Naghan (Chahar Mahal Bakhtiari-High Zagros, Iran) Earthquake of 6 april 1977. A preliminary field report and a seismotectonic discussion

Abstract

The Naghan earthquake of magnitude 6 (Ms) occurred on 6 April1977 in the mountanious area of Chahar Mahal Bakhtiari in the HighZagros, south of Shahr-e-Kord. It killed 348 people, injured about 200,and caused destruction over an area of 150 Km2. The shock damagedbeyond repair 2,100 houses and killed 0.7°/o of livestock in the area;eight schools collapsed and 37 were damaged. The maximum intensityot' the main shock did not exceed VIII (MM).The earthquake was associated neither with any fresh surface faulting,nor with reactivation of the existing faults and salt domes atsurface. Heavy rain fell before and during the earthquake and thedestruction was more extensive where the saturated clay content of thealluvium was higher; it was also more extensive in water-logged areasor the areas with a shallower water table. Landslides occurred on steepslopes and destroyed or damaged some villages built on slopes. Severalaftershocks caused additional damage to the already destroyed ordamaged villages.The Naghan earthquake was another instance of the « subsedimentaryZagros-Type Earthquake » in the High Zagros part of the Zagros ActiveFolded Belt, indicating that the re-adjustment of the unexposed metamorphosedPrecambrian Basement at depth caused no tectonic deformation(surface faulting) at the top of the sedimentary cover. This wasdue to the presence of the Upper Precambrian Hormoz layers (Saltdeposits) acting as a slippage zone along the decollement surface of theZagros at depth.