Analysis of the 1997 Zirkuh (Ghean-Birjand) aftershock sequence in east-central Iran

In this study, the 1997 Zirkuh (Ghaen-Birjand) locally recorded aftershock sequence was analyzed. On the basis of the distribution of aftershocks, a NW-SE trend vertical faulting with an extension of at least 90 km could be estimated. The fault rupture causing the Zirkuh earthquake apparently initiated in the epicentral area and propagated unilaterally to southeast. The cross-section of aftershocks along the fault suggests that the aftershocks had a depth range of 20 km, indicating that the seismic activity was taking place within the upper crust and the seismogenic layer, in this region, had a thickness not greater than 20 km. The distribution of aftershocks suggested that the western block acted as hanging wall during the source process of the main shock. The time-space distribution of aftershocks showed two distinguished gaps coincided with the observed gaps on the surface faulting. It was concluded that the first gap acted as a barrier during the main shock and aftershocks, while the deeper part of second gap was ruptured either during the main shock or the aftershocks. The time-frequency pattern of aftershocks decay followed the Modified Omori relations for the first 10 days while the Omori model was best fitted for the later stage of the aftershock sequence.