Active thrust front of the Greater Caucasus: The April 29, 1991, Racha earthquake sequence and its tectonic implications

Abstract

Although fault‐bounded thrust sheets are common in the geological record, seismic evidence for their motion is sparse. The April 29, 1991, Racha earthquake (Ms = 7.0), the largest instrumentally recorded earthquake in the Greater Caucasus, is one of the largest recent earthquakes 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 seismographs and various other instruments, we locate 1952 aftershocks occurring between May 7 and June 30, 1991. The aftershocks form a zone ∼70 km long and 10–25 km wide striking E‐W, following the Racha ridge at the southern boundary of the Greater Caucasus thrust system. Teleseismic body waves are inverted for source parameters of the mainshock and the two largest aftershocks. The solutions show thrust faulting with centroid depths of 3–10 km, comparable to depths of locally recorded aftershocks (∼2–12 km). The shallow‐dipping nodal plane, the aftershock distribution, and surface geology demonstrate that the main event was caused by faulting on a thrust system dipping NNE at 20°–31° bounding the southern slope of the Greater Caucasus. This fault system thrusts the Greater Caucasus structures south over the Dzhirula basement massif. The inferred fault geometry suggests that the active fault is either a detachment between sediments and Dzhirula basement or cuts through the basement at shallow depths. The 1500‐m‐high Racha ridge overlies the aftershock zone and is a likely consequence of repeated similar earthquakes. Hence the 1991 earthquake sequence shows that the western Greater Caucasus is accommodating plate convergence at a rate possibly comparable to the eastern Greater Caucasus (a few millimeters per year). Along‐strike geological discontinuities above and below the thrust surface correspond to the eastern end of the mainshock rupture area. No strong evidence for transfer structures could be found along strike, suggesting that differences in collisional style between the western and eastern Greater Caucasus may reflect differences in mechanical properties rather than differences in convergence rate. A June 15, 1991, event and its aftershocks, southeast of the primary aftershock zone along strike, show fault planes and slip vectors rotated ∼41° clockwise from the mainshock. This rotation is consistent with an along‐strike change in direction of the thrust front, near 44°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.

Cenozoic-Recent tectonics and uplift in the Greater Caucasus: a perspective from Azerbaijan

“…The Greater and Lesser Caucasus are seismically active zones linked to the rapid and non-uniform plate convergence between Arabia and Eurasia (Philip et al 1989;Jackson 1992;Priestley et al 1994;Triep et al 1995;Jackson et al 2002;Allen et al 2004Allen et al , 2006 (Fig. 1).…”

Caucasus Seismic Information Network: Data and Analysis Final Report

“…It buried at least one village on the southern slope with landslide rock debris. The aftershock zone was 10-25 km wide and more than 70 km in length, interpreted to lie beneath the southern slope (Triep et al, 1995 and 5. This fault was proposed by Milanovsky (1968Milanovsky ( , 1977, Philip et al (1989) to be a major crustal transform fault that extends further SW, joining the Anatolian system of major faults.…”

“…The predominant mechanism for these small magnitude earthquakes is volcanic activity and thus cannot be assigned to individual faults. These earthquakes should be considered independent of the potential BKFZ; any difference in seismicity is possibly due to a transition in mechanical properties (Triep et al, 1995).…”

“…These studies indicate, however, that there is little internal shortening within the Greater Caucasus and that the present rate of shortening across the Caucasus cannot account for all the observed strain in the region. and 10 km (Triep et al, 1995). Overall, large magnitude earthquakes are mainly located near the margins of the range, and seismicity is higher to the east of the Caucasus mountains (Jackson, 1992) beneath the Lesser Caucasus (Ritzwoller et al, 1998;Hearn and Ni, 1994;Al-Lazki et al, 2004).…”

“…However, incorporating the seismic shortening from the 1991 Racha event and appropriate aftershocks eliminates the proposed differential across the BKFZ (Triep et al, 1995).…”