Tecfonop~ysics, 34 (1976) T17-T22 Q Elsevier Scientific Publishing Comply,
Amsterdam - Printed in The Netherlands
A note on some features of global seismicity and global distribution of tectonic fractures
B. GADOMSKA’ and H. TUOMIN~N3
‘Zns~it~te of Seismolo~, Uniuersi~y of Helsinki, Helsinki ~Finland) ‘Z~sti~ute of Geophysics, Polish Academy of Sciences, Warsaw ~Poiand) ‘Department of Geology and Mineralogy, University of Helsinki, Helsinki (Finland) (Submitted January 23,1976;
revised version accepted June 14, 1976)
ABSTRACT Vesanen, E., Miiki, M-L., Teisseyre, R., Gadomska, B. and Tuominen, H., 1976. A note on some features of global seismicity and global distribution of tectonic fractures. Tectonophysics, 34: T17-T22. Some features of the global retribution of earthquakes are discussed, and symmetry in the global distribution of tectonic fractures is noted. A new notion of seismic chimneys is defined and their role in global tectonics is considered. Deep horizontal displacements in the distribution of deep-focus earthquakes are also pointed out.
The aim of this study is to point out some characteristic features of the global distribution of earthquakes and to explain some relationships between seismic activity and global geodynamic processes. An analysis of surface and depth distribution of earthquake foci forms the basis of this study. Three-dimensional models constructed for almost all seismic regions with deep and intermediate-form earthquakes were used. It was assumed that the main seismic regions are associated with great tectonic fractures. The fractures form a certain system, within which geodynamic processes lasting even several geodynamic cycles, can develop. The system is characterized by a certain type of symmetry. When global distribution of earthquakes is considered (Fig.l), several symmetrical features can be distinguished: (1) Seismic belts symmetric in couples and removed from each other by about 180”: A-A and C--C against B--B and D-0; these belts correspond to great tectonic lineaments. (2) Symmetrically situated triple junctions. (3) Areas of so-called seismic chimneys. The system is bounded by two circum-polar belts of relatively low seismicity. These belts are approxima~ly elliptic and pe~endicul~ to each other.
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Detailed studies of the fracture distribution suggest that some of them form quite regular great circles (Tuominen et al., 1973). It should be noted that an unique terrestrial shear pattern has been discussed by Richards (1973). His pattern, characterized by a certain symmetry, is based on a simple model of the earth in the form of a rotating sphere containing a solid spherical shell surrounding a plastic interior. In the system of great tectonic lineaments, fractures traversing these lineaments should also be included (e.g. fracture series intersecting the Mid-Atlantic Ridge). Different velocities of horizontal displacements can occur along the neighbouring fractures of a traverse system and this will lead to some horizontal deformations on the earth’s surface. The simple theory of plate tectonics is not fully adequate to explain global geodynamic processes. The mechanism of these processes must be more complex, and it is responsible for both shallow phenomena and deep mobile systems. The complex character of the mechanism of geodynamic processes connects thermal convection (or advection) with differentiation and phasetransformation processes. Geodynamic processes cannot be restricted to relatively thin layers forming the lithospheric plate only --- they must go deeper, encompassing layers of considerably greater thickness (some hundreds of kilometres). This conclusion is based on two factors: (1) the role of seismic chimneys in global tectonics; and (2) the depth distribution of earthquake foci in the circumPacific belt. A continuous, almost vertical region containing earthquake foci is called the seismic chimney. Its horizontal size is usually rather small, with the exception of shallow near-surface,layers (see Fig.4). An analysis of three-dimensional models of earthquake distribution shows, that seismic chimneys are associated with several island-arc structures. The maximum depth of chimney penetration varies in different seismic regions. In the region of the Sunda Islands and the Mariana Islands it goesdown to about 500 km and 600 km, respectively. In the regions of the South Sandwich Islands, the Aleutian Islands, New Britain and the Carpathian arc (Vrancea zone) seismic activity does not extend deeper than 200-240 km. It should be noted that all the listed chimneys are situated in areas of island-arc structures, formed during the Alpine orogenesis. The nature of seismic chimneys is not entirely clear. They could be considered as pillars, relatively stable during geodynamic processes. Seismic activity associated with chimneys could be connected with very deep lateral inhomogeneities in the upper mantle. Let us assume that some rigid old material of the mantle resists the flow of more viscous material (Fig.2). Then along planes A and B, which bound the rigid block, relative displacements would originate, and at the junction of these planes (point 0 in Fig.2) a crack will develop. This scheme is called the dislocations + crack mechanism of earthquakes, described by Teisseyre (1970). The scheme is significantly different from that generally accepted as the formation mechanism of triple-junctions. In the case of the dislocations -+ crack model there are two slip planes and the crack is formed at their junction.
