ABSTRACT The megacity of Tehran, the political, economic, and military center of Iran, is exposed to a risk of large-magnitude earthquakes originating on several adjacent and inner-city active faults. The city lies at the southern foot of the central Alborz Mountains, which frame the South Caspian Basin and have been the source of damaging historical left-lateral strike-slip and reverse-fault earthquakes. The most recent destructive earthquake in Alborz was the Rudbār left-slip earthquake of M w 7.3 on 20 June 1990 northwest of Tehran, taking more than 40,000 lives and destroying three cities. This earthquake was in a seismic gap, and its source fault did not show clear geomorphic signs of being active prior to the earthquake. East of Tehran, the 22 December 856 Komesh (Dāmghān) earthquake had a magnitude previously estimated at M s 7.9, with estimated losses of 40,000–200,000 lives. Our reevaluation of historical, archaeological, and structural evidence reduces estimates of both magnitude and losses, similar to the 1990 Rudbār earthquake. The latest earthquake to affect the present Tehran metropolitan area was the Lavāsānāt earthquake on the central section of the Moshā fault, on 27 March 1830, with its epicenter located ~30 km northeast of the city, which had a magnitude of M w ~7.0–7.4. Prior to this event, the Ruyān earthquake north of Tehran struck the same section of the fault on 23 February 958, with a magnitude previously estimated as M s 7.7, although our reevaluation reduces the magnitude to around M w ≥7.0 (7.0−7.4). Both the 958 and 1830 earthquakes along the central segment of the Moshā fault, with an interval of 872 yr, might have loaded the North Tehran fault system near the cities of Tehran and Karaj, as well as the faults underneath the metropolitan area. The North Tehran fault system west of Tehran might have sustained an earthquake of M w ~7.0 in May 1177. The earthquake histories of the Niāvarān, Darakeh, Farahzād, and Kan left-lateral strike-slip faults (part of the North Tehran fault system at the mountain front north of the city), the inner-city Mahmudieh and Dāvudieh south-dipping reverse faults, the central-eastern section of the North Tehran fault system (now within the metropolitan area) as well as blind thrusts under the city are unknown. Except for the 1830 distant earthquake, no medium-to large-magnitude earthquake (M w 6.5–7.5) has occurred within the Tehran metropolitan area during the past 839 yr along the faults beneath the metropolitan area or in the immediate vicinity. This may indicate a >839-yr-long period of strain accumulation within a long interseismic period between large-magnitude earthquakes in Tehran. With the active-fault hazard to the rapidly growing population along several faults, it is necessary for the government to: (1) conduct extensive paleoseismic trenching to identify the most hazardous of Tehran’s faults, previous rupture areas, average coseismic slip rates, earthquake magnitudes, and average recurrence periods of earthquakes from at least eight fault systems within the metropolitan area; (2) deal with extensive corruption of the construction and building-inspection industries; and (3) enforce the 1969, 1988, and 1999 Iranian Code for Seismic Resistant Design of Buildings. As with the 12 January 2010, M w 7.0 Haiti earthquake, losses from the next Tehran earthquake of M w ≥7.0 could exceed 100,000 people. It is necessary to prepare and implement an earthquake management master plan as a disaster prevention tool, enforce the building code with transparency, and retrofit public structures and infrastructure in order to mitigate earthquake risk in Tehran and protect the lives of ~15 million people (roughly 20% of the country’s population) living in the Tehran and Alborz (Karaj city) Provinces. Considering the continuous threat of earthquake hazard in Tehran and the suburbs, we should immediately prepare the necessary plans to mitigate the earthquake risk by utilizing the scientific and engineering techniques derived from earth sciences; we should do this before an earthquake strikes. —Professor Setrāk Ābdāliān, 1951, Tehran (translated from Persian)

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