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The northeastern of the South China Sea (SCS) has undergone tectonic inversion from a Mesozoic convergent continental margin to a Cenozoic divergent continental margin. The remnant subduction-related structures, such as volcanic arc and forearc basin, likely preserve key clues to reveal the dynamic mechanism of this tectonic inversion. We applied ocean bottom seismometers (OBS) data, combining with multi-channel seismic data, gravity data, drilling data, and petrologic observations, to reveal geophysical and structural features of the Dongsha Basin, a Mesozoic basin in the northeastern SCS. The Mesozoic strata are characterized by large thickness (~1–8 km), relatively high P-wave velocities (3.5–5.5 km/s), high densities (2.55–2.65 g/cm3), alternating folds, and thrust faults, which are significantly different from the features of Cenozoic strata. The Dongsha Basin is flanked by a Mesozoic granitic arc to the northwest and an inferred Palaeo-Pacific subduction zone to the southeast, indicating that it is geographically located in the Mesozoic forearc region. Considering the similar Mesozoic strata detected in the southern margin of the SCS and the contemporaneous accretionary complex outcrops observed in the Palawan Continental Terrane, we propose that the Mesozoic strata in these two regions originated from a common Mesozoic forearc basin, which was broken by the forearc rifting and subsequently separated by the spreading of Cenozoic SCS. Accordingly, a continuous Palaeo-Pacific subduction system is reconstructed, including the “arc-forearc-accretionary wedge” tectonic sequences. Reviewing the mechanisms of forearc rifting globally and the retreat of volcanic arcs onshore South China, we speculate that rollback of the Palaeo-Pacific subduction slab might be one of the causes for the forearc rifting and the initial extension of the SCS.

Offshore–onshore seismic survey is one of the main methods to study crustal structures in offshore–onshore transitional zones. At present, the seismic waves commonly used in imaging are the crustal refraction (Pg), the crustal reflection from the Moho (PmP), and the upper-mantle refraction (Pn) waves. The propagation distances of Pg and PmP are commonly less than 210 km, and Pn propagates with an apparent velocity of ∼8 km/s. In 2015, two offshore–onshore wide-angle seismic lines with a length of ∼350 km were acquired in the Pearl River Estuary. In addition to Pg, PmP, and Pn, a new seismic phase was observed, which has a long propagation distance (offset of ∼170–290 km), low apparent velocity (∼5.85 km/s), and low frequency (∼4–7 Hz). Similar seismic phases have been widely reported in previous offshore–onshore and reservoir seismic surveys, but the understanding of these phases is still limited. Herein, we used both raytracing and waveform modeling methods to identify the new seismic phase as the secondary Pg phase, which reflects from the surface (named Pg2Pg). We also discuss favorable conditions for Pg2Pg, including (1) a thin sedimentary layer with low velocity at the surface in which the reflection of Pg occurs, which can reduce the incidence angles and hence increase the energy of the reflected waves; (2) a sedimentary basement dipping toward the sea at the positions of the air gun shots, which focuses seismic waves; (3) relatively smooth interfaces of the medium, which can reduce the scattering of Pg2Pg; and (4) air guns that can excite low-frequency signals, which can reduce the attenuation of seismic waves. Checkerboard tests and practical applications show that Pg2Pg can significantly improve upper-crustal resolution, especially for onshore areas. Our research promotes the data mining of offshore–onshore seismic surveys, which is important for obtaining more detailed crustal structures.

The northeastern margin of the South China Sea experienced continental rifting followed by magmatism in the Cenozoic. This evolution has strongly influenced the heterogeneity of its crustal structure. To gain insights into the spatial variation in crustal structure, data from 30 ocean bottom seismometers were used to forward model ten P-wave velocity models along both dip and strike directions. The Cenozoic strata (1.7–3.5 km/s), the Mesozoic strata (3.5–5.5 km/s), the crystalline crust (5.5–7.5 km/s) and high velocity lower crust (7.0–7.5 km/s) were identified based on P-wave velocities. Overall, the crust thins from ~25 km on the continental shelf to ~15 km on the slope, with thicker crust underneath the Dongsha Basin (a Mesozoic basin) but thinner crust underneath the pre-Cenozoic NW-trending fault zones. This difference in thickness is possibly controlled by different strength of inherited structures, i.e., the Dongsha Basin and the fault zones. Furthermore, a widespread high velocity lower crust layer, which resulted from the intrusion of a mafic sill complex, was observed over the shelf to slope. Mafic intrusions within the lower crust have uplifted the caprock on the shelf, and magma eruptions have formed many post-rift seamounts on the slope. The evidence suggests that post-rift magmatism overprinted the vertical crustal structure. We therefore propose that the heterogeneous crustal structures in this passive margin are an integrated result of segmental extension followed by magmatic overprinting.

