Microseismicity and seismotectonics of the Firuzkuh and Astaneh faults (East Alborz, Iran)

In 2007 and 2008, we installed two local seismological networks of 10 stations around the two active faults of Firuzkuh and Astaneh, which are part of the Sharud fault system located in Central Alborz, ~ 150 km east of Tehran, Iran. These networks were deployed for about 7 months each and recorded 785 and 1194 earthquakes, respectively. We analyzed the seismicity and focal mechanisms together with data from the permanent network of the Institute of Geophysics (University of Tehran). The tectonics of this area shows a pull-apart relay (Bashm and Chashm faults) between the 2 left-lateral strike–slip faults of Firuzkuh and Astaneh. Firstly, we computed the velocity structure which minimizes the travel time residuals and obtained a sedimentary 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 the North Semnan fault is dipping north and both the Chashm and Astaneh faults are vertical. The Garmsar, Bashm and North Alborz faults seem to be less active. Focal mechanisms associated with the Chashm and Astaneh faults show mainly strike–slip motion (left-lateral on a NE–SW trending active plane) with no difference between the 2 faults, suggesting they behave similarly.

Research highlights
► This paper show the results regarding the seismicity of 2 active faults located close to Tehran which is the capital of Iran
► It shows that the pull-apart basin located between the 2 faults is only a surface feature.
► At depth, the 2 faults are linked by a strike–slip relay fault
► This suggests that the 2 faults of Firuzkhu and Astaneh could join and create an earthquake of magnitude greater than 7