Polarization-Feature-Based Near-Field Back-Projection (PNBP) to constrain the rupture velocity of the 2023 M_W7.8 Kahramanmaraş Earthquake

Rupture-velocity constraints are fundamental to both earthquake source physics and strong ground-motion hazard assessment, yet teleseismic back-projection (BP) from different groups commonly reports different rupture velocities for the same event, which sometimes also deviate from the near-field estimation. We develop an innovative polarization-feature-based near-field back-projection method (PNBP) based on the near-field seismograms, in which we use linearly polarized phases on near-fault horizontal records to locate radiation points on the fault plane. Applied to the 6 February 2023 M_W7.8 Kahramanmaraş mainshock (Türkiye), PNBP resolves bilateral rupture propagation with a clear velocity contrast, with average velocities of ~4.1 km/s to the northeast and ~1.4 km/s to the southwest from the initial point of the main ruptured segment. Comparison with published teleseismic BP results suggests that near-field constraints provide complementary information, particularly for rupture segments that are less well resolved in far-field images, and synthetic tests further indicate that PNBP can trace continuous rupture fronts. These results support the use of near-field polarization information to refine rupture-velocity estimates and to improve the physical interpretation of BP-derived features in complex, multi-segment ruptures.