Jiří Vaněk

Magma Intrusions and Earthquake Swarm Occurrence in the Western Part of the Bohemian Massif

We are proposing a hypothesis that earthquake swarms in the West Bohemia/Vogtland seismoactive region are generated by magmatic activity currently transported to the upper crustal layers. We assume that the injection of magma and/or related fluids and gases causes hydraulic fracturing which is manifested as an earthquake swarm at the surface. Our statements are supported by three spheres of evidence coming from the western part of the Bohemian Massif: characteristic manifestations of recent geodynamic activity, the information from the neighbouring KTB deep drilling project and from the 9HR seismic reflection profile, and the detailed analysis of local seismological data. (1) Recent manifestations of geodynamic activity include Quaternary volcanism, rich CO2emissions, anomalies of mantle-derived3He, mineral springs, moffets, etc. (2) The fluid injection experiment in the neighbouring KTB deep borehole at a depth of 9 km induced hundreds of micro-earthquakes. This indicates that the Earths crust is near frictional failure in the western part of the Bohemian Massif and an addition of a small amount of energy to the tectonic stress is enough to induce an earthquake. Some pronounced reflections in the closely passing 9HR seismic reflection profile are interpreted as being caused by recent magmatic sills in the crust. (3) The local broadband seismological network WEBNET provides high quality data that enable precise localization of seismic events. The events of the January 1997 earthquake swarm are confined to an extremely narrow volume at depths of about 9 km. Their seismograms display pronounced reflections of P- and S-waves in the upper crust. The analysis of the process of faulting has disclosed a considerable variability of the source mechanism during the swarm.We conclude that the mechanism of intraplate earthquake swarms generated by magma intrusions is similar to that of induced seismicity. As the recent tectonic processes and manifestations of geodynamic activity are similar in European areas with repeated earthquake swarm occurrence (Bohemian Massif, French Massif Central, Rhine Graben), we assume that magma intrusions and related fluid and gas release at depths of about 10 km are the universal cause of intraplate earthquake swarm generation

Earthquake distribution and volcanism in Kamchatka, Kurile Islands, and Hokkaido Part 1: Kamchatka and northern Kuriles

SummaryThe morphology of the Wadati-Benioff zone in the region of Kamchatka and Northern Kuriles, based on the distribution of 1102 earthquake foci, verified the existence of an intermediate depth aseismic gap and its relation to active andesitic volcanism. A system of deep seismically active fracture zones, genetically connected with the process of subduction, was delineated in the continental plate and confirmed by the results of deep seismic sounding. Two of these fractures, dipping toward the subduction zone, may be considered as the principal feeding channels for active and Holocene volcanoes of the continental volcanic belts of Kamchatka.

Morphology of the Wadati-Benioff zone, andesitic volcanism, and active fracture zones in Central America

SummaryThe morphology of the Wadati-Benioff zone in the region of Central America, based on the distribution of 1377 earthquake foci, verified the existence of an intermediate aseismic gap and its relation to active andesitic volcanism, and the non-uniformity of subduction due to the hampering effect of the main structural features of the subducting Cocos plate. Four deep seismically active fracture zones, genetically connected with the process of subduction, and three fracture zones manifesting the possible boundary between the Americas and Caribbean plates were identified in the continental wedge.

Experimental study of a pulse method for measuring elastic parameters of rocks on samples

РезюмеВ статье описываются методы повышения прпменимости и надежности метода прозвучивания при определении упругих постоянных твердых тела)путем детальной классификации наблюдаемой волновой картины возникающей в образце,б)возможиостей применения электромеханпческих преобразователей, диаметр которых меньше диаметра образца,в)обобщением метода прозвучивани я для образцов другой формы чем цилиндрической. Приведены необходимые соотношения для времен прихода отдельных волновых групп и удобный метод расчета скоростей продольных и поперечных волн. Таким образом возможно определить полный комцлект упругнх постоянных с высокой точностыо. Интересной проблемой является дополнение кинематнческих исследований изучением динамических свойств отдельных волновых групп. Дальнейшие исследования будут направлены на решение этой задачи экспериментальным путем.

