Coseismic and postseismic displacements from the 1978 Mw 7.3 Tabas-e-Golshan earthquake in eastern Iran

We use optical image correlation of historical aerial photographs, and modern satellite images to investigate the 1978 Mw 7.3 Tabas-e-Golshan thrust earthquake in eastern Iran. Correlation of images between 1974 and 1991 reveals a near-surface shortening component of ∼2.9 m across the margin of the Tabas fold, which is a combination of coseismic and postseismic deformation. Correlation of images between 1991 and 2013 shows a further ∼0.3 m of postseismic shortening. Using six pre-earthquake aerial photographs acquired in 1956 and stereo SPOT-6 imagery from 2013, we also generate pre- and post-earthquake digital elevation models (DEMs) for one of the main fold segments. Differencing of the two DEMs reveals a height change of ∼4.7 m. Elastic dislocation modelling of the 1974–2013 displacement field requires 7 m slip on a 50° dipping fault, extending from a depth of 0.1 km to 6 km at its base (the majority of slip, ∼6.5 m, occurred prior to 1991). Our results, combined with previous InSAR observations, indicate time-decaying shallow postseismic afterslip. It is likely that most of the afterslip occurred prior to 1991. The slip appears to dissipate in the near surface, and is accommodated as a narrow band of flexural slip on bedding planes. Comparison of the fault slip model with terrace heights measured from the SPOT-6 DEM suggests that the Tabas fold system may exhibit characteristic slip behaviour. Such behaviour would require a magnitude Mw 7.3 earthquake every ∼3500 years, based on the previously estimated shortening rate of ∼1.0 mm/yr. This study highlights the usefulness of historical imagery in investigating past earthquakes, thus providing new information about historical faulting in continental regions