Evaluation of seismic stress changes in the northern part of the Zagros in Iran

Abstract

The Zagros tectonic zone is one of the most active seismic regions in the convergence process of the Arabian and Eurasian plates. In this region, the northern part shows less seismic activity, but it has gained more attention after the Ezgeleh earthquake on November 12, 2017, with M_w = 7.4. In this study, the stress inversion of focal mechanisms has been used to quantitatively and qualitatively analyze the stress. The b-value in the northern part of the Zagros region has been calculated in three morphotectonic units. Moreover, in order to evaluate spatial and magnitude distribution of earthquakes in the Zagros region, we used maximum likelihood method. According to the results of the inversion of 154 focal mechanisms with a moment magnitude equal or greater than 5, the maximum stress axis is generally preserved in the northeast-southwest direction in each of the five subregions. Deviations in stress axis from the convergence axis of the Arabian plate and the Iranian continent were happened. Comparison of the relative stress values before (R = 0.49) and after (R = 0.91) the Ezgeleh earthquake shows that the differential stress (σ₁-σ₃) in zone 2 has reached the maximum value before the main shock. Based on the morphotectonic classification in the Zagros region to Imbricated Zone, Simply Folded Belts, Mountain Front Fault, we calculated the seismic parameter b, equal to (1.06, 1.10, 1.13) in each part, respectively. These results show that the distribution of stress in the entire Zagros area is not uniform, and the level of seismicity varies according to the geological structure of the region. About the stress ratio (R), as can be seen in Tables 3 and 4, in zone 2, the amount of R is very low before the main shock (R = 0.49). However, in complete catalog, this value reached to (R = 0.91). Therefore, the stress ratio (R), can be another factor that shows the full of stressed region. When R value decreased, it means that the differential stress (σ₁-σ₃) increased (Eq. 1) in that region.