Results Of Deep Seismic Sounding Research Articles

Seismotectonic scheme of Central and Eastern Europe

The 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.

Optimization of Earth crust profiles by collocation

All gravimetric problems dealing with the inner gravity field of the Earth are ill-posed, ambiguous. Thus, we must complement the gravity data with other information, particularly with the results of deep seismic soundings and with the notions of the g-'.ologists of the structure of the Earth's crust if we want to solve our problems. Our task, however, is not solved for the actual Earth, but for its model. The most important information for us is the distribution of densities within the Earth's body. Thz construction of an uniform normal vertical density profile is of no use, since at least in the upper layers of the Earth's body the density is not a function of the d¢-pth only, but depends on the geological structure of the crust and varies from point to point. According to our opinion it would be better to use a different procedure. We have at our disposal the characteristics of the outer Earth's gravity field, and we are able to estimate its accuracy. We know the depths of the interfaces discovered by deep seismic soundings and, finally, there are some geological hypotheses about the structure of the Earth's crust in the place under consideration. Our task is now to process these three quite different sets of information using a suitable mathematical apparatus. We consider the new methods of mathematical statistics to be the most expectant for this purpose. In preparing for the extensive task mentioned above, we started with a quite simple case. We tried to derive a mathematical aid for optimizing the Earth's crust profiles, designed by seismologists and geologists. The collocation [1] was used to solve the problem. This method includes adjustment, filtering and data prediction.

Epeirogenic studies in India with reference to recent vertical movements

Epeirogenic movements of regional extent have been manifest and recorded in several parts of India from Late Proterozoic to present times through the Tertiary and Recent. The main features of these movements, especially Recent vertical movements, in typical areas in the platform and coastal areas of peninsular India are described and the salient results of the study, qualitative and quantitative, of these zones are outlined. Quantitative measurements of Recent vertical movements have been made largely by geodetic methods and precision levelling in the plateau areas as also in the mobile, orogen zones of the Himalayan and sub-Himalayan tracts and by sea-level measurements through an extensive network of coastal automatic tide-gauge stations. The annual variations of sea level have been studied for several Indian ports. The land uplift or subsidence in relation to mean sea level over the Indian coast has also been studied. The application of geophysical techniques in the identification and delineation of regions of epeirogenic movements has been discussed and illustrated by the results in two type areas, viz., the coastal sedimentary belt of Madras and the vast Deccan Trap territory in western India. In the latter area, blanketed by extensive basaltic lavas over a vast region, the gravity data have brought out a number of major zones of subsidence and uplift which have been corroborated by the results of refraction seismic soundings. One of these major zones of subsidence is in the Koyna region which witnessed a major disastrous earthquake a few years ago. The results of deep seismic sounding along a traverse across the southern part of the Indian peninsula now in progress have also been briefly mentioned. Five important areas have been selected for quantitative measurements of current epeirogenic movements, namely the Shillong Plateau in northeast India, the Chhotanagpur Plateau and Singhbhum region in Bihar, the Aravalli region of Rajasthan, the Narmada Sone Rift in central India and the Deccan Trap territory including the Koyna earthquake zone.

Crustal structure of central and southeastern europe by data of explosion seismology

Results of deep seismic sounding in Central and Southeastern Europe are presented. Crustal thickness on the territory in question varies from 20 to 65 km. Areas with thicker crust are generally confined to highland structures (the Carpathians, the Dinarides, etc) and to places of ancient Proterozoic guosynclinal regions (the Ukrainian shield) where mountainous structures have been completely washed out. Thin crust is associated with massives and occasionally with depressions (the Black Sea depression and others).

Results of deep seismic soundings along international profile VII in Czechoslovakia and poland

The paper deals with the results of DSS measurements along international profile VII, carried out by Czechoslovak and Polish geophysicists in 1970 – 71. The profile situation is shown in Fig. 1. By 1971 part of the profile in the region of the Bohemian Massif between points 1 and 3 and in Poland between points 5 and 7 had been surveyed (Fig. 2). The seismograms were used to construct the travel-time curves of the fundamental types of waves PK, PM, Pn (Fig. 4). The mean velocities were computed from the travel-time curves of the reflected waves (PM and PK) and the refracted waves (Pg). Isolines of the mean velocities could be constructed for the region of the Bohemian Massif (Fig. 6). The velocity data found were used for the depth interpretation of the travel-time curves of the principal types of waves and to construct a seismic section (Fig. 8). In the region of the Pre-Sudeten block the thickness of the crust was found to be 34–37 km, and in the Sudeten it increased to 40 km. Towards the south the thickness of the crust gradually reduces to 30 km in the system of the Lužice faults. In the Bohemian Cretaceous the thickness of the crust is about 30 km. Further to the south, in the region of the Moldanubicum, the thickness of the Earth's crust increases rapidly, and at the southern border of the Central Bohemian pluton it reaches values of about 42 km.

