Seismogenic Structures in the Ma River Fault Zone, Vietnam: Insights from Focal Mechanisms of Microearthquakes and Local Stress States

Focal mechanisms of microearthquakes (M-L < 4) are important in understanding tectonics of seismogenic structure, especially in areas with high seismic potential. However, one of the major difficulties in obtaining a reliable focal mechanism for a microearthquake is deriving sharp P- and S-wave arrivals from a waveform with a low signal-to-noise ratio compared with those from larger earthquakes. Another challenge is the uneven distribution of seismic stations, which results in high uncertainty of focal mechanisms of microearthquakes. To address these problems, we used waveform inversion to improve the accuracy of determining focal mechanisms. High-quality waveform data recorded by 12 temporal broadband stations deployed in the Ma River fault zone, northern Vietnam, are used to conduct a waveform inversion to obtain reliable fault-plane solutions. In addition, a frequency-wavenumber integration method and a cut-and-paste method are applied to estimate the optimum focal mechanisms. This procedure is shown to effectively increase the accuracy of focal mechanisms obtained for microearthquakes. Our results are congruent with local geological structures and indicate a complex fractured crust in the Ma River fault zone. By studying local stress states, we claim that the Ma River fault zone is a wrench fault system with high seismic potential.