Acknowledgements
This work has been supported by the China Postdoctoral Science Foundation (2019M650317) and the National Natural Science Foundation of China (Grants 41904043; U1939202). Waveforms are downloaded from the data management centers of the Incorporated Research Institutions for Seismology (IRIS, http://www.iris.edu) and the China National Seismic Network at Institute of Geophysics, China Earthquake Administration (SEISDMC, doi:10.11998/SeisDmc/SN). Figures are produced by the Generic Mapping Tools of Wessel and Smith (2013). We thank Professor Jian Wang at the Institute of Geology and Geophysics, Chinese Academy of Sciences for help in using PickNet. The authors acknowledge the course ‘English Presentation for Geophysical Research of Peking University (Course #01201110) for help in improving the manuscript.