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VOL. 6, NO. 9 GEOPHYSICALRESEARCH LETTERS SEPTEMBER1979 THE TABASEARTHQUAKE OF SEPTEMBER16, 1978 IN EAST-CENTRALIRAN A PRILIMINARY FIELD REPORT A. Mohajer-Ashjai and A. A. Nowroozi Atomic Energy Organization of Iran Abstract. The largest historic magnitude The mezoseismalintensity did not exceed IX on earthquake of inland Iran occurred at 1SH, 3SM the Mercalli Scale and the preliminary epicenter and S3.0S (GMT) on Sept. 16th, 1978. This event given by USGS - (33.21N, 57, 3SE, D = 33 km) and measured 7.7 in Richter scale and left behind AEOI both fall at a distance of 40 to S0 km to over iS,000 casualties in township of Tabas, a the southeast of the macroseismic epicenter, out- traditional oasis at the edge of east Iranian side the area of maximum damage (Figure 2). For desert. The area of total destruction extend- AEOI's instrumental epicenter, a total of 23 ed for 1,000 km2 along the western foot of the stations in Iran were used, which resulted in Shotori Range. Other earthquake effects such the following parameters: as fresh thrust faulting, extensive landslid- Origin Time (GMT) : 15 H 35 M 53.05 ing, stone up-throw and collapse of under- Latitude : 33.35 N Longitude : 57.40 E ground irrigation channels were observed with- Depth : S km Magnitude : 7.7. in the mezoseismal area. A principal contrac- The Tabas plain, Shotori Range and northern tion to the east could be taken as the causa- Lut desert are major geomorphic elements of tive agent, which is in accord with the general the area. The Shotori Range is a north-south tectonic regime of the area. running, fault bounded, strongly tectonized and uplifted feature between two regions of consid- Introduction erable rigidity, known as Lut and Tabas blocks (Stocklin, 1968). On the 16th of September, 1978, a destruc- In contrast to the general seismicity tive earthquake of Ms = 7.7 occurred in Tabas of most parts of eastern Iran, there is no doc- area, east central Iran (Figure 1). This shock umented historic shock near the epicentral area killed over iS,000 people and caused total des- of this earthquake. In the 20th century, a few truction in an area of SO by 20 km in dimension instrumentally located events of small magnitude along the western foothills of the Shotori Range. were observed about 40km to the north of Tabas, ß '' -*•"*- e•--•.•. 19&B •SHAO -'/1953 • / 0 • _ • 0 • • •e •o ' •1•. 1 SEISMICITY OF NE,IRAN 6 _Y (•9OO_ •978) 5. 6 SCALE • Eerthqueke Fe•I 0 S0 1• 150 •0•m. Hejor Queternery Or Active Feult Copyright 1979 by the American Geophysical Union Paper number 9L0391. 689 0094-8276/79/009L-0391501.00 690 Mohajer-Ashjai: The Tabas Earthquake o 36 30' o Shirgeht ?arasht o Robat - ß - Dahane Azmighan Kerond Boshruyeh o Eresk A.E.O.I. Day h_uk ! •arghub C / / / I 4, /" ' ' 3300 5700 15 30• SCALE 0 5 10 15 20 25 Kin. FIG:2 ISOSEISklAL MAP OF TABAS EARTHQUAKE SEPT_ 16_1978 Fig. 2. Isoseismai map and fresh faulting and fracturing associated with the Tabas Earthquake of September 16, 1978. • Instrumentalepicenter • Villages slightly damaged ß Villages with total destruction O Strongly felt no damage • Villages with major damageor destruction - ._. Fresh and reactivated thrust faulting • Villages partially damaged •:C•7.%Secondary fractures and faulting which are insignificant compared with the severe aftershocks show a random distribution over an earthquake experienced in adjacent active areas area of 60 by 40 km along the western foothills such as Torbat-e-Heidarieh in 1923, Birjand in of the Shotori-Range where the fresh faultings 1946 and Dasht-e-Bayaz in 1968 (Figure 1). were observed. The full results of this work Nevertheless, on the basis of neotectonic con- will be published separately. siderations the Tabas area was designated as having the potential for severe damage, on the Earthquake Effects and Damage to Structures seismic zoning map of Iran prepared by Mohajer- Ashjai and Nowroozi (1978). The traditional building style is based on Post earthquake observation in Tabas area con- easily available sun dried clay bricks and sisted of aftershock monitoring for a period of heavy arch roofs, which totally destroyed with- one monthand observation of earthquakeeffect in in the mezoseismalarea. The only exceptions terms of surface faulting and damagedistributions. were a few domestructures, locally used to An array of 9 portable stations was installed in cover water storages. Traditional architecture the epicentral area 4 days after occurrenceof and the historic style of housingin the town the mainshock. OverS0 eventswere recorded of Tabaswere among the archeological17pro- daily and initial location of 300 well recorded tected areas of Iran. Therefore, most of the Mohajer-Ashjai: The Tabas Earthquake 691 Fig. 3 Maximum offset of fresh faulting Fig. 4 Rejuvenation of a Quaternary fault observed in the field. (Measured throw 3.5km east of Tabas. is 30cm