The ?Moho? as a transition zone

Results of deep seismic soundings from various areas of the world are presented. Much weight is put on recent European investigations. Seismic refraction surveys are found to yield reliable information on the depth and undulation of the Mohorovicic discontinuity (Moho). Wide angle reflection data are used for the derivation of an overall velocity gradient zone of a few kilometers in many continental areas. From near vertical and subcritical reflection data a stepwise character of the gradient zone is deduced. The overall picture from different seismic investigations shows the Moho as a laminar transition zone of a few kilometer thickness. There is a general stepwise increase in the velocity of compressional (P) and shear waves (S), possibly often interrupted by layers with lower velocities, until values around 8 km/s are reached for theP-waves in the uppermost part of the mantle. Petrologic explanations of these features deal with layers of partial melts, crystallisation seams, intrusions and some peeling of mantle matter. Problems of the evolution, shifting, and subduction of the Moho are discussed in the light of plate tectonics.

Geological interpretation of the gravity data for a profile across the Pacific Ocean

This paper presents a model of geological interpretation of gravity observations along the profile across the Pacific Ocean from the Japan trench across the Hawaiian ridge to the Middle America trench crossing eleven geological regions (the total length of the profile is about 14,000 km). The interpretation was made by comparing the gravitational attractions of the combined layer of the hydrosphere and the crust and upper mantle from sea level to the depth of isostatic compensation by using the fundamental principles of the theory of isostasy. The density of the crust and mantle beneath the M discontinuity was determined from the results of deep seismic sounding (DSS) and the correlation between compressional velocities and densities of rocks. A statistical analysis was made of the data obtained by many scientists to improve the correlation for different rocks at pressures of 0.001–10 kb. Density distribution curves are presented for different rocks and different velocity intervals. The lateral density inhomogeneities in the lower part of the combined layer from a depth of 50 km to the depth of isostatic compensation are supposed to cause regional gravity anomalies of the first order determined from satellite observations, whereas density inhomogeneities of the combined layer of the hydrosphere and the crust and upper mantle to a depth of 50 km account for residual gravity anomalies obtained from free-air anomalies (the normal gravity formula derived by Helmert (1901–1909)) after the regional first-order anomalies are subtracted from the satellite data. For the profile across the Pacific Ocean we obtained curves of regional gravity anomalies of the first order, residual gravity anomalies, ocean-floor topography, variations of the apparent density of the combined layer of the crust and mantle to a depth of 50 km below sea level, variations of the apparent density of the upper-mantle layer to the same depth, variations of heat flow, and a model of a density section to a depth of 50 km below sea level. The density of the crustal rocks varies from 2.7 to 3.0 g/cm3. The upper-mantle layer has its highest density (3.42 g/cm3) directly beneath the M discontinuity under the Hawaiian ridge (from DSS) and under the Japan trench (under the continental slope). Compact rocks lie beneath the northwest Pacific basin and the mid-Pacific mountains and also in fracture zones. The upper-mantle rocks of the lowest density (3.13 g/cm3) lie beneath the east Pacific ridge, the Hawaiian ridge (at a greater depth), and in the form of isolated horizontal lenses beneath the mountains of the Darwin uplift. Anomalously high values of the heat-flow intensity were found. A suggestion is made that the upper-mantle rocks with a density of 3.13 g/cm3 west of the Hawaiian ridge are genetically related to the rocks of about the same density lying at the base of the third crustal layer (density obtained from DSS data). This interesting region requires geophysical investigation and deep drilling to reach the rocks with a 3.13-g/cm3 density at a depth of about 3 km. These studies will throw light on the processes of transformation of rocks in the crust and upper mantle.