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 (Hanu and Vank, 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.     

A comment on determining the induced magnetic field in bodies of arbitrary shape

Summary The authors prove that the solution of Laplace's equation under Neumann's boundary conditions can be transformed to an advantage into determining the field which is generated by charges, induced by the external field at surfaces of discontinuity.     

Geodynamic interpretation of the earthquake distribution in the kermadec subduction zone

Summary The 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.     

The use of laboratory measurements of electric resistivity of rocks for evaluating the magnetotelluric method

Summary A profile model of electric resistivity as a function of depth was compiled, the initial parameters being the values of the electric resistivity determined experimentally for basaltic and eclogitic rocks from the Bohemian Massif. The curves of the apparent resistivity were computed for this model and the measure of information evaluated for the various model layers. The dominant influence of the subsurface layer was proved.     

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

Summary The 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.     

Day-time ionospheric disturbances of corpuscular origin in the midlatitudeD-region

¶rt;m uu nau mu m a nu a¶rt;u ¶rt; D-amu u. a¶rt; m nu u u. u¶rt;a a a mu nma u nma mu m (20 ¶rt; 150 ).     

Heidornit Na2Ca3[Cl · (SO4)2 · B5O8(OH)2] ein neues Bormineral aus dem Zechsteinanhydrit Mit einer röntgenographischen Untersuchung

Ohne Zusammenfassung     

Frequency limitation to the theoretical investigation of the transmission of hm waves through the ionosphere

Summary As a result of investigating the energy flow, transmitted by short-period HM waves, the existence of a frequency limitation was found at the theoretical investigation of the propagation of these HM waves.