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Four destructive M7+ earthquakes and 18 M6+ earthquakes demonstrate that the Littoral Fault Zone (LFZ) is one of the most seismically active regions in the offshore area of South China. However, the location, geometry, and structural characteristics of the LFZ, which spans over 1000 km, have not been properly examined. In this study, we investigate the precise position of the LFZ offshore East Guangdong (EGD) and map the fault structure using recently acquired high-resolution multi-channel seismic (MCS) reflection data. The LFZ consists of an array of normal faults that collectively form a terraced sidewall fault zone linked to the continental rifting of the South China Sea (SCS). This fault zone is approximately 35–75 km wide and runs parallel to the shoreline. The primary faults of the LFZ EGD segment correlate to a high-gravity gradient zone and serve as the northern boundary of the Zhu-I Depression. During the syn-rift phase, the LFZ experienced significant intra-plate deformation, resulting in fault throws of up to 1000 m. This deformation was locally influenced by pre-existing structures, giving rise to a listric geometry at depth. In the post-rift period, the LFZ exhibited inherited activity along its rift-related faults. The presence of growth fault structures suggests that the LFZ remained active throughout the Cenozoic, with numerous active faults extending practically to the seafloor. The negative flower structure confirms the strike-slip character of the LFZ, which is influenced by the modern tectonic stress field. Intense multi-phase Cenozoic magmatism, including igneous intrusion, volcanism, and lava flows, has been detected in the offshore area of Nan'ao Island. The frequent earthquakes in the study area are primarily attributed to far-field stress release resulting from the subduction of the Philippine Sea Plate beneath the Eurasian Plate. Additionally, the intense magmatism weakens crustal structures, further facilitating stress release and the frequent occurrence of earthquakes. • The location, geometry, and structure of the Littoral Fault Zone were revealed in the offshore area of East Guangdong. • Tectonic evolution of the LFZ is at 3 stages, which are related to the movement of the surrounding Plates. • Widespread magmatism was found in the waters near Nan'ao Island, assumed to be linked to the frequent earthquakes. [ABSTRACT FROM AUTHOR]

The land-sea transition zone in the northern South China Sea (SCS) records important information from the continental rifting to the seafloor spreading. The crustal structure is the key to explore the deep tectonic environment and the evolution of the SCS. In 2015, the onshore-offshore 3D deep seismic experiment was carried out on the Pearl River Estuary (PRE). Explosions and air guns were used as sources on land and at sea respectively in this experiment.Onshore seismic stations and Ocean Bottom Seismographs (OBSs) synchronously recorded the seismic signals. We focus on an onshore-offshore seismic profile (L2, SE-trending) along the eastern side of the PRE. By modelling the seismic travel times, we constructed a P-wave velocity model along the profile. The model shows that the sediment on land is thin and has seismic velocities of 4.5–5.5 km/s. In contrast, thickness of the offshore sediment gradually increases to more than 4.0 km, and the velocities vary between 2.0 km/s and 4.5 km/s. The onshore and offshore crustal velocities are 5.8–6.8 km/s and 5.5–6.8 km/s, respectively. At depth between 15 km and 20 km, a low-velocity layer (LVL; only 5.9 km/s) is detected, pinching out under the Littoral Fault Zone (LFZ). The LVL has probably accommodated the crustal extension beneath the land area, resulting in low extent of the crustal thinning. A slightly uplifted Moho exists beneath the Dongguan fault depression zone, representing a place where hot mantle materials ascend. Localized thickening of the sediments and rapid thinning of the crust characterize the LFZ, and it can be regarded as a tectonic boundary between the South China (SC) with normal continental crust and the northern SCS margin with extended continental crust. [ABSTRACT FROM AUTHOR]

The Pearl River Delta (PRD) onshore-offshore area lies in the transition zone between South China and northern continental margin of the South China Sea. To better understand the tectonic and deformation patterns at depth, we developed a 3-D high-resolution crustal shear wave velocity (Vs) and azimuthal anisotropy model by ambient noise tomography in this region. The data sets comprise ∼30 days of seismic ambient noise recorded by 124 land seismic stations and 28 Ocean Bottom Seismometers (OBS). Our results reveal a significant lateral contrast in azimuthal anisotropy in the crust. The fast axes of the azimuthal anisotropy in land show an approximately NW-SE direction, which may be mainly controlled by the regional stress field. In the offshore PRD, however, the fast axes of anisotropy strike nearly in the NE-SW direction, likely dominated by the shearing of the littoral fault zone. Combined with other previous results, the anisotropic model reveals three major crustal deformation processes in this region since the Yanshanian. Furthermore, an upper-middle crustal frozen mafic magma chamber with high Vs (∼3.85–4.0 km/s) is imaged beneath Shenzhen, Zhuhai, and Hong Kong. It is interpreted as a source of intense magmatic activities in and around Hong Kong during the late Mesozoic. Our new model provides important constraints for understanding the crustal architecture and the multistage deformation processes in and around the PRD onshore-offshore area. • The results reveal two distinct anisotropy patterns and complex deformation processes in the Pearl River Delta onshore-offshore area. • The anisotropic directions agree well with the strike of the littoral fault zone, showing structural sources in the large fault system. • An upper-middle crustal frozen mafic magma chamber with high Vs is imaged beneath the Shenzhen, Zhuhai, and Hong Kong. [ABSTRACT FROM AUTHOR]