a, Study region and locations of broadband seismic stations (triangles) used in this study. The thick red lines give the location of the profiles shown in Fig. 2 in the main text. The black lines (numbered from 1 to 10) give the location of the profiles shown in Extended Data Figs. 2 and 3, to demonstrate the non-uniqueness of the phases X1 and X2. Black dashed curves show the Wadati-Benioff zone obtained from the distribution of intermediate/deep earthquakes. The red symbols indicate the Cenozoic active volcanoes in this region. b, Spatial distribution of earthquakes used in receiver function study, colored by their back azimuth. c, Histogram shows the number of receiver functions from different back azimuth.