Multibeam bathymetric, seismic and borehole data are used to investigate a large-scale strike-slip structure, the Baiyun-Liwan Fault Zone, in the northern South China Sea. This fault zone comprises NW- to NE-striking faults and negative flower structures that were generated by oblique extensional displacement. Notably, the interpreted data reveals that the Baiyun-Liwan Fault Zone was active during the Cenozoic, recording intense magmatism, and accommodating significant intraplate deformation during progressive continental rifting and ocean spreading. It bounds two distinct crustal segments and played a significant role in segmenting the northern margin of the South China Sea. The geometry of faults and strata within the Baiyun-Liwan Fault Zone also controlled local sediment routing and depocentre evolution during the Cenozoic. As basement and syn-rift structures change markedly across the Baiyun-Liwan Fault Zone, we propose this structure to be inherited from a lithospheric-scale fault zone separating the Mesozoic arc from forearc-related terrains. We therefore stress the importance of pre-existing structures in the development of rifted margins, with the example provided by the Baiyun-Liwan Fault Zone having profound implications for palaeogeographic reconstructions in the South China Sea. At present, the Baiyun-Liwan Fault Zone is incised by the Pearl River Canyon and eroded by recurrent submarine landslides, forming a major area of sediment bypass towards the abyssal plain.

The origin and evolution of magmatism in rifting margins are a fundamental geological subject, and remain the focus of intense studies. Unlike the classical magma-poor and magma-rich rifting margins, the northern margin of the South China Sea (SCS) exhibits a strong post-rift magmatism, but the evolution and mechanism of this behavior are still poorly understood. In this work we investigate the detailed structural features of the post-rift magmatism in the crust of the SCS northern margin. Our analysis is based on wide-angle and multichannel reflection seismic data, combined with previous seismological results. Our results show a wide distribution of shallow igneous sills and intrusions. These features are spatially and tectonically linked to the lower crustal high-velocity bodies (HVBs) and submarine volcanism, indicating a causative connection between the three features. Considering the existence of an obvious low-velocity branch of the Hainan plume in the northern SCS and uplift of the lithosphere-asthenosphere boundary, we propose that the HVBs reflect consolidated mafic intrusions which formed the lower crustal reservoirs feeding the overlying igneous sills and intrusions. In the SCS northern margin where abundant extensional and detachment faults can act as magmatic channels, such a dynamic process might cause post-rift volcanism. Our results suggest that the crustal magmatic system of post-rift volcanism has a multilevel upward migration mode, and the HVBs in the lower crust could be the product of post-rift magmatism, further indicating that the SCS northern margin had a magma-poor property at the rifting phase but has undergone a strong magmatic rejuvenation by the subsequent mantle plume.

The Pearl River Estuary (PRE) is located at the onshore-offshore transition zone between South China and South China Sea Basin, and it is of great significant value in discussing tectonic relationships between South China block and South China Sea block and seismic activities along the offshore active faults in PRE. However, the researches on geometric characteristics of offshore faults in this area are extremely lacking. To investigate the offshore fault distribution and their geometric features in the PRE in greater detail, we acquired thirteen seismic reflection profiles in 2015. Combining the analysis of the seismic reflection and free-air gravity anomaly data, this paper revealed the location, continuity, and geometry of the littoral fault zone and other offshore faults in PRE. The littoral fault zone is composed of the major Dangan Islands fault and several parallel, high-angle, normal faults, which mainly trend northeast to northeast-to-east and dip to the southeast with large displacements. The fault zone is divided into three different segments by the northwest-trending faults. Moreover, the basement depth around Dangan Islands is very shallow, while it suddenly increases along the islands westward and southward. These has resulted in the islands and neighboring areas becoming the places where the stress accumulates easily. The seismogenic pattern of this area is closely related to the comprehensive effect of intersecting faults together with the low velocity layer.

