SEISMOTECTONIC OF THE WESTERN ALBORZ (IRAN) DEDUCED FROM MICROEARTHQUAKE SEISMICITY

A dense seismological network of 30 stations was installed in the epicentral area of the 1990 June Roudbar-Tarom earthquake to precise the geometry and the kinematics of the fault system in this area. The analysis of the microseismicity recorded during 7 weeks reveals that the crustal structure beneath the Rudbar region consists of an ~6-km thick sedimentary layer (~5.4 km/sec) above an 8-km thick upper crust (Vp=5.9 km/sec), overlaying a slow (Vp=6.3 km/sec) 21-km lower crystalline crust. Location of selected events confirms seismic activity of the Zard-Goli, Kelishum and Kabateh and Majil faults. We observe several alignments of the microseismicity roughly oriented NE-SW which can be associated either to the bent termination of some known faults like Manjil in the region or to the active structures with the same orientation especially those located north of Rudbar. Distribution of the focal depths indicates that in the west the seismicity is vertical and probably related to the Kabateh rupture. In the central parts, the dip of seismicity progressively changes from 45° to vertical. Reverse faulting is associated with the Eastern part of the Manjil fault, dipping northward. The calculated focal mechanisms confirms the partitioning of oblique convergence of the Central Iran toward South Caspian basin accommodates by reverse faulting on the Manjil fault and strike-slip faulting on the Zard-Goli fault.