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

Abstract

SUMMARY 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 (∼16°) 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 ∼10 km. Again, the surface geomorphology indicates a region of folding above an eastward‐dipping thrust fault, which is ∼10 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.

Fault mechanics and post-seismic deformation at Bam, SE Iran

“…The sharp step in elevation across the escarpment requires that dip-slip fault motion extends to near the surface. Similar structures have been recognized elsewhere in Iran such as at Tabas-e-Golshan (Walker et al 2003), Ferdows (Walker et al 2003), Sefidabeh (Berberian et al 2000), Rigan (Walker et al 2013) and Shahdad (Berberian et al 2001).…”

State of stress in the northern Tabas block, east-central Iran, as inferred from focal mechanisms of the 1978 Tabas earthquake sequence

“…Because the value of the parameter Φ is equal 0.5, this stress regime does not contradict the presence of active strikeslip faults and focal mechanisms of earthquakes. This result is consistent with Walker et al [30]. Accordingly, body-wave seismograms for two earthquakes in September 1968 near Ferdows (150 km east of Tabas) indicated thrust faulting at depths of about 10 km.…”

The 18 August 2014 Mw 6.2 Mormori, Iran, Earthquake: A Thin-Skinned Faulting in the Zagros Mountain Inferred from InSAR Measurements

“…Previous studies attributed MFF to the rigid Precambrian basement of the belt that is exposed in the western part of the Arabian shield (Falcon, 1967;Husseini, 1988;Konert et al, 2001); however, the role of the basement fabrics and their seismicity are still not well known. For some cases in the eastern part of the Iranian continental plateau, the causal relationship between blindreverse faulting and surface folding has been well documented (Walker et al, 2003;Walker, 2006), but this is not the case for the Zagros mountains, in which the thick sedimentary cover is decoupled from the old rigid basement by the extensive Hormuz salt. The sedimentary cover of the Zagros mountains is 5-15 km thick, which includes platform carbonates, marls, sandstone, and shale with some evaporate horizons as decollements in different levels (Bahroudi and Koyi, 2003;Sherkati et al, 2006).…”