Source characteristics of the 2019 Wang Nuea earthquake and depth-dependent seismotectonics of intraplate faulting in northern Thailand

Abstract

The 2019 Mw 4.9 Wang Nuea earthquake provides a valuable case study for understanding intraplate faulting and depth-dependent seismotectonics in northern Thailand. This study integrates full-waveform inversion, aftershock relocation, and morphotectonic analysis to investigate the source characteristics of the mainshock and the distribution of associated seismicity. Results indicate that the mainshock occurred as a shallow strike-slip event on an unmapped, steeply dipping fault within the Phayao Fault Zone (PYFZ), at a depth of approximately 7 km. Aftershock focal mechanisms reveal a systematic variation with depth, including reverse faulting at shallow levels, strike-slip faulting at intermediate depths, and normal faulting at greater depth. These findings suggest a vertically stratified deformation regime, controlled by inherited structural fabrics, lithological contrasts, and variations in crustal rheology. The results are consistent with a regional stress field dominated by NNE–SSW compression in the upper crust, transitioning to an extensional regime at depth. Comparative analysis with the 2014 Mw 6.3 Mae Lao earthquake supports this depth-dependent behavior and underscores the role of crustal heterogeneity in shaping intraplate faulting in northern Thailand.