Epidemic Type Aftershock Sequence (ETAS) and Coulomb Stress Modeling of the 2010 Rigan and 2021 Fin Doublet Earthquakes in SE Iran

This study investigates the triggering mechanisms of large aftershocks and doublet earthquakes on the Iranian Plateau using separate statistical and physical approaches. The seismicity of the Rigan (2010.12.20, M_\mathrmW=6.5 , and 2011.01.27 M_\mathrmW=6.2 ) and Fin-Bandar Abbas (2021.11.14, M_\mathrmW=6.1 , and M_\mathrmW=6.3 ) doublets is analyzed to determine whether temporal clustering patterns and static stress changes are consistent with the occurrence of the second mainshock in each sequence. The Epidemic-Type Aftershock Sequence (ETAS) model is applied to quantify spatiotemporal clustering, background seismicity rates, and aftershock productivity. Results show that regions of elevated triggered-event probability, as obtained from the retrospective ETAS analysis, are located around the eventual epicenters. Results show that the aftershock productivity \left(Ae^\alpha (m_i-M_C)\right) and clustering coefficients w(x,y) increase prior to the second large event in both sequences, and spatial hotspots of high triggered event probability coincide with the eventual epicenters. Temporal and spatial residual analyses further indicate that ETAS captures key features of the evolving seismicity leading up to the second mainshock. Coulomb static stress change modeling is used independently to examine physical stress interactions. In both doublets, the second mainshock occurred within prominent lobes of positive Coulomb stress generated by the first event, and aftershock migration patterns align with stress-enhanced regions. Together, these results indicate that statistical clustering processes and static stress loading jointly govern the development of doublets in Rigan and Fin. The findings highlight the value of complementary statistical–physical assessments for understanding earthquake triggering and improving short-term seismic hazard evaluations.