Active faulting in the Birjand region of NE Iran

We use satellite imagery and field observations to investigate the distribution of active faults around Birjand in eastern Iran to determine how the transition between conjugate zones of faulting can be accommodated by diffuse active faulting. In the south of the study area, right‐lateral strike‐slip faults of the Sistan Suture Zone end in thrusts which die away westward from the strike‐slip faults. These thrust terminations appear to allow a northward change to E‐W thrusting in central parts of the study area. The introduction of E‐W thrusting is, in turn, likely to facilitate a change to E‐W left‐lateral faulting north of the study region. The relatively diffuse pattern of active faulting at Birjand relates to the regional transition between N‐S and E‐W strike‐slip faulting in northeast Iran, which involves a change from nonrotational to rotational deformation. The change from N‐S to E‐W faulting is likely to result from the orientation of preexisting structures in Iran and western Afghanistan, which are roughly parallel to the active fault zones. The features described at Birjand also show the influence of preexisting structure on the location and style of active faulting at a local scale, with the position of individual faults apparently controlled by inherited geological weaknesses. Very few modern earthquakes have occurred in the region of Birjand and yet destructive events are known from the historical record. The large number of active faults mapped in this study pose a substantial seismic hazard to Birjand and surrounding regions.