A large set of seismic events recorded along the southern flank of the central Alborz in the Tehran, Iran, region during 1996–2008 were relocated using the double-difference technique and an appropriate velocity model. The relocated hypocentral locations improve significantly in comparison with those routinely determined by the Iranian Seismological Center. The distribution of 1402 relocated events delineates coherent features corresponding to major and some unknown faults. Seismicity associated with the Mosha fault forms a prominent plane dipping toward ∼20° N. South of the Mosha fault, there are two distinct tightening of events, one with strike gently turning clockwise relative to the Mosha fault trend and another with a west-to-east trend. Relocations confirm the existence of activity northeast of the Firuzkuh fault. This cluster extends approximately parallel to the Firuzkuh fault strike with a dip to the south–southeast. Also, there are probably several faulting systems around the Parchin fault with different strikes. Cross sections indicate that the southeastern part of Parchin fault dips toward the northeast. Our relocations for events of the mountain front south of the Taleghan fault delineate a distinct south and southwestern dipping seismicity zone extending up to 30 km.