Azimuthal Directivity Evident in the Regional Amplitude and Duration of Ground Motions from the 2025 Mw 7.7 Mandalay Earthquake

Abstract The 28 March 2025 Mw 7.7 Mandalay earthquake produced long-period ground motions that propagated throughout mainland Southeast Asia. Although the supershear rupture along the Sagaing fault has been documented, its influence on regional shaking has not yet been quantified. Earlier work by Ornthammarath et al. (2025) focused mainly on long-period basin amplification in Bangkok. The present study expands the analysis to quantify amplitude and duration characteristics using 30 three-component strong-motion recordings from Myanmar and Thailand. We analyze amplitude and duration residuals relative to Next Generation Attenuation-West2 ground-motion and duration models. The results reveal a pronounced azimuthal pattern: stations located between 110 and 155 azimuth degrees from the rupture fault aligned with the Mach fronts of the southward-propagating supershear rupture exhibit large positive spectral acceleration residuals and strongly negative D5–95 duration residuals. In contrast, stations southwest and west of the rupture (e.g., Yangon and Nyaung U, respectively) show negative amplitude residuals and positive duration residuals. This bilateral contrast closely matches the expected Mach cone radiation pattern of a supershear rupture, demonstrating that regional ground motions (150–600 km) may retain the geometric imprint of the rupture-front speed. These findings suggest that supershear-related directional effects may persist beyond the near-fault region and contribute to far-field amplification patterns, with potential implications for incorporating rupture dynamics and a fault-to-site azimuth parameter into ground-motion models used in seismic hazard assessments of large strike-slip faults in similar tectonic environments.