Fig.2. Scheme of crack formation near a rigid block in a flow of more viscous material: horizontal cross-section. Boundaries of the rigid block are marked by A and B.
Fig.3. Depth distribution of earthquake foci across the South Pacific. Deep convection is marked by dashed lines, while the arrows indicate the direction of the convection flow.
Displacements along the crack compensate the vector balance of the slip vectors along the block side planes. This mechanism explains the occurrence of earthquakes in a restricted region near the block edge. This region corresponds to the seismic chimney. A tensile-type crack can originate even at greater depths when filling of the crack by viscous material flowing around the block is assumed. The flow of material around the rigid block could be connected with global horizontal movements, and the maximum depth of chimney penetration would define the m~imum thickness of a layer encompassed by these movements. The depth distribution of earthquake foci in the South Pacific area (Tonga Islands and South America) is of special interest. A cross-section of the South Pacific is shown schematically in Fig.3. More detailed cross-sections were made by Vesanen et al. (1975). It can be seen that in the region of the Tonga Islands there are almost vertical seismic zones of considerable thickness. The first zone extends from the earth’s surface down to a depth of about 300 km and it is displaced eastwards from the second zone situated at depths from about 300 km to 600 km. Small but distinct seismic activity observed at a depth of about
300 km implies that there are probably horizontal slip planes, along which these two zones are displaced. A similar pattern of earthquake distribution can also be found on some cross-section given by Woj~z~-Gadomska (1973). The depth distribution of earthquake foci in,South America is similar to that in the Tonga region (Teisseyre et al., 1974). In South America, however, a seismic zone extending from the surface to a depth of 300 km is displaced westwards from a deeper zone situated at depths from 400 km to 600 km. It should be noted that according to the model 12 of McDonnell (1965) viscosity decreases to considerably small values at a depth range of 200-400 km. At the same range an increase of dislocation flow is observed (Wojtczak-Gadomska, 1973). These facts additionally confirm the reality of deep horizontal discontinuities in the upper mantle. These deep horizontal displacements could probably be connected with the convection occurring at greater depth (see Fig.3). The special role the Hindu Kush region in the global fracture system should also be briefly mentioned. The three-dimensional model of earthquake distribution shows that in this region also there is a seismic chimney (Fig.4) penetrating down to a depth of about 300 km. At smaller depths earthquakes are not restricted by a narrow chimney but form local seismic belts associated with the main tectonic structures of the region. Detailed analysis of fault dis-
Fig.4. Schematic diagram of the seismic zone in the Hindu Kush region. The centre of the seismic chimney is marked by C; 1, 2, 3 and 4 denote local seismic belts.
placements, based on fault-plane solutions of selected earthquakes, and of fracture distribution (Vesanen et al., 1975) suggests that the zone of seismic chimney in Hindu Kush forms a certain kind of convergence pole of the fracture system.
REFERENCES McConnell, R.K., 1965. Isostatic adjustment in a layered Earth. J. Geophys. Res., 20: 5171-5188. Richards, T.Ll., 1973. A terrestrial shear pattern? Pure Appl. Geophys., 110: 2012-2021. Teisseyre, R., 1970. Crack formation and energy release caused by the concentration of dislocations along fault planes. Tectonophysics, 9: 547-557. Teisseyre, R., Wojtczak-Gadomska, B., Vesanen, E. and Miiki, M-L., 1974. Focus distribution in South America deep-earthquakes regions and their relation to geodynamic development. Phys. Earth Planet. Inter., 9: 290-305. Tuominen, H.V., Aarnisalo, J. and Soderholm, B., 1973. Tectonic patterns in the Central Baltic Shield, Bull. Geol. Sot. Finland, 45, pt. 2: 205-217. Vesanen, E., Teisseyre, R., Gadomska, B., Tuominen, H. and Maki, M-L., 1975. Some remarks on the relevance of seismicity, seismic chimneys and fracture system to geodynamics. Publs. Inst. Geophys. Pol. Acad. Sci., 93: 177-196. Wojtczak-Gadomska, B., 1973. Distributions of the released seismic energy and the number of earthquakes in deep structures of the Pacific area. Publs. Inst. Geophys. Pol. Acad. Sci., 65, pp.193.