SEISMIC SOURCE STUDY OF THE RACHA-DZHAVA (GEORGIA) EARTHQUAKE FROM AFTERSHOCKS AND BROAD-BAND TELESEISMIC BODY-WAVE RECORDS - AN EXAMPLE OFACTIVE NAPPE TECTONICS

The Racha-Dzhava earthquake (Ms = 7.0) that occurred on 1991 April 29 at 09: 12:48.1 GMT in the southern border of the Great Caucasus is the biggest event ever recorded in the region, stronger than the Spitak e arthquake (Ms= 6.9) of 1988. A field expedition to the epicentral area was organised and a temporary seismic network of 37 stations was depl oyed to record the aftershock activity. A very precise image of the af tershock distribution is obtained, showing an elongated cloud oriented N105 degrees, with one branch trending N310 degrees in the western pa rt. The southernmost part extends over 80 km, with the depth ranging f rom 0 to 15 km, and dips north. The northern branch, which is about 30 km long, shows activity that ranges in depth from 5 to 15 km. The com plex thrust dips northwards. A stress-tenser inversion from P-wave fir st-motion polarities shows a state of triaxial compression, with the m ajor principal axis oriented roughly N-S, the minor principal axis bei ng vertical. Body-waveform inversion of teleseismic seismograms was pe rformed for the main shock, which can be divided into four subevents w ith a total rupture-time duration of 22 s. The most important part of the seismic moment was released by a gentle northerly dipping thrust. The model is consistent with the compressive tectonics of the region a nd is in agreement with the aftershock distribution and the stress ten sor deduced from the aftershocks. The focal mechanisms of the three la rgest aftershocks were also inverted from body-wave records. The April 29th (Ms = 6.1) and May 5th (Ms = 5.4) aftershocks have thrust mechan isms on roughly E-W-oriented planes, similar to the main shock. Surpri singly, the June 15th (Ms = 6.2) aftershock shows a thrust fault strik ing N-S. This mechanism is explained by the structural control of the rupture along the east-dipping geometry of the Dzirula Massif close to the Borzhomi-Kazbeg strike-slip fault. In fact, the orientation and s hape of the stress tensor produce a thrust on a N-S oriented plane. Na ppe tectonics has been identified as an important feature in the Cauca sus, and the source mechanism is consistent with this observation. A h idden fault is present below the nappe, and no large surface breaks we re observed due to the main shock. The epicentral region is characteri zed by sediments that are trapped between two crystalline basements: t he Dzirula Massif, which crops out south of Chiatoura, and the Caucasu s Main Range north of Oni. Most, if not all, of the rupture is control led by the thrusting of overlapping, deformed and folded sediments ove r the Dzirula Massif. This event is another example of blind active fa ults, with the distinctive feature that the fault plane dips at a gent le angle. The Racha Range is one of the surface expressions of this bl ind thrust, and its growth is the consequence and evidence of similar earthquakes in the past.