ACTIVE THRUST FRONT OF THE GREATER CAUCASUS - THE APRIL 29, 1991, RACHA EARTHQUAKE SEQUENCE AND ITS TECTONIC IMPLICATIONS

Although fault-bounded thrust sheets are common in the geological reco rd, seismic evidence for their motion is sparse. The April 29, 1991, R acha earthquake (M(S) = 7.0), the largest instrumentally recorded eart hquake in the Greater Caucasus, is one of the largest recent earthquak es in continental thrust belts and provides evidence on mechanisms of thrust sheet motion. Using data from a deployment of program for Array Seismic Studies of the Continental Lithosphere (PASSCAL) digital seis mographs and various other instruments, we locate 1952 aftershocks occ urring between May 7 and June 30, 1991. The aftershocks form a zone si milar to 70 km long and 10-25 km wide striking E-W, following the Rach a ridge at the southern boundary of the Greater Caucasus thrust system . Teleseismic body waves are inverted for source parameters of the mai nshock and the two largest aftershocks. The solutions show thrust faul ting with centroid depths of 3-10 km, comparable to depths of locally recorded aftershocks (similar to 2-12 lan). The shallow-dipping nodal plane, the aftershock distribution, and surface geology demonstrate th at the main event was caused by faulting on a thrust system dipping NN E at 20 degrees-31 degrees bounding the southern slope of the Greater Caucasus. This fault system thrusts the Greater Caucasus structures so uth over the Dzhirula basement massif. The inferred fault geometry sug gests that the active fault is either a detachment between sediments a nd Dzhirula basement or cuts through the basement at shallow depths. T he 1500-m-high Racha ridge overlies the aftershock zone and is a likel y consequence of repeated similar earthquakes. Hence the 1991 earthqua ke sequence shows that the western Greater Caucasus is accommodating p late convergence at a rat;: possibly comparable to the eastern Greater Caucasus (a few millimeters per year). Along-strike geological discon tinuities above and below the thrust surface correspond to the eastern end of the mainshock rupture area. No strong evidence for transfer st ructures could be found along strike, suggesting that differences in c ollisional style between the western and eastern Greater Caucasus may reflect differences in mechanical properties rather than differences i n convergence rate. A June 15, 1891, event and its aftershocks, southe ast of the primary aftershock zone along strike, show fault planes and slip vectors rotated similar to 41 degrees clockwise from the mainsho ck. This rotation is consistent with an along-strike change in directi on of the thrust front, near 44 degrees E longitude, and demonstrates strong local structural or topographic control on slip direction. The rotation requires along-strike shortening within the Greater Caucasus thrust system at a rate comparable to the rate of thrusting.