High-Resolution Crustal Imaging of Afghanistan from Earthquake Inter-Event Interferometry

High-resolution imaging of crustal structure is critical for understanding deformation processes, lithospheric rheology, and seismic hazard in continental collision zones. Afghanistan and its surrounding regions lie at the junction of the Indian, Eurasian, and Arabian plates, hosting diverse tectonic environments that include major strike-slip fault systems, thick foreland and intracontinental basins, hot orogenic cores, and ongoing continental subduction beneath the Hindu Kush. Despite frequent damaging earthquakes, existing seismic velocity models are limited by sparse station coverage and the sensitivity of conventional methods to long wavelengths or near-vertical ray paths, resulting in poor resolution at seismogenic depths. Here, we address these limitations using earthquake inter-event interferometry, which exploits dense inter-source ray coverage to enhance lateral resolution in regions with uneven seismic networks and provides new constraints on crustal structure across Afghanistan and adjacent areas.

We analyzed a high-quality regional dataset of earthquakes recorded between 2006 and 2019 across Afghanistan, eastern Iran, Pakistan, Tajikistan, Turkmenistan, and Uzbekistan. Vertical-component seismograms were processed to retrieve empirical Green’s functions from both earthquake–station surface waves and inter-event correlations of Rayleigh-wave coda. Inter-event interferometry was applied under stationary-phase and minimum-separation conditions, and all paths were stacked using phase-weighted stacking to enhance signal coherence. Rayleigh-wave group velocities were measured with a multiple-filter technique and inverted for period-dependent two-dimensional group-velocity maps using fast-marching surface-wave tomography, with regularization optimized through L-curve analysis and resolution assessed by checkerboard tests. Local dispersion curves were then inverted for one-dimensional shear-wave velocity profiles and assembled into a quasi-three-dimensional Vs model extending to ~60 km depth.

The resulting shear-wave velocity model reveals pronounced lateral and vertical heterogeneity that closely tracks major tectonic provinces. Foreland and intracontinental basins appear as shallow low-velocity domains reflecting thick, weakly consolidated sediments, with contrasting depth evolution between rapidly strengthening foreland crust and basins that retain mid-crustal weakening near major fault systems. The Pamir and western Himalayan collision zone is characterized by a strong upper crust overlying laterally extensive middle- to lower-crustal low velocities, interpreted as thermally and fluid-weakened thickened crust associated with shortening, partial melting, and ductile flow. Farther east, the Hindu Kush exhibits narrow, localized low-velocity anomalies vertically linked to intense intermediate-depth seismicity, consistent with fluid-controlled weakening above the north-dipping Indian lithosphere. To the south, the Makran subduction zone forms the most vertically continuous low-velocity system, extending from the shallow accretionary prism to Moho-proximal depths and reflecting thick sediment accumulation, underplating, hydration, and high pore-fluid pressures above the subducting Arabian plate, bounded by a cold, mechanically strong backstop at the Sistan–Makran transition.

This study presents the first regional application of earthquake inter-event interferometry in Afghanistan, providing new constraints on crustal rheology, fault and suture architecture, and an improved seismic velocity framework for geodynamic analysis, earthquake location, and hazard assessment in a complex continental collision zone.