The 1957 Alborz earthquake struck northern Iran's Mazandaran province on 2 July 1957. It had a moment magnitude (M_w ) of 7.1 and occurred at a focal depth of 15 km (9.3 mi). The thrust-faulting shock was assigned a maximum Modified Mercalli intensity of IX (Violent). It devastated 120 villages in the Alborz Mountains and caused an estimated 1,500 fatalities. The earthquake also triggered landslides including one that dammed the Haraz River. Some damage was also reported in Tehran, Qaem Shahr and Sari. The total damage was estimated at US$25 million.

The Iranian plateau is a region of highly deformed crust wedged between the Arabian Plate to the southwest and Eurasian Plate to the northeast. Crustal deformation is dictated by oblique-convergence of 22 cm (8.7 in) per year between the two tectonic plates. Deformation is accommodated by reverse and strike-slip faulting. Off the country's Makran coast, the seafloor undergoes active subduction. In northern and southern Iran, thrusting allows convergence through crustal thickening to take place. Most of the convergence is accommodated along fold and thrust belts of the Zagros, Alborz, Kopet Dag mountains, and in eastern Iran, where seismicity is high. Seismicity in the central plateau is low due to its geology consisting of rigid and aseismic tectonic blocks such as the Central Iran, Lut Block and Southern Caspian blocks.

The earthquake was the result of shallow thrust faulting along the Larzaneh Fault, a shallow south-dipping thrust fault in the central Alborz Mountains. The earthquake's rupture dimensions was an area of fault estimated at 50 km (31 mi) by 25 km (16 mi). It was the second largest instrumentally recorded earthquake in the Alborz, behind the 1990 Manjil–Rudbar earthquake. The earthquake's fault plane solution indicate it occurred along a northwest–southeast trending reverse fault. The corresponding Mercalli intensity isoseismal lines showed an east–west trend and greater damage to the south implying the fault trended east–west and dipped southwards direction. The shock occurred near a section of the Larzaneh Fault that connects with another opposite-dipping reverse fault.

No evidence of surface rupture associated with the earthquake were found, partly because the area of maximum damage was inaccessible and rocklides cut-off routes through the mountains. However, some fault movement was observed along the Amir Fault Zone, a south-dipping reverse fault. Fresh cracks were reported along a road that crosses the fault. Nicholas Ambraseys also documented 3 km (1.9 mi) of surface faulting which were not coseismic. These features were possibly the result of landslides. Several large fissures extending for several kilometers in the mountainous area, north of Sangchal, thought to be surface ruptures were fractures from a landslide.

The earthquake killed over 1,500 people and levelled 120 villages. The meizoseismal area was located between the Haraz and Talar rivers; encompassing the dehestans of Band Pay, Beh, Dala Rustaq and Chalav. The villages of Sangchal, Nandal, Chaliyasar, Nasal, Andvar and Pardameh were the hardest-hit. In Burun, Varzaneh, Shanguldeh, Nal and Dinan, rockfalls and landslides contributed to further devastation. A 1 km (0.62 mi) long reservoir formed in the Haraz River near Aliabad after a landslide obstructed and formed a 20 m (66 ft) high dam across the river. Engineered infrastructures along Haraz road had little damage. In Bayjan, the abutments of an 80 m (260 ft) long masonry arch bridge sunk, and its deck fractured. Cracks appeared inside a tunnel connecting Kuhrud and Bayjan. Minor damage was observed at a tunnel near Nur and on a wooden bridge in Aliabad.

Damage beyond the meizoseismal area was distributed over a wide area though less severe. The severity of damage decreased dramatically with distance towards the northwest and southeast compared to the northeast and southwest. Additional damage occurred in Pul-i Safis and Shirgah, and several homes were destroyed in the town of Utu situated along the Kaslian River. Caved roofs were reported in Polur and Fasham, while some homes in Tehran sustained damage. Some modern constructions in Qaem Shahr and Sari also sustained damage. Heavy damage occurred in an imamzadeh at Vāneh and the Emamzadeh Hashem Shrine.

• List of earthquakes in 1957
• List of earthquakes in Iran

Sources

• The International Seismological Centre has a bibliography and/or authoritative data for this event.