Rayleigh wave phase velocity and azimuthal anisotropy of central North China Craton derived from ambient noise tomography

The phase velocity and azimuthal anisotropy of Rayleigh wave are obtained by ambient noise tomography based on the continuous seismic waveforms of 306 seismic stations deployed in the central North China Craton under the project ChinArray Phase III. The results show that the phase velocity anomaly and azimuthal anisotropy at short periods are mainly related to the surface structure. The basins exhibit low velocity anomalies and orogenic belts show high velocity anomalies. The fast wave direction is mainly along NE-SW direction, consistent with the fault strike and tectonic strike. At the middle and long periods, there are significant differences in phase velocity anomaly and azimuthal anisotropy in the northern and southern segments of Shanxi Rift. The northern segment shows a large-scale low velocity anomaly, while the southern segment turns into high velocity anomaly. The significant low velocity anomaly in the northern segment is related to the magmatism of the Quaternary Datong volcanic group. The change of its central position with depth may represent the crustal upwelling channel of mantle thermal material. The north-south difference of phase velocity anomaly may indicate that magmatism is only active in the northern segment. The azimuthal anisotropy is weaker in the northern segment and mainly along the NE-SW direction, while in the southern segment the azimuthal anisotropy is stronger and gradually rotates to the E-W direction. The fast wave direction of the surface wave corresponding to the middle and lower crust is consistent with the direction of the maximum compressive stress in this region. It is inferred that the crustal azimuthal anisotropy of the Shanxi Rift is mainly affected by the crustal stress field, but the northern segment is also affected by magmatism. Compared with the previous SKS splitting results, it is revealed that in the northern segment the fast wave direction of the crust and the uppermost mantle is not consistent with that of the mantle. Considering that this area is affected by magmatism, the source of surface wave azimuthal anisotropy is more complicated. The deformation pattern of crust and mantle needs to be further studied. On the other hand, the lower crust and uppermost mantle in the south-central segment show stable high velocity anomaly. The fast wave direction of surface wave gradually rotates to E-W direction, which is roughly consistent with the SKS splitting fast wave direction and accords with the vertical coherent deformation model.