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



The Naghan earthquake of magnitude 6 (Ms) occurred on 6 April
1977 in the mountanious area of Chahar Mahal Bakhtiari in the High
Zagros, south of Shahr-e-Kord. It killed 348 people, injured about 200,
and caused destruction over an area of 150 Km2. The shock damaged
beyond repair 2,100 houses and killed 0.7°/o of livestock in the area;
eight schools collapsed and 37 were damaged. The maximum intensity
ot' 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 at
surface. Heavy rain fell before and during the earthquake and the
destruction was more extensive where the saturated clay content of the
alluvium was higher; it was also more extensive in water-logged areas
or the areas with a shallower water table. Landslides occurred on steep
slopes and destroyed or damaged some villages built on slopes. Several
aftershocks caused additional damage to the already destroyed or
damaged villages.
The Naghan earthquake was another instance of the « subsedimentary
Zagros-Type Earthquake » in the High Zagros part of the Zagros Active

Folded Belt, indicating that the re-adjustment of the unexposed metamorphosed
Precambrian Basement at depth caused no tectonic deformation
(surface faulting) at the top of the sedimentary cover. This was
due to the presence of the Upper Precambrian Hormoz layers (Salt
deposits) acting as a slippage zone along the decollement surface of the
Zagros at depth.

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Published by INGV, Istituto Nazionale di Geofisica e Vulcanologia - ISSN: 2037-416X