Study of the seismicity rate and Coulomb stress changes associated with the April 9th, 2013 Kaki-Shonbe earthquake (Mw=6.3) and the spatial distribution of aftershocks

Nowadays, the effect of an earthquake in triggering of other events in the surrounding areas is completely accepted. This effect in triggering future events and spatial distribution of aftershocks can be explained using the Coulomb stress changes theory. Occurrence of April 9th, 2013 earthquake with moment magnitude of 6.3 in Bushehr province that followed by an aftershock with 5.4 magnitude after 14 hours in its vicinity, convinced us to examine Coulomb stress change theory for this region of Iran related to this event using the Coulomb 3.4 software. We calculated Coulomb stress changes associated with the Kaki-Shonbe earthquake on surrounding faults and investigated the effect of transferred stress on spatial distribution of aftershocks. We also calculated the seismicity rate changes in the study area and investigated its correlation with Coulomb stress changes. For calculation of Coulomb stress changes, we used a half-space with Poison ratio equal 0.25 and shear modulus about of 800 kbar. The effective coefficient of friction in our calculations was 0.4 that is appropriate for these kinds of faults. We also used a number of about 1,100 earthquakes with magnitude more than 4, from 1913 to October 2016, to calculate the seismicity rate changes.
The Kaki-Shonbe Mw 6.3 earthquake occurred on 9 April 2013 (11:53 UTC, 16:23 local time) in the Zagros Simply Folded Belt in south-western Iran and its largest aftershock was triggered after 14 hours. The epicenter location was 20 km northeast of the town of Kaki, and the earthquake resulted 40 fatalities and 860 injured. Reverse slip on two along-strike, southwest dipping fault segments were found by analyzing satellite interferometry data. The main shock rupture initiated at the lower northern end of the larger northwest segment and slip on the smaller southern segment is likely aseismic. At first, to investigate the effect of the Kaki-Shonbeh earthquake on occurred aseismic slip on the southeast fault plane, we calculated the Coulomb stress changes related to this event on this fault plane by applying slips on the parts of causative fault of main shock. Our results showed that the transferred stress on most part of this fault plane is positive especially in the places that experienced aseismic slip. The aseismic displacement on this fault can be due to the displacement on the causative fault of Kaki-Shonbe earthquake and it is acceptable because of the tectonics of the study area and prevailing stress system. Investigation of the effect of Coulomb stress changes on the distribution of aftershocks showed that more than 80 percent of aftershocks have occurred in places where stress changes were positive. In other word, lots of the aftershocks have occurred in places where the transferred stresses due to co-seismic slip on the northwest fault segment and aseismic slip on the southeast fault segment were increased.
We calculated the Coulomb stress changes due to April 9th earthquake and aseismic slip on the southeast segment on the active faults in the study area. The obtained results indicate that the occurred slips on these fault segments increased the stress in some part of the Zagros Mountain Front Fault (MFF), Zagros Fore-deep Fault (ZFF), and the northern part of the Borazjan Fault. Coulomb stress changes due to these slips show a good correlation with calculated seismicity rate changes in the study area. The Borazjan earthquake epicenter, occurred on November 28th, 2013 with moment magnitude of 5.6, is located in the region that both Coulomb stress changes and seismicity rate changes increased and had positive amounts.