P-wave velocity and azimuthal anisotropy of the crust and uppermost mantle beneath Northeast China

Summary Northeast China hosts one of the largest intraplate Cenozoic volcanic provinces and has experienced multiple collisions during the Paleozoic, extension during the Late Jurassic–Early Cretaceous and compression during the Pliocene. Tectonism in Northeast China has been largely controlled by subduction of the western Pacific plate since the Mesozoic and is typically characterized by the formation of multiple intraplate volcanic groups and sedimentary basins. The mechanism underlying the tectonism in this area, particularly the origin of the Cenozoic intraplate volcanoes and the evolution of the supersedimentary Songliao basin, remains controversial. To address these issues, we conducted seismic tomography to image the P-wave velocity and azimuthal anisotropy of the crust and uppermost mantle beneath Northeast China. In this study, we adopt an eikonal equation-based traveltime tomography method to invert high-quality P-wave first arrivals. We manually pick 21,006 P-wave first arrivals from 890 regional earthquakes recorded by 426 broadband seismic stations. Four prominent features are revealed by tomographic inversion. First, the results reveal a significant low-velocity anomaly in the uppermost mantle of the Changbaishan volcano and strong azimuthal anisotropy with E–W-oriented fast velocity directions distributed along the low-velocity anomaly. This feature indicates horizontal flow in the uppermost mantle beneath the Changbaishan volcano. Second, a prominent low-velocity anomaly is present in the mid-lower crust below the Changbaishan volcano, implying the presence of a magma chamber. Third, the crust of the Wudalianchi volcano is characterized by a nearly normal velocity structure and strong azimuthal anisotropy with N–S-oriented fast velocity directions, suggesting that magmatic activity has little effect on the crustal structure. Fourth, a widespread distinct low-velocity anomaly in the mid-lower crust beneath the Songliao basin reflects the mechanically weak properties of the mid-lower crust.