Seismological and field observations from the 1990 November 6 Furg (Hormozgan) earthquake: a rare case of surface rupture in the Zagros mountains of Iran

We document a unique example of a co-seismic surface rupture in the Zagros mountains of SE Iran. Using P- and SH-waveform inversion, aftershock distributions and field observations we show that the Mw 6.4 1990 November 6 Furg (Hormozgan) earthquake was associated with ∼15 km of south-facing surface ruptures with an average vertical displacement of ∼1 m. Earthquakes of Mw 6-7 are common in the Zagros mountains of Iran. However, no co-seismic surface ruptures associated with thrust faulting have been reported before now, and blind thrusting appears to be more common. Source parameters determined from seismology and measurements from the surface ruptures indicate slip on a fault dipping ∼30°N. Although there is likely to be considerable uncertainty in at least some locations, the aftershock activity measured in the month following the earthquake appears to be broadly confined to a diffuse area around the northward-dipping fault. The geomorphology of the epicentral region contains indications of both long-term Quaternary uplift and cumulative fault scarps from repeated earthquakes. The Furg earthquake probably occurred on a reactivated section of the High Zagros fault, a geological suture from the early history of the Iran-Eurasia collision. The 1990 earthquake is likely to have ruptured to the surface because of its unusual location in the High Zagros zone, away from the present-day focus of seismic activity in the Simply Folded zone of the Zagros. It is possible that other sections of the High Zagros sutures still have the potential to generate earthquakes. © 2005 RAS