A high-precision laser gravimeter for measuring the absolute value of gravitational acceleration by the free-fall method has been built in the Siberian Branch of the USSR Academy of Sciences (Novosibirsk). The use of a free-fall interferometer illuminated by a He-Ne laser stabilized by an iodine absorption cell ('"I2) together with high-speed electronic counters and a rubidium frequency standard, plus constructive measures aimed at reducing the action of non-gravitational forces, provide an accuracy of measurements better than lo-' m.s-2. In practice, the relative measurement error in g, as determined from convergence of the results of repeated measurements, is below 4 parts in l o 9 (4 xThe gravimeter is used for geophysical experiments and in the investigation of non-tidal variations of the Earth's gravitational field. m.s-').

The fifth in the series of International Comparisons of Absolute Gravimeters (ICAG) was held at the Bureau International des Poids et Measures (BIPM) in November 1997. Fifteen absolute gravimeters participated in the comparison. The mean gravity value obtained at station A (0.9 m) at the BIPM was found to be 980 925 707.8 µGal with a standard uncertainty of 2.8 µGal. This is consistent with the results obtained in previous comparisons at this site. Conclusions based on the analysis of the present results and proposals for future activities are presented.

A ballistic gravimeter has been constructed using a laser interferometer to measure the vertical distance traversed by a free-falling body and a rubidium frequency standard to measure elapsed time. The error in distance and time measurement are estimated to be less than 2 x 10(-8) and 2 x 10(-11) respectively. The systematic error in measuring gravitational acceleration, after corrections have been made, does not exceed 0.02 mgal.

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Modern methods for determination of gravity values make it possible to obtain measurements with the accuracy up to 10 -9 from g 0 of the normal value (up to 1 microgal = 10 m/sec 2 ). While all the systematic and periodic effects are excluded, a question is raised about stability of the gravity field of the Earth over time. Changes of the altitude (the Earth's radius) with time can be estimated with an accuracy of 0.1 mm by modern space geodetic techniques, such as VLBI method. Our experi ments for evaluation of stability of the gravity values over the past decades are based on the data obtained by Russian and foreign observatories using absolute ballistic laser gravimeters. The results put a limit of 10 -10 per year to changes of the Earth's radius. These estimations can be useful for testing hypotheses in tectonics.Measurements of nontidal variations of gravity (∆g), which were obtained from 1992 to 2012 at the Talaya seismic sta tion (located in the southwestern part of the Baikal region), are interpreted together with GPS observation data. At the Ta laya seismic station, the linear component of gravity variations corresponds to changes in the elevation of this site. The corre lation coefficient is close to the normal value of the vertical gradient of gravity. At this site, coseismic gravity variations at the time of the Kultuk earthquake (27 August 2008, Mw=6.3) were caused by a combined effect of the change of the site's elevation and deformation of the crust. Our estimations of the coseismic effects are consistent with results obtained by mod eling based on the available seismic data.Key words: monitoring of gravity variations, changes in the Earth's radius, rift zone, GPSmonitoring, earthquake.Recommended by V.A. San'kov Citation: Timofeev V. Yu., Kalish Ye.N., Stus' Yu.F., Ardyukov D.G., Arnautov G.P., Smirnov M.G., Timofeev A.V., Nosov D.A., Sizikov I.S., Boiko E.V., Gribanova E.I. 2013. Gravity variations and recent geodynamics of the southwestern part of Baikal region.