Focal Mechanisms from Waveform Inversion and Multiple Velocity Models: Phayao Fault Zone, Northern Thailand

Northern Thailand, situated within the complex Sunda Tectonic Plate, shows significant seismic activity due to its proximity to major tectonic boundaries; however, it often has lower to medium magnitudes. One of the delineated active fault zones is the Phayao Fault Zone (PFZ), which generated an Mw 6.3 earthquake in 2014. Its further south located Pan Segment is also active, but with lower magnitudes. However, understanding and characterizing its seismicity is essential for ongoing seismic hazard assessment of the area. To overcome the challenges, waveform inversion techniques in combination with multiple velocity models were employed, with the aim to characterize the seismic source parameters of earthquakes in this area. Hypocentres were determined with exceptional precision and subsequently validated by applying a velocity model that demonstrated the highest double-couple percentage. This indicates the model's efficacy in precisely calculating hypocentral parameters in this specific geological context. Our findings unveil a complex interplay between right-lateral strike-slip and reverse faulting mechanisms, consistent with a transpressional tectonic regime in the Pan Segment. This regime reflects the accommodation of regional compressional stresses superimposed on the dominant strike-slip motion along the Phayao Fault Zone, thereby yielding a significant contribution to seismic hazard assessment in Northern Thailand. The study also underscores the need for further research to refine these models and methodologies, thereby enhancing our understanding of the seismic characteristics of earthquakes in such regions. Methodologies and insights gained here could serve as a model for characterizing seismic source parameters in other understudied low-seismicity regions globally.