A STUDY OF THE BARISAKHO-GEORGIA-EARTHQUAKE OF 1992-OCTOBER-23 FROM BROAD-BAND SURFACE AND BODY WAVES

We study the 1992 October Georgian earthquake located in the central p art of the Great Caucasus. This is the last strong (M-s = 6.5) shallow earthquake of a series of three that struck this region in the two-ye ar period of 1991-1992. In order to determine the source parameters we analysed surface and body waves from the IRIS and GEOSCOPE very broad -band digital networks. The fundamental mode of surface waves was retr ieved using frequency-time analysis (FTAN), a technique that makes inv ersion more stable and enables the measurement of the spectral feature s from the signal in the presence of stationary noise. We used long-pe riod surface waves (60-150 s) to obtain the seismic moment (M-0 = 4.8 x 10(18) N m) and the focal mechanism (strike theta = 300 degrees, dip delta = 15 degrees and rake lambda = 135 degrees). The space-time dis tribution of the source moment density tensir was determined from inte rmediate-period (35-60 s) surface waves. We found the following result s: duration = 6 a, rupture velocity = 3.2 km s(-1), major and minor ax es of the fault surface l(max) = 32 km and l(min) = 20 km, respectivel y. For the inversion of body waves we performed a shape waveform model ling using the gradient method of Nabelek. We considered the source as a point source of 6 s duration. The results of body-wave inversion (M -0 = 3.0 x 10(18) N m, theta = 300 degrees, delta = 17 degrees and lam bda = 137 degrees) are in good agreement with estimates from surface w aves. One of the nodal planes of the fault-plane solution corresponds to a thrust fault which agrees with the general seismotectonic feature s of the region of the Kartli and Kokheti ridges in the central part o f the Great Caucasus. The southern slope of the Great Caucasus strikes WNW-ESE and the underlying crystalline basement plunges towards the N NE under the main axial zone. The source plane coincides with the loca tion of the contact between the crystalline basement and the upper-blo ck overthrust.