The 1994 Sefidabeh (eastern Iran) earthquakes revisited: new evidence from satellite radar interferometry and carbonate dating about the growth of an active fold above a blind thrust fault

In 1994, three shallow earthquakes of Mw∼ 6 occurred close together on blind thrusts near Sefidabeh in eastern Iran. In an earlier study of the teleseismic waveforms, the geomorphology and the faulting in the epicentral region, it was suggested that these earthquakes were associated with the growth of a ridge above a blind thrust fault system, whose activity could be detected by its effect on the surface drainage. In this study we present a SAR interferogram that precisely determines the location and amount of coseismic surface displacements, showing that the earthquakes in the Sefidabeh sequence probably occurred on en-echelon fault segments associated with three stepping ridges. We also present U/Th dates of ∼100 ka for lake deposits uplifted by the growing ridge. From the cumulative, dated uplift and knowledge of the surface displacements due to an earthquake sequence, we estimate that ∼120 such events have occurred in the past 100 ka, with an average recurrence interval of 830 yr, and an average convergence rate of 1.5 mm yr−1 on the Sefidabeh thrust; each estimate has an uncertainty of a factor of two, either way. We argue that the Sefidabeh fault originally formed by coalescence of many small fault segments, and has grown in length at about 2 cm yr−1 in the past 100 ka. Though the coseismic surface deformation observed in the SAR interferogram closely resembles folding, the overall topography does not, because of inherited topography associated with earlier geological deformation. In spite of this, the activity of the buried thrust fault can easily be detected by its effect on the surface drainage: a significant lesson when interpreting landscapes that are not entirely due to the present-day deformation.