In 1981 June and July two large earthquakes occurred on the Gowk Fault System in south-east Iran. Both earthquakes were associated with surface faulting showing a combination of reverse and right lateral strike-slip motion on parallel, adjacent faults striking north-south and dipping both east and west. Such motion can be seen to have occurred in the recent past and is responsible for the formation of elongated alluvium-filled depressions in the footwall region common to both the east- and west- dipping faults. The earthquake of June 11 involved motion along about 15 km of the Gowk Fault System with a moment of about 1.0 × 1026 dyne cm. The earthquake of July 28 was larger and occurred farther north with movement along at least 65km of the Gowk Fault System and a moment of about 7 × 1026 dyne cm. Surface faulting in both earthquakes was complex and spread over several kilometres width on en echelon fault segments as well as on faults dipping both east and west. The long-period WWSSN seismograms of these two mainshocks are unusually complicated and clearly made up of several individual subevents. Sensible interpretation of these waveforms without guidance from the surface faulting and the long-period SRO moment tensor inversion of Dziewonski & Woodhouse would be impossible. However, a combination of these three data sources allows a geologically plausible interpretation of the rupture processes in these earthquakes to be made.
Although the faulting in both mainshocks was on the same large-scale feature (the Gowk Fault), the rupture zones of the two earthquakes did not, apparently, overlap and were separated by a gap of 5 km along strike in which both coseismic and recent geological displacements are very small. Rupture in both mainshocks appears to have nucleated near this gap and to have propagated away from it. Other such gaps are visible along the irregular displacement of the northern fault break, and may have been the nucleation positions for individual subevents of the July 28 earthquake. Recognition of such nucleation sites is of obvious importance to earthquake prediction efforts.