Microseismicity and seismotectonics of the Firuzkuh and Astaneh faults

abstract Article history:Received 20 September 2010Received in revised form 18 February 2011Accepted 10 April 2011Available online xxxxKeywords:IranAlborzSeismicityEarthquakes In 2007 and 2008, we installed two local seismological networks of 10 stations around the two active faults ofFiruzkuh and Astaneh, which are part of the Sharud fault system located in Central Alborz, ~150 km east ofTehran, Iran. These networks were deployed for about 7 months each and recorded 785 and 1194earthquakes, respectively. We analyzed the seismicity and focal mechanisms together with data from thepermanent network of the Institute of Geophysics (University of Tehran). The tectonics of this area shows apull-apart relay (Bashm and Chashm faults) between the 2 left-lateral strike–slip faults of Firuzkuh andAstaneh.Firstly,wecomputedthevelocitystructurewhichminimizesthetraveltimeresidualsandobtainedasedimentary cover of 4 km overlying an 8 km-thick upper-crust with a Moho located at a depth of 33.5 km.Secondly, we located the seismicity which is distributed mainly on the Astaneh fault and Chashm fault.Seismicity is also associated with the North Semnan fault located farther south. Cross sections show that theNorthSemnanfaultisdippingnorthandboththeChashmandAstanehfaultsarevertical.TheGarmsar,Bashmand North Alborz faults seem to be less active. Focal mechanisms associated with the Chashm and Astanehfaults show mainly strike–slip motion (left-lateral on a NE–SW trending active plane) with no differencebetween the 2 faults, suggesting they behave similarly.© 2011 Elsevier B.V. All rights reserved.