Spatial variation of seismicity parameters for the 2017 ezgeleh earthquake: insights into seismotectonic stress distribution

Abstract

This study analyzes the spatio-temporal variations of seismic parameters (b-value, Z-value, and Log (M₀)) associated with the 2017 Mw 7.3 Ezgeleh earthquake in the Zagros fold-and-thrust belt. Using a catalog of 13,981 events (2006 − 2024) and a density-based clustering method, clustered and background seismicity were separated. The results show significant precursory reductions in b-values (to ~ 0.35) and negative Z-values (to ~ − 4.2) along major fault zones such as the High Zagros Fault, indicating progressive stress accumulation and micro-fracturing. Following the mainshock, both parameters show a clear recovery, with b-values increasing to ~ 1.0 and Z-values becoming positive, reflecting stress release and aftershock activity. Spatial analysis of Log (M₀) reveals dominant seismic energy release in the western part of the study area near Ezgeleh and Sarpol-e Zahab (values from ~ 18.5 to > 19.5). In contrast, the eastern region around Kermanshah and the Shirizi and Morvarid faults exhibits markedly lower values (~ 12 to 14.5). This pattern identifies the western segment as the primary zone of stress accumulation and release. The integrated approach demonstrates the utility of these parameters for understanding stress distribution and fault segment behavior before and after a major thrust earthquake.