2. REGIONAL SEISMOTECTONIC SETTING
2.1. Regional Structure
As previously stated, the region under consideration is situated in the High Zagros (Thrust Zone) of the Zagros Active Folded Belt (Fig. 2). The geological history of this Belt is comparatively simple, as a relatively quiet sedimentation continued from Upper Precambrian (Infracambrian) to Pliocene. The Paleozoic sediments overlie a shield-like basement of Precambrian age. Regional disconformities occur at the top of the Aptian, the Cenomanian-Turonian, the Cretaceous, and the Eocene (Stocklin 1968, James and Wynd 1965, Berberian 1976 b).
Evaporites have been prominently involved in Zagros tectonism. Jurassic and Miocene evaporitic sequences in the Zagros Active Folded Belt are zones above which disharmonic folding is reported, and similar movements have been postulated at the Hormoz Salt belts of Upper Precambrian age, near the base of the Paleozoic platform rocks, or on top of the metamorphosed Precambrian Basement (the metamorphosed Precambrian Basement does not crop out in Zagros). This important evaporite layer separates the sedimentary column from the basement. In addition, many large Hormoz Salt diapirs, some now active, reach the surface in the Zagros Active Folded Belt (Dunnington 1968, Stocklin 1968 a, b, Falcon 1974, Berberian 1976 b).
The Zagros sediments covering the margin of the Arabian plate were folded mainly during the Main Zagros Orogeny (Wallachian diastrophism) at the end of Pliocene-Pleistocene times, when up to fourteen parallel anticlines and synclines were developed. These structures generally have a northwest-southeast direction, but in the extreme southeast (Fars-Hormozgan provinces) the trend of the fold axes gradually swings towards the east. The spectacular NW-SE striking folds (long, broad structures that are assymmetric or overturned towards the southwest, where they gradually die out) are cut by thrust and high-angle reverse faults that are parallel to the folds but dip both northeast and southwest. These faults appear to be structurally related to the folds, and like them to be the results of Plio-Pleistocene and Quaternary compression.
These deformations also generated along the northern side of the mountain range a major tectonic disconformity, the Main Zagros Reverse Fault, which separates the Zagros domain, in the southwest, from the Central Iranian domain in the northeast.
According to Falcon (1974), the general picture of the Alpine Orogeny in the Zagros Belt is of a series of waves of depressions and uplifts advancing from the northeast towards the southwest. It is thought that the Thrust Zone-Central Iran boundary was close to the northeast margin of the Arabian basement shield at least from the initiation of the Alpine movements.
From the crustal point of view the Simply Folded Belt contains simple folds caused by the rucking up above the decollement of the Phanerozoic column during one erogenic phase. The Thrust Zone is more complex because it has been rucked up twice: the first phase of folding and fauting affected the Thrust Zone in the Cenomanian Turonian to Middle Eocene interval, dominantly in Upper Cretaceous times. The Middle Eocene strata are of flysch facies, while pre-Middle Miocene movements are relatively insignificant in the Simply Folded Belt. In the latter zone only a single phase of folding of Mio-Pliocene age has affected the rocks (Falcon, 1974).
Fig .4. ERTS_A (Landsat-A) imagery of the Naghan(Chahar Mahal Bakhtiari) region in High Zagros The Naghan region of 6 April 1977 earthquake is shown by a circle.
2.2. Regional Seismicity
The very high twentieth century seismicity of the Zagros Active Folded Belt is characterized by a large number of shocks in the magnitude 5 or 6 range, and a very
small number with magnitude equal to or greater than 7 (Fig. 5). The precision of depth calculation of the Zagros is very poor, and without special studies little can be said on the subject, except that most earthquakes occur in the crust, and that they do not appear to exceed 100 km. Series of earthquakes resembling swarms also seem to be characteristic of the Zagros (Berberian 1976 a, b).
The total thickness of the sediments above the metamorphosed Precambrian basement cannot be more than 10 to 15 (or a possible maximum of 18) kilometres. Thus the earthquakes occurring within this depth range may be related to deformation of the sediments and or movements of the salt layers and their diapirism (?).
However, many earthquakes occur at greater depths, and salt diapirism would not necessarily be associated with thrust faulting, whereas the focal mechanisms of most earthquakes are thrust faults. It would therefore appear that most of the earthquakes are subsedimentary and probably related to the basement or upper mantle (Haghipour et al. 1972) and that reactivation of some basement faults during readjustment of the basement causes subsedimentary earthquakes within the Zagros Active Folded Belt.
2.3. Previous Destructive Earthquakes of the Naghan Region
In 1976, July 27 an earthquake of magnitude 5 was strongly felt at Shalamzar. The instrumental epicentre was located at 31.60N, 50.36E and the estimated focal depth was 77 km.
The only known semi-destructive earthquake in Naghan region is the Sarpir Earthquake of 21 September 1975. The instrumental epicentre of this shock, which took place at 14.16.37.8 GMT (17.46.36 local time), is located at 31.599N, 51.037E. The macroseismic epicentre was located at 31.63N, 51.13E (see southeastern corner of Fig. 6). The magnitude was 5.2 (Mb), the estimated focal depth 33 km (N) and the maximum intensity VI-VII (MM). The earthquake destroyed Sarpir village and damaged Dorahan and Deh Bagh. Two people were killed in Sarpir.
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