Evaluation of NLLoc positioning method and seismogenic structure analysis of Luanzhou MS 4.3 earthquake

NLLoc is a nonlinear search positioning method. In this study, we use simulated arrival time data to quantitatively evaluate the NLLoc method from three aspects: arrival time picking accuracy, station distribution, and velocity model. The results show that the NLLoc method exhibits high positioning accuracy and stability in terms of arrival time picking accuracy and station distribution; however, it is sensitive to the velocity model. The positioning accuracy is higher when the velocity model is smaller than the true velocity. We combined absolute and relative positioning methods. First, we use the NLLoc method for absolute positioning of seismic data and then the double difference positioning method for relative positioning to obtain a more accurate relocation result. Furthermore, we used the combined method to locate the earthquake sequence after collecting dense seismic array data on the Luanzhou M_S 4.3 earthquake that occurred on April 16, 2021, in Hebei Province. By fitting the fault plane with the relocated earthquake sequences, the results show that the strike and dip angles of the seismogenic fault of the Luanzhou M_S 4.3 earthquake are 208.5° and 85.6°, respectively. This indicates a high-dip angle fault with North–North–East strike and North–West dip directions. Furthermore, we infer that the seismogenic fault of the Luanzhou M_S 4.3 earthquake is the Lulong fault.