Spatial and Depth-Dependent Patterns of Seismicity in the Baghlan Region, Afghanistan

Abstract This study examines spatial, depth-dependent, and temporal variations in the Gutenberg–Richter b-value in the Baghlan region of northern Afghanistan, focusing on the Andarab and Khanjan fault systems and the 25 March 2002 Nahrin earthquake (Mw 6.1). An earthquake catalogue for 1959–2024 was compiled from the International Seismological Centre (ISC) and United States Geological Survey (USGS) databases. After magnitude homogenization, declustering with the Reasenberg algorithm, and magnitude completeness assessment using the maximum curvature (MAXC) method, 1,232 earthquakes with Mw ≥ 4.3 were selected from 1,301 events. The bvalue was estimated using the maximum-likelihood method, and its spatial distribution was examined using a moving-window approach. The results show spatial variability, with b-values ranging from 0.80 ± 0.03 to 1.40 ± 0.16. Lower values (0.60–0.80) occur at depths of approximately 10–45 km, suggesting higher differential stress and variations in crustal mechanical properties. Most earthquakes occur at depths of 0–40 km, with the highest frequency at 15–20 km. Low b-values are concentrated between 36°–36.5°N and 69°–70°E, particularly near the Andarab and Khanjan faults. A b-value of about 0.80 is also identified near the 2002 Nahrin earthquake epicentre. These findings highlight the influence of regional tectonic structures and complex crustal heterogeneity on earthquake-size distribution in Baghlan.