UDC 550.312

GEOPHYSICS

A. Sh. FAITELSON, E. A. AZARKINA

SECULAR CHANGES IN GRAVITY IN UKRAINE

(Presented by Academician A. V. Peive, March 13, 1970)

Repeated measurements of gravity at the same points, carried out at intervals of several years, suggest that during this time gravity undergoes changes on the order of tenths of a milligal. Although these changes often lie within the limits of measurement accuracy, indirect confirmation of their reality is provided by a sufficiently good correlation over large areas between the distribution of these changes and the arrangement of elements of the geological setting (for example, tectonic elements) of the region under study. Such a correlation may be explained by the existence of a connection between present-day deep-seated processes in the Earth’s interior, which affect the gravitational field, and the indicated setting. Consideration of the material by area (and not at individual points, or even along profiles) substantially increases the objectivity of the correlation with the geological setting and the reliability of attributing discrepancies in gravity to secular variations (¹, ²).

In 1962 and 1969, under the direction of E. A. Azarkina (the Spetsgeofizika Trust), observations of gravity were carried out by the same method at 36 points uniformly distributed over the territory of Ukraine. The second series of observations was conducted specifically to establish secular variations. The root-mean-square error of determining the points in 1962 was ±0.06 mGal (Worden gravimeter), and in 1969 ±0.05 mGal (gravimeters of the Institute of Physics of the Earth, USSR Academy of Sciences). Thus, the error in determining the differences in gravity acceleration is ±0.08 mGal. No dependence was noted between the detected differences \(\Delta g\) and the measured increments of gravity \((g - g_{\text{ref}})\). This indicates that the influence of possible errors in the scale value of the gravimeters used may be neglected. From the results of repeated observations, differences \(\Delta g\) over a period of 7 years from +0.4 to −0.5 mGal were obtained relative to the initial point in Kiev, where the value of gravity \((g_{\text{ref}})\) was conventionally assumed to be unchanged. On the compiled map of differences \(\Delta g\) (Fig. 1), extensive areas of positive and negative values stand out, with a very smooth transition between them.

A correlation is observed between the detected discrepancies in gravity and the tectonic elements of Ukraine (³), which makes it possible to explain these discrepancies to a considerable extent by secular variations \(\Delta g_t\) (in our case \(\Delta g_t = g_{1969} - g_{1962}\)). The area of positive \(\Delta g_t\) values covers the Ukrainian Shield and the zone of subsidence of the basement toward the Ciscarpathian trough. Northeast of this area, on the Voronezh Massif, the values of \(\Delta g_t\) decrease to −0.5 mGal. The Dnieper-Donets aulacogen is characterized by transitional values of \(\Delta g\), although on the whole it seems to be marked by some increase in \(\Delta g_t\), especially in the southeastern part—in Donbass. Negative \(\Delta g_t\) values characterize the zone of subsidence of the basement toward the Black Sea depression.

Between the two noted series of observations (1962 and 1969), interesting work was carried out to reveal secular variations along the Kharkov—Lvov profile (⁴), with an interval between measurements of one

year (1965–1966). Between the results obtained along this profile and those described in the present work, agreement is observed only in the most general outlines. This makes it possible to recommend that future work be carried out not along a profile but over an area, which, as was already noted, makes it possible

Fig. 1. Diagram of isolines of \(\Delta g_t\) for Ukraine. The mean interval between the initial and repeated observations is 7 years.
1 — values of \(\Delta g_t\) in mGal; 2 — isolines of \(\Delta g_t\) at intervals of 0.2 mGal; 3 — uplifts (\(A\) — Ukrainian Shield, \(B\) — Voronezh Massif); 4 — Dnieper–Donets aulacogen; 5 — regions of Alpine folding.

to judge more objectively the role of secular variations in the obtained differences in the force of gravity (especially if these differences are close to the magnitude of the error).

The authors express their gratitude to Corresponding Member of the USSR Academy of Sciences V. V. Fedynskii for assistance in organizing the work on carrying out the repeated observations and for discussion of its results.

Received
5 II 1970

CITED LITERATURE

1. A. Sh. Faitelson, DAN, 188, No. 3, 579 (1969).
2. A. Sh. Faitelson, DAN, 189, No. 6 (1969).
3. Tectonic Map of Eurasia, A. L. Yanshin, ed., Moscow, 1965.
4. G. T. Sobakar, Reports of the Academy of Sciences of the Ukrainian SSR, Series B, Geological, Geophysical, Chemical and Biological Sciences, No. 9, 781 (1968).