Microseismic evidence of slip partitioning for the Rudbar‐Tarom earthquake ( M s 7.7) of 1990 June 20 in NW Iran

SUMMARY The focal mechanism of the destructive earthquake at Rudbar in northern Iran on 1990 June 20 was an unexpected left‐lateral strike‐slip motion on a previously unknown fault, the Baklor–Kabateh–Zard‐Goli fault, within a complex system of reverse faults. We analysed microseismicity recorded by a dense local network deployed for 7 weeks, 8 yr after the main shock, to help evaluate the subsurface geometry and kinematics of the active fault system. In the west, the 1998 microseismicity seems to be related to the reverse Manjil. In the centre, we observe both reverse faulting associated to the Manjil fault and strike‐slip faulting associated with de Zard‐Goli rupture whose dip progressively changes from 45°N to vertical. In the east, the reverse faulting is located south of the Manjil fault and strike‐slip faulting to the vertical Zard‐Goli fault. We suggest that because the only stable geometry that accommodates large partitioning motion prevents the strike‐slip fault to intersect the reverse fault, the active reverse fault jumps southwards beneath the Shahrud fault.