Seismically active fracture zones and distribution of large accumulations of metals in the central part of Andean South America

Abstract The analysis of the geometry of distribution of earthquake foci in the central part of Andean South America between 18° and 34°S made the delineation of several seismically active fracture zones in the continental wedge overlying the subducting Nazca plate possible. Correlation of their position with the distribution of hypogene accumulations of metals revealed that the majority of large mineral deposits and mining districts are situated in the outcrops of these fracture zones. We present geometrical documentation (map of epicentres, vertical and longitudinal cross sections) of the most important fracture zones and data on mineralogical composition, genetic type and available radiometric ages of mineral deposits. Sixteen mining districts in Chile, and 24 in Argentina, were attributed to the seismically active fracture zones. Major mining districts and individual large mineral deposits occur in six seismically active fracture zones roughly parallel to the axis of the Peru-Chile trench (Carachas-Portillo, Choquelimpie, Iquique, Domeyko, Río Blanco-Los Bayos and Farellones F.Z.), in six fracture zones roughly perpendicular to the trench (El Salvador, Maricunga, Jaroma, Ujina, Tumbaya and Incahuasi-León Muerto F.Z.) and in two fracture zones oriented at an angle of about 45° in relation to the direction of the presently active Andean subduction (Aconcagua and Sierra del Volcán F.Z.). The occurrences of large mineral deposits of different ages show that these fracture zones were also active in the geological past and represent sites of permanent re-opening of paths allowing ore-bearing solutions and long-term accumulation of large amounts of metals to occur in relatively restricted domains of the Earths crust. The mining districts with dated mineral deposits are arranged into four periods of hypogene mineralization: Upper Miocene-Pliocene, Upper Oligocene-Middle Miocene, Upper Eocene-Middle Oligocene, Lower Paleocene-Upper Eocene. These periods of metallogenic activity correlate well with four supposed Andean subduction cycles active in the Tertiary. The occurrence of mineral deposits of different ages in recently active fracture zones can be used as an important evidence in favour of long-term spatial permanence and activity of these zones and as a guide for the discovery of further mineral deposits hidden under young sedimentary and volcanic cover in the fracture zones.

Seismic Activity Around and Under Krakatau Volcano, Sunda Arc: Constraints to the Source Region of Island Arc Volcanics

There is general agreement that calc-alkaline volcanic rocks at convergent plate margins are genetically related to the process of subduction (Ringwood, 1974; Maaloe and Petersen, 1981; Hawkesworth et al., 1997). However, opinions on the mode and site of generation of primary magma for island arc volcanism differ substantially. The site of generation of calc-alkaline magma is thought to be either in the mantle wedge (Plank and Langmuir, 1988; McCulloch and Gamble, 1991) or in the subducting slab (White and Dupré, 1986; Defant and Drummond, 1990; Edwards et al., 1993; Ryan and Langmuir, 1993). We present seismological evidence in favour of the latter concept. A distinctive seismicity pattern around and under the Krakatau volcano was identified during systematic studies of the SE Asian convergent plate margins by means of global seismological data. A column-like cluster of events, probably associated with the dynamics of the volcano, is clearly separated from the events in the Wadati-Benioff zone. The accuracy of hypocentral determinations of the events of the cluster does not differ from the accuracy of the events belonging to the subducting slab. The depths of the cluster events vary from very shallow to about 100 km without any apparent discontinuity. On the other hand, there is a pronounced aseismic gap in the Wadati-Benioff zone directly beneath the volcano at depths between 100-150 km. The Krakatau cluster connects this aseismic gap to the volcano at the surface. The pervasive occurrence of earthquakes in the continental wedge between the subducting slab and the Earth surface bears witness to the brittle character of the continental lithosphere and casts doubt on the existence of large-scale melting of mantle material. The aseismic gap (Hanuš and Vaněk, 1985), interpreted by us as a partially melted domain occurring in subducted slabs in practically all active subduction zones that reach depths greater than 100 km, is here used as evidence for the location of the primary source region of island arc volcanics in the subducting plate.

