Role of Fault Geometry in Generating Backward-migrating P-wave Radiation During the 2025 Mw 7.7 Myanmar Earthquake

Kotaro Tarumi , Kazunori Yoshizawa 

The Mw 7.7 Myanmar earthquake of 28 March 2025 ruptured the Sagaing Fault system over ~450 km and exhibited complex rupture behavior, including intermittent supershear propagation and backward-migrating high-frequency (HF) P-wave radiation. We image the rupture evolution using multi-frequency teleseismic P-wave back-projection (BP) (0.05–0.5, 0.1–1.0, and 0.3–2.0 Hz) and compare the results with near-fault strong-motion data recorded at Nay Pyi Taw (NPT). Our BP results reveal two key features: (1) two distinct episodes of backward-migrating HF radiation initiated at the Sagaing–Meiktila and Meiktila–NPT fault junctions, with apparent migration speeds of 6.0–6.5 km/s, and (2) a pronounced HF radiator concentrated on a short splay-fault branch adjacent to the NPT segment. These HF radiators are preferentially located near the edges of high slip-rate patches in a P-wave slip model. The near-field displacement record at NPT shows a negative pulse following the quasi-static offset, likely representing a stopping phase generated by abrupt rupture arrest. Our results suggest that spatial variations in fault geometry, including segment junctions and fault branching, played an important role in rupture deceleration and termination, generating backward-migrating stopping-phase radiation.