Seismotectonic aspects of the 2025 Kunar earthquake, Afghanistan: insights from field observations and InSAR-based remote sensing

Active deformation in eastern Afghanistan occurs across a complex fault network associated with the India-Eurasia collision zone, yet the role of subsidiary structures remains poorly constrained. The 31 August 2025 Mw 6.0 Kunar earthquake provides an opportunity to investigate their role within the Kunar Fault System. This earthquake caused severe damage to unreinforced masonry and mudbrick structures, triggered over 104 landslides, and generated > 22 aftershocks with M ≥ 4.0. The earthquake resulted from thrust faulting on the north-northwestern segment of the Kunar fault system within the Hindu Kush ranges. This fault system forms a broad deformation zone characterized by predominantly left-lateral to oblique motion, with reverse slip accommodated on subsidiary thrust faults, particularly along the northern margin of Kunar valley. Field investigations document widespread surface deformation along active thrust zones, including open ground cracks and landslides. Surface cracks are concentrated in Mazar-Dara Valley of the Nurgal District, where most landslides and casualties occurred. Ascending and descending InSAR interferograms reveal contrasting deformation patterns between the mainshock and a Mw 5.6 aftershock on 4 September 2025. The aftershock exhibits a compact deformation field, approximately elliptical in shape (∼5 km × 9 km), and with a NEE-trending long axis consistent with reverse faulting on a moderate-sized structure. The peak Line-of-Sight displacement of ∼ 15 cm was located west of Ghaziabad village, which is west of the mainshock. In contrast, the mainshock produced a broader (∼12 km × 9 km) and spatially heterogeneous deformation field dominated by uplift with minor subsidence, reaching a maximum LOS displacement of ∼ 22 cm east of Ghaziabad. The complexity of the mainshock deformation may reflect rupture involving structurally segmented or multiple fault strands, while the detailed geometry of the seismogenic fault remains to be further resolved by future geodetic fault modelling efforts.