LAUSR

Southeast of Iran experienced eight destructive earthquakes during 30 years from 1981 to 2011. Six of these events with M > 6.5 were fatal and caused great human and financial losses in the region. The 1981 July 28 (Mw 7.2) Sirch earthquake with 65 km surface rupture was the largest event in this region since 1877 and with other three earthquakes occurred in Golbaf-Sirch region during 17 years. The 26 December 2003 (Mw 6.6) Bam earthquake was one of the most destructive events in the recorded history of Iran. There were more than 26,000 killed, 30,000 to 50,000 injured people, and more than 100,000 were homeless. We calculated the static coulomb stress changes due to this earthquake sequence (four earthquakes) between 1981 and 1998 on the Golbaf-Sirch right-lateral fault and the Shahdad reverse fault and a slow slip on the Shahdad fault. Our calculations showed positive stress changes due to previous events on the ruptured plane of next earthquake. For example, the rupture plane of the 14 March 1998 (Mw 6.6) Fandoqa earthquake received a maximum positive stress change about 2.3 MPa. Also, some parts of the surrounding faults received positive stress changes due to these events. Stress changes on the planes of other four events until 2011 were calculated in this study. The 26 December 2003 (Mw 6.6) Bam earthquake and the 20 December 2010 (Mw 6.5) first Rigan earthquake received negligible (about thousandth (0.001)) negative stress changes in this sequence. The last event in our study area, the 27 January 2011 (Mw 6.2) second Rigan earthquake, experienced more than 0.5 MPa coseismic coulomb stress changes especially in its hypocenter and according to our calculations, it is mostly due to the first Rigan event. By using well-located aftershocks of the Rigan earthquake, we investigated the correlation between coulomb stress changes and aftershocks distribution. Calculated coulomb stress changes due to these two events on the optimally oriented strike-slip faults for the first event showed that most of the well-located seismicity occurred in regions of stress increase and majority of them concentrated near the ruptured plane where the stress changes are in the highest value. Based on our computation for the second event, it would be concluded that most of the aftershocks located in the places that imposed stress are positive and some of them are in places where the imposed stress changes are zero or very small. So, there is a good correlation between coulomb stress changes and aftershocks distribution for both Rigan events. Calculating imparted coulomb stress changes that resolved on the nodal planes of the Rigan first event aftershocks has also been considered to examine whether they were brought closer to failure or not by using different fault friction. Various values of effective coefficient of friction (0.2, 0.4, and 0.8) were used to find the best value of fault friction that produces the highest gain in positively stressed aftershocks. Based on these calculations, majority of aftershocks received positive stress changes by increasing the effective coefficient of friction.