Active Thrust Front of the Greater Caucasus - the April 29, 1991, Racha Earthquake Sequence and Its Tectonic Implications

Publication Type  Journal Article
Year of Publication  1995
Authors  Triep, E. G.; Abers, G. A.; Lernerlam, A. L.; Mishatkin, V.; Zakharchenko, N.; Starovoit, O.
Journal Title  Journal of Geophysical Research-Solid Earth
Volume  100
Issue  B3
Pages  4011-4033
Journal Date  Mar 10
ISBN Number  0148-0227
Accession Number  ISI:A1995QM54700018
Key Words  arc-continent collision; fault plane solutions; intraplate earthquakes; deep earthquakes; seismicity; beneath; inversion; himalaya; geometry; adjacent
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 (M(S) = 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 similar to 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 (similar to 2-12 lan). 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 degrees-31 degrees 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 rat;: 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, 1891, event and its aftershocks, southeast of the primary aftershock zone along strike, show fault planes and slip vectors rotated similar to 41 degrees clockwise from the mainshock. This rotation is consistent with an along-strike change in direction 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.

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Qm547Times Cited:23Cited References Count:70

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