TEHRAN: EARTHQUAKE TIME BOMB

Tehran lies at the southern foot of the Alborz Mountains, which frame the southern Caspian Basin and have been the source of major historical strike-slip and reverse-fault earthquakes. The most recent was the Rudbar left-slip earthquake of M_w 7.3 on 20 June 1990 northwest of Tehran, taking more than 40,000 lives. This earthquake filled a seismic gap, and its source faults did not show geomorphic signs of being active prior to the earthquake. East of Tehran, the 22 December 856 Damghan earthquake had a magnitude estimated at M 7.9 with claimed losses 40,000-200,000 people, but re-evaluation of the evidence greatly reduces estimates of both magnitude and losses. In contrast, Tehran itself, with a rapidly-growing population of 12-15 million, appears to be in a seismic gap. The only earthquake in modern times in the Tehran metropolitan area struck the Mosha fault north and northeast of the city with an earthquake of M_s ≈ 7-7.3 on 27 March 1830. Other active faults include the North Tehran thrust, which may have sustained an earthquake of M_s ≈ 7.2 in May 1177. The Ruyan earthquake northwest of Tehran may have struck the Taleqan fault on 23 February 958 with a magnitude reevaluated here as M_s 7.1. The Holocene earthquake histories of the Niavaran and Darakeh left-slip faults and the Mahmudieh and Davudieh south-dipping reverse faults within the city and the Kahrizak, Pishva, and Parchin faults in the southeastern suburb of Ray are unknown. Earthquake hazards to Tehran from active faults indicate the necessity for the Iranian government to (1) conduct extensive paleoseismic trenching to identify the most hazardous of Tehran’s faults, (2) deal with extensive corruption of the construction and building-inspection industries, with a Corruption Perception Index rating by Transparency International of 28 out of 100, and (3) enforce the Iranian Code for Seismic Resistant Design of Buildings (1969, revised 1988). Losses from the next Tehran earthquake of M_w > 7 could exceed 100,000.