A study of the Barisakho, Georgia, earthquake of 1992 October 23 from broad-band surface and body waves

Abstract

SUMMARY We study the 1992 October Georgian earthquake located in the centra) part of the Great Caucasus. This is the last strong (Ms = 6.5) shallow earthquake of a Series of three that struck this region in the two-year 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 retrieved using frequency-time analysis (FTAN), a technique that makes inversion more stable and enables the measurement of the spectral features from the signal in the presence of stationary noise. We used long-period surface waves (60-150 s) to obtain the seismic moment (Mo =4.8~10'~ N m) and the focal mechanism (strike 8=300, dip 6 = 15 and rake A= 135). The space-time distribution of the source moment density tensor was determined from intermediate-period (35-60 s) surface waves. We found the following results: duration = 6 s, rupture velocity = 3.2 km s-I, major and minor axes of the fault surface I,,, = 32 km and Zmin = 20 km, respectively. For the inversion of body waves we performed a shape waveform modelling using the gradient method of NabElek. We considered the source as a point source of 6 s duration. The results of body-wave inversion (Mo = 3.0~ 10 N m, 8 = 300°, 6 = 17 and 1 = 137) are in good agreement with estimates from surface waves. One of the nodal planes of the fault-plane solution corresponds to a thrust fault which agrees with the general seismotectonic features of the region of the Kartli and Kokheti ridges in the central part of the Great Caucasus. The southern slope of the Great Caucasus strikes WNW-ESE and the underlying crystalline basement plunges towards the NNE under the main axial zone. The source plane coincides with the location of the contact between the crystalline basement and the upper-block overthrust.