Spatial–temporal variations of b-value and DC-value prior to the November 12, 2017, Mw 7.3 earthquake in Sarpol-e Zahab, Zagros, Iran

Abstract

The occurrence of the 7.3 magnitude Sarpol-e Zahab earthquake on November 12, 2017, in the Zagros region underscored the importance of conducting a comprehensive seismicity analysis in this area. In this study, we have analyzed the spatiotemporal variation of the seismic b-value and D_C-value in the vicinity of the Sarpol-e Zahab earthquake and surrounding region using a unified and homogeneous earthquake catalog spanning 01/01/2006–10/31/2024 belong to the Iranian Seismological Center (IRSC). In the tectonically active parts of the study area, b-values range from 0.4 to 1.4. According to the results, prior to the mainshock, the b-value exhibited low values in the vicinity of the Sarpol-e Zahab earthquake, suggesting significant stress accumulation and an imminent rupture. In contrast, the D_C-value displayed higher values, indicating increased fault heterogeneity and crack complexity as the fault approached failure. Immediately after the mainshock, the b-value increased sharply, corresponding to stress release and a dominance of smaller aftershocks, whereas, the D_C-value decreased. The temporal b-value curve shows a decreasing trend before the occurrence of the November 12, 2017, Mw 7.3 earthquake. Temporal analysis of the b-value also reveals a clear increase post-mainshock, indicating stress redistribution and ongoing fault healing. The concentration of low b-values (b < 0.5) between 10 and 20 km depth suggests a potential for stress concentration in this depth range. These findings demonstrate the complementary roles of b-value and D_C-value in characterizing seismic behavior and fault evolution. In the tectonically active regions of the study area, which correlate to the reverse/thrust and strike-slip active faults, the b-values and D_C-values range from 0.4 to 1.4 and from 1.5 to 4, respectively. The observed patterns provide valuable insights into pre-seismic stress buildup, post-seismic stress relaxation, and faulting processes.