Earthquake faulting and bedding thrust associated with the Tabas-e-Golshan (Iran) earthquake of September 16, 1978

Abstract

abstract The Tabas-e-Golshan earthquake of magnitude Ms = 7.7 occurred in a region of east central Iran known to have been quiescent for at least 11 centuries. The shock was associated with 85 km of discontinuous thrust faulting at the surface along an existing but unrecognized late Quaternary fault (the Tabas Fault). Nearly all the surface breaks followed the obvious scarps created by the previous faulting. An extensive zone of “bedding thrusts” (bedding-plane slip) was also developed in the Neogene clay deposits of the overthrust block, east of the main fault zone. Minimum vertical uplift (throw) and slip measured at about 150 and 300 cm, respectively. The deformation caused by this earthquake indicates crustal thickening of the region, i.e. uplifting along the earthquake fault. The Tabas Fault is one of the principal structures by which the western flank of the Shotori mountains is raised about 2 km above the alluvial deposits of the Tabas compressional graben. It is a multi-role deep-seated Precambrian fault and one of the major features during Paleozoic and Mesozoic, in the western part of the fault-controlled subsiding sedimentary basin of Shotori. The earthquake killed more than 20,000 people and severely damaged or destroyed about 90 villages together with the town of Tabas. This catastrophic event emphasizes once more the importance of mapping recent faults and the careful field study of structures associated with low-magnitude buried earthquakes. In the case of Tabas, the lack of historical seismic damage over the past 11 centuries meant that there was no adequate assessment of the seismicity and seismic hazard on this basis. A comparison of the Plio-Pleistocene and Late Quaternary crustal deformation with that associated with this earthquake indicates the intra-plate deformation to be essentially unchanging since that time, and the regional stress field to be not greatly changed since the late Alpine phase. The present state of knowledge about regional tectonics and seismicity suggests the country to be a broad zone of compressional deformation, the present deformation being taken up mostly by the existing late Quaternary mountain-bordering reverse faults inherited from the old geological times. The seismicity is widespread along several faults and the continental crust is thickening and shortening in the NE-SW direction.