The velocity of elastic waves in diabase and greywacke under pressures up to 4 kilobars

РезюмеСтатья посвящена изучению поведения упругих параметров горных пород, в частности скоростей продольных и поперечных волн, при высоких всесторонних давлениях до 4килобар и одностороннем давлении до 1килобар. Исследования проводились на двух представительных типах горных пород пржибрамского рудного района, диабазах и граувакках, с целью создать основы для применения ультразвуковых методов изучения горного давления в этой области.Измерения при всесторонних давлениях производились в лаборатории высоких давлений ИФЗ АН СССР в Москве. Скорости упрутих волн измерялись методом прозвучивания посредством электронной аппаратуры ГИ ЧСАН с разрешающей способностью 0,02мксек на экране электронно-лучевой трубки. Для возбуждения и приема упругих колебаний применялисьХ срезы кварца, собственной частотой от 1 до 2,5мгц. При расчетах скоростей вводились поправки на время пробега импульса аппаратурой.Полученные результаты измерений приведенные в таб. 4 и графиках сравниваются с данными других авторов (фиг. 18, 19). У изучаемых пород наблюдаются небольшие изменения скоростей продольных и поперечных волн с давлением, которые достигают максимального относительного изменения 5% при давлениях до 4килобар. Одновременное измерение скоростей продольных и поперечных волн позволило определкть изменения постоянной Пуассона σ с давлением. Отмечается, что зависимость σ от давления может принимать более сложную форму, чем до сих пор обычно предпологалось.

Deep seismically active fracture zones in ecuador and northern peru

РезюмеСuсmемa 8 сеŭсмuческu aкmuвных рaзломных зон былa обнaруженa нa основaнuu рaсnре¶rt;еленuя очaгов землеmрясенuŭ в конmuненmaльноŭ лumосфере Эквa¶rt;орa. Былu оnре¶rt;елены nоложенuе u шuрuнa нa nоверхносmu, mольщuнa, уклон u мaксuмaльнaяглубuнa оm¶rt;ельных рaзломных зон. Сущесmвовaнuе эmuх зон незaвuсuмо nо¶rt;mвер¶rt;aюm нaблю¶rt;енuя uсmорuческuх рaзрушumельных землеmрясенuŭ u соnосmaвленuе зон с nоверхносmноŭгеологuеŭ, mекmонuкоŭ, вулкaнuзмом uгu¶rt;роmермaльнымu nроявленuямu.SummaryA system of 8 seismically active fracture zones was delineated on the basis of the distribution of earthquake foci in the continental lithosphere of Ecuador. The position and width of the outcrop, thickness, dip and maximum depth of the individual fracture zones were estimated and correlated with surface geological and tectonic phenomena, volcanism and hydrothermal manifestations. The existence and strike of the fracture zones was independently confirmed by the occurrence of historical disastrous earthquakes.

Earthquake distribution and volcanism in Kamchatka, Kurile Islands, and Hokkaido Part 3: Southern Kuriles and Hokkaido

РезюмеМорфологuя зоны Wadati-Benioff в облaсmu Юужных Курuльскuх о-вов u Хоккaŭ¶rt;о, основaннaя нa рaсnре¶rt;еленuu 4015 очaгов землеmрясенuŭ, nоmвер¶rt;uлa сущесmвовaнuе nерехо¶rt;ноŭ aсеŭсмuческоŭ зоны u ее связь с aкmuвным aн¶rt;езumовым вулкaнuзмом. Былa нaŭ¶rt;енa u оnuсaнa зонa naлеосуб¶rt;укцuu, aкmuвuзuровaннaя nромежуmочнымглубuнным сmолкновенuем с aкmuвноŭ зоноŭ суб¶rt;укцuu.SummaryThe morphology of the Wadati-Benioff zone in the region of Southern Kuriles and Hokkaido, based on the distribution of 4015 earthquake foci, verified the existence of an intermediate depth aseismic gap and its relation to active andesitic volcanism. A paleosubduction zone activated by an intermediate depth collision with the active subduction zone was found and described.

Geodynamic interpretation of the earthquake distribution in the kermadec subduction zone

SummaryThe morphology of the Wadati-Benioff zone in the Kermadec region, based on the distribution of 1100 earthquake foci, verified the existence of an intermediate aseismic gap and its relation to active andesitic volcanism, and the non-uniformity of subduction due to the hampering effect of the main structural features of the subducting Pacific plate. Two cycles of the recently active subduction in the Tonga-Kermadec island arc were found.