Surface expression of thrust faulting in eastern Iran: source parameters and surface deformation of the 1978 Tabas and 1968 Ferdows earthquake sequences

Previously unrecognised thrust faults in eastern Iran were responsible for a destructive earthquake at Tabas (1978, September 16), which produced over 80 km of distributed and discontinuous surface ruptures above a series of low anticlinal hills to the west of a major range-front. Analysis of long-period body-wave seismograms shows a simple rupture on a gently dipping (similar to16degrees) thrust, with a slight right-lateral component. This is compatible with the locally recorded aftershock distribution. Body wave analysis of two later, smaller events show similar source orientations. Several indicators of long-term active folding at Tabas can be recognised in the geomorphology, and surface ruptures from 1978 are consistent with co-seismic fold growth. Drainage incision also indicates uplift at depth on thrust faults dipping eastwards beneath the folds. Body-wave seismograms for two earthquakes near Ferdows, 150 km east of Tabas, in September 1968 also show thrust faulting at depths of similar to10 km. Again, the surface geomorphology indicates a region of folding above an eastward-dipping thrust fault, which is similar to10 km west of a fault bounded range-front. In both Tabas and Ferdows, the active faulting appears to show Quaternary migration away from the range-front, possibly in response to stresses produced by the elevated topography. The identification of zones of active fault-related folding is important for earthquake hazard assessment and also for an understanding of the local tectonics. We conclude that the structures which gave rise to the 1968 and 1978 earthquakes could have been recognised beforehand from the clear signals in the geomorphology, had we known what to look for at that time.