Large earthquakes that occur away from plate boundaries (i.e., large intraplate earthquakes), though rare, could cause heavy damage. Understanding their causes could help with seismic hazard assessment. In this study, we image the seismogenic structure under the 1969 Ms6.4 Yangjiang intraplate earthquake within the stable South China Block with a high-resolution three-dimensional shear-wave velocity model constructed from ambient noise tomography based on a high-density seismic array. The model images relatively low velocities at a fault intersection region from the surface to at least 13 km depth in the Yangjiang seismogenic zone. Given spatial links among the faults, surface seawater, and low velocities, we interpret the low velocities to reflect the presence of seawater-filled highly-fractured rocks in fault zones. We infer that the seawater infiltration could have elevated pore pressure in the long term and lowered effective stress, as evidenced by low earthquake b values at depths of 8-13 km. This process could explain the generation of the Ms6.4 Yangjiang earthquake. We further hypothesize that the large intraplate earthquakes within stable plates are prone to occur in regional weakening regions that tend to build up stress. Given sufficient tectonic stress accumulation, the long-term hydrologically-driven crustal stress variations via increasing pore pressure could play an essential role in triggering large intraplate earthquakes. This study could help seismic hazard assessment in the Guangdong-Hong Kong-Macao-Greater-Bay-Area, China.

Based on the teleseismic records of the dense broadband seismic array, the crustal anisotropy parameters of the west Ordos block and its adjacent regions were determined with P wave receiver functions. The results indicate that the dominant direction of the fast wave is N-S in the Alxa block, NEE-SWW in north Ordos block, N-S in south Ordos block, and E-W in the Yangtze block. The fast wave direction of crust anisotropy in north Ordos block is distinctly different from the south Ordos block and the Alxa block. The Dabashan thrust fold belt is a visible boundary of crust anisotropy, which implies that the crust anisotropy kept the information of ancient South China block and North China block. Comparing with the direction of the fast wave from the SKS splitting, we tentatively discussed the interaction mode between the crust and mantle. The crust and mantle are decoupled in south Ordos block and coupled in north Ordos block. Combining the results of previous geophysical studies, we presented a crust-mantle interaction model to explain the geophysical observations. The most prominent features of the model are the horizontal eastward expansion of the mantle material in the southern Ordos and the vertical upwelling of the mantle material in the northern Ordos. The different modes of movement of the mantle material led to the deep contrasting structures of north and south Ordos, including the crust anisotropy. The mantle upwelling also implies that north Ordos block might be currently experiencing craton destruction.

Based on the massive teleseismic waveforms collected by the China Array II project, we investigate the 410 km and 660 km discontinuities (referred to as ‘410’ and ‘660’) beneath the northeastern Tibetan Plateau. After all of the 95747 radial receiver functions are stacked into each 0.5x0.5° grid, the P410s and P660s arrival times are Picked. Referring to the local 3-D tomographic model, the ‘410’ and ‘660’ depths are 416±0.6 km and 676±0.8 km, respectively, and the mantle transition zone (MTZ) thickness is ~260 km. Considering the remote effect of the subducted Indian Plate and Pacific Plate, we focused on the depressed two discontinuities. In particular, along the northeastern boundary of the Tibetan Plateau, the ‘410’ is confirmed as a dynamic barrier to hot asthenospheric flow, which escapes from the Tibetan Plateau and is obstructed by the rigid cratonic lithosphere in its surrounding area. The ‘660’ is obviously affected by cold anomalies that are delaminated or removed from the thickened lithosphere. In the western NCC(North China Craton), combining with the depressed ‘660’ and relatively larger P660s-P amplitude ratio, we speculate that a certain amount of water has accumulated in the lower MTZ, while the low Vs velocity around the 660 km depth may be related to melting. According to previous results, we propose a mantle convection model driven by the edge-derived convection of the subducted Pacific Plate. The cratonic lithosphere beneath the Ordos block may also undergo slow modification.

To better constrain the amalgamation of the East and West Cathaysia blocks, we deployed a short-period dense seismic array oriented nearly north-south in the southwestern Cathaysia block. According to the 997 teleseismic receiver functions recorded by 527 node geophones, we find that the Mohorovicic discontinuity (Moho) is slightly uplifted towards the coastal Cathaysia block. The most obvious feature is the high Vp/Vs ratio and deepen Moho beneath the Sanshui basin. Based on the common conversion point (CCP) migration images, we deduce that there may be distributed with the extension of the Gaoyao-Huilai deep fault, and it is supported by the offset of Moho and occurrence of the Mw 4.3 earthquake. Combined with other evidences, we further concluded that a weak suture zone is constrained beneath along the Zhenghe–Dapu and Gaoyao–Huilai deep faults, which may indicate the remnants of the amalgamation of the East and West Cathaysia blocks.