic service gradually disintegrated during World War I. After the war, seismology progressed in the newly constituted states Czechoslovakia, Poland and Yugoslavia in broader, all-European collaboration.