across a low angle thrust) houses in this town were several hundred years faulting was observed 2.5 km east of Khosrow- old and the oldest mosqueof the town has ap- Abad and 4 km east of Dehashk, which extend at parently survived for 700 years with no serious NIOE for about 40 km showing a pure thrust move- damage. This is another indication for lack of ment with throw of about 30 cm (Figure 3). Up previous seismic activities, or possibly a very to 11 parallel sets of similar but smaller long return period for major shock around the faults and fractures were developed all within Tabas area. Apart from houses built in the a belt of about 2 km wide. The most westerly traditional style, the area contains some better member showed over-thrusting of the eastern built, masonry schools, three small bridges, a block over the west along a low angle fault plan new hospital, and a few elevated metal water dipping 1S degrees to the east. At about 600 tanks. Although,these engineeringstructures meterstowards the east, anotherprominent fresh were not specially designedfor major earthquakes, fault showeda reversed senseof dip-slip across none of them had total collapse or were as bad- which the western block over-thrusted the east- ly dmnagedas the traditional building. Among ern one along a 29 degree dipping plane. The other important earthquake effects were collapse later faulting was conspicuously related to re- of underground irrigation channels and extensive juvenation of an important Quanternary thrust land sliding. In several cases, stones were fault showing cummulated offset of 170 cm with thrown up and over turned near Tabas and Kurit, a total throw of 100 cm (Figure 4). A typical indicating high vertical acceleration in the or- cross section of the above fault system indica- der of 1.0g. ted several horst blocks uplifed by thrust movements in opposite directions (Figure 5). Fresh Faults and Fractures Similar but much larger scale tectonism was observed in older geological formations within Several sets of fresh faulting and fractur- the Shotori Range (Stoklin, et.al., 1965). ing appeared on the ground surface within the Another set of thrust faulting and fractur- area of maximumdamage, some of which could be ing was observed 5 km northeast of Tabas airport, of vibratory nature. The most prominent fresh striking at N40W with dip angle of about 30 - FIG. 5 SCHEMATIC STRUCTURAL SECTIONS THROUGH SHOTORI RANGE KUH_E_SHOTORI TABAS PLAIN E RECENTALLUVIU•••- SLIGHTLYFOLDED NEOGENE FOLDED AND FAULTEDPRE NEOGENESEDIMENTARY ROCKS SCALE 0 1 2 3 4 5 KM. FRESH EARTHQUAKE FAULTS AND FRACTURES 692 Mohajer-Ashjai: The Tabas Earthquake (1972), observed dextral movement along the North Anatolian fault, coupled with subduction at the eraton arc, requires a sideway movement of the Western Turkish plate to the west, in order to escape the northsouth compression between Arabia and the Causasus. A somewhat symmetrical situation seems to occur for Central Iran, which moves eastward, with dextral move- ment along the northwest striking faults, which includes the Main Recent Fault system of Zagros, Uromieh Dokhtar dextral shear Zone (khoury and Mohajer-Ashjai 1978), and ruh-Banan fault; and sinistral movement along ENE fault system, of the Great ravir (Doruneh) fault and Shahrud 'fault complex (Figure 6). The faults mentioned above are, however, only sampies of a much more extensive population of faults which prevade the whole of Iran and subdivided the "plates" into subplates and the subplates into minor blocks. On a more local scale, the Lut FIG.6 NEOTECTONIC NODEL OF IRAN and Tabas blocks appear as masses within the interior of the Central Iran that move even ...... MAJOR OROQRAPHIC BELT further to the east and could have given rise • COHTRACTION BOUNDARY to the Shotori-Range and the recent large MAJOR SHEAR FAULT ZOHE magnitude earthquake in Tabas area. i /•'COMPRESSIVEMOTION Acknowledgements. We are thankful to our degree to the northeast. This fracture system colleagues M. Payman, A. Kabiri, Gh. Taghi- closely follows the western flank of Neogene Zadeh for their participation in the field pro- anticline at the foot of the Shotori Range. A gram and contributions to the isoseismal map. series of tension and secondary fractures occurred randomly in northern and central parts President of the Atomic Energy Organization of the damagearea mainly at E-W to NE direction. of Iran is acknowledged for his kind permission to publish this paper. Conclusions and Neotectonic Implications References The Tabas earthquake of September 16, 1978 is not only the largest event of inland Iran, Khoury, S.G. and Mohajer-Ashjai, A review of but it is one of the most complicated shocks the geologic and tectonic evolution of observed in recent years. Initial evidence Zagros and central Iran. J. Petrol. Soc.- such as considerable distance between instru- Iran, V.70.

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