Essential features of seismotectonics of Central segment of the Alpine Belt

Category Tectonic & Seismotectonic
Group GSI.IR
Location 4th internetional Conference on Seismology
Holding Date 11 March 2008
General:
     Structural zoning of Central segment of Alpine Belt, including Great and Lesser Caucasus, Kopet-Dagh, Elbours, Zagros, Central Iran upland, Turan-Skiphian and Arabian plates consists of foredeeps, external and internal fold systems, Central Iran Stable massif and surrounding ancient and young platforms. Deep faults separate these essential tectonic units. The main geodynamic moving force for the fold-Fracture deformation of Central segment of Alpine Belt From GPS data is the interaction of the Arabian, Skihpian and Central Iranian lithosphere plates. The Arabian plate displacement to the northeast due to spreading the Red sea rift at the recent and modem stages, and also spreading of the Central Iran region subsiding and the recent volcanism cause the intensive compression stress in the Zagros fold system. Similarly the Kopet-Dagh and Caucasus fold zones are compressed between Turan-Skiphian and Central Iran platforms. This development type results the domination of compressive stress in outer zones of the fold region, being as a result of orogenic uplift and increasing of the axial zone. Stress field is characterized by horizontal orientation of the compression axis across the external and internal fold systems. In such field most active From geological and seismological points of view are reverse boundary faults, separating the above mentioned main tectonic zones, and shear diagonal faults, separating blocks of the earth's crust inside fold zones and foredeeps.
Few recent earthquakes, occurred in some segments of central sector of the Alpine Belt in the end of XXth Century were studied very carefully: seismic ruptures were mapped and excavated, aftershocks were observed by temporal seismic station networks. In the Alpine Belt author took part in epicentral investigations of rupture of Kum-Dagh, 1983, Ms=5,9 and Burun,1984, Ms=6,2, earthquakes in Western Turkmenistan, Spitak disastrous earthquake in Lesser Caucasus (Armenia) in 1988, Ms=7,0, Racha earthquake in Great Caucasus (Georgia), 1999 Ahram Mw=5.2 and Kareh Bas Mw=6.2 earthquake in Central Zagros (Iran). Tectonic position of the sources, seismological and geological manifestations of shocks allow the compiling the seismotectonic model of each earthquake source and
identify the active faults responsible for every event occurrence.
The excavation of trenches across the rupture of above mentioned earthquakes has been carried out. All these studied areas are the typical tectonic active regions of Alpine Tectonic Belt such as the KopetDagh, the Lesser and Great Caucasus, Zagros fold systems. At least three very strong (Mmax=7.5 and more) prehistoric earthquakes with the recurrence interval about 8000-14000 years occurred in Western Turkmenistan in Late Pleistocene. Five seismic shocks were discovered in trenches in the North-Western Caucasus and four events in the Central Caucasus for 10000 years. Thus, recurrence interval there estimated about 2000 years for these regions. Three seismic events with recurrence interval about 10000 years were recognized in the Northern Armenia (Spitak) seismoactive zone. Seismic offsets with amplitude 0.5-0.7 m, created colluvial wedge, were discovered in zone of the Kazerun-Borazdjan active fault in the Central Zagros. The earthquakes had magnitude about 7 and The main results of this research are as follow: 1) strong and major earthquake sources are the stable structures in the geological environment; 2) seismic shocks repeatedly occur in the same source zone; 3) in the Central segment of the Alpine Belt during last 26000 years there are both periods of seismic activation (0-11 000, 17000-19000 and 25000-26000 years b.p.) and the periods of seismic quiescent (12000-16000 and 20000-24000 years b.p.); 4) thus, the Holocene is period of seismic activation.

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