We used earthquake catalogs recorded by Guangdong Seismological Network from 2008 to 2014 to resolve the spatial variations of b-values in the coastal area of Guangdong, particularly in three key research areas (Yangjiang, Heyuan, and offshore Nanao Island) with strong seismicity. Our results revealed that b-values exhibited significant spatial variations, and zones with low b-values could indicate the most likely seismogenic area of large earthquakes. We observed three clear low b-value patches in the offshore Nanao Island. We found a distinct high b-value peak at the depth of 11 km and two minimum peaks at about 14 and 7-8 km in the Yangjiang area. The overall b-values generally decrease with depth in the Heyuan area. The spatial variations of b-values reflect tectonic anomalies; that is, the 'low-high-low' distribution of b-values in the offshore Nanao Island and the Yangjiang area may indicate the anomaly of the crustal structure with a weak layer. The b-values of reservoir-induced seismicity are obviously lower than that induced by tectonism. This finding indicates that the reservoir area is generally at high stress state under the condition of high pore pressure. We inferred that large earthquakes might be prone to occur at 10-12 km depth in the offshore Nanao Island, at 12-15 km depth in the Yangjiang area, and at the lower part of the seismic activity zone in the Heyuan area. Moreover, the upstream area of the Xinfengjiang reservoir is the most likely area of future large earthquakes in the Heyuan area.

A cluster of earthquakes occurred in the Taiwan Shoal region on the outer rise of the Manila Trench. Although most were of small to medium magnitudes, one strong earthquake occurred on September 16, 1994. Several previous studies have provided important information to progress our understanding of this single earthquake. However, little is currently known about the earthquake cluster, and it is necessary to investigate the deep crustal structure of the Taiwan Shoal region to understand the mechanisms involved in controlling and generating it. This study presents a two-dimensional seismic tomographic image of the crustal structure along the OBS2012 profile based on ocean-bottom seismograph (OBS) data, which exhibits a high-velocity anomaly flanked by low-velocity anomalies in the upper crust beneath the Taiwan Shoal. In this study, 765 earthquakes (Richter magnitude ML > 1.5) occurring between 1991 and 2015 were studied and analyses of earthquake epicenters, regional faults, and the crustal structure provides an improved understanding of the nature of active tectonics in this region. Results of analyses indicate firstly that the high-velocity area represents major asperities that correspond to the location of the earthquake cluster and where stress is concentrated. It is also depicted that the earthquake cluster was influenced by fault interactions. However, the September 1994 earthquake occurred independently of these seismic activities and was associated with reactivation of a preexisting fault. It is also determined that slab pull is resisted by the exposed precollision accretionary prism, and the resistive force is causing accumulation of inplane compressive-stress. This may trigger a future damaging earthquake in the Taiwan Shoal region.

The northern margin of the South China Sea, as a typical extensional continental margin, has relatively strong intraplate seismicity. Compared with the active zones of Nanao Island, Yangjiang, and Heyuan, seismicity in the Pearl River Estuary is relatively low. However, a ML4.0 earthquake in 2006 occurred near Dangan Island (DI) offshore Hong Kong, and this site was adjacent to the source of the historical M5.8 earthquake in 1874. To reveal the seismogenic mechanism of intraplate earthquakes in DI, we systematically analyzed the structural characteristics in the source area of the 2006 DI earthquake using integrated 24-channel seismic profiles, onshore–offshore wide-angle seismic tomography, and natural earthquake parameters. We ascertained the locations of NW- and NE-trending faults in the DI sea and found that the NE-trending DI fault mainly dipped southeast at a high angle and cut through the crust with an obvious low-velocity anomaly. The NW-trending fault dipped southwest with a similar high angle. The 2006 DI earthquake was adjacent to the intersection of the NE- and NW-trending faults, which suggested that the intersection of the two faults with different strikes could provide a favorable condition for the generation and triggering of intraplate earthquakes. Crustal velocity model showed that the high-velocity anomaly was imaged in the west of DI, but a distinct entity with low-velocity anomaly in the upper crust and high-velocity anomaly in the lower crust was found in the south of DI. Both the 1874 and 2006 DI earthquakes occurred along the edge of the distinct entity. Two vertical cross-sections nearly perpendicular to the strikes of the intersecting faults revealed good spatial correlations between the 2006 DI earthquake and the low to high speed transition in the distinct entity. This result indicated that the transitional zone might be a weakly structural body that can store strain energy and release it as a brittle failure, resulting in an earthquake-prone area.

The extensive intraplate seamounts are obvious features in the northern South China Sea (SCS). However, the distribution, volume, and origin of these seamounts are not well understood, which greatly hinders our understanding of magmatism in the SCS. Based on high-resolution bathymetric data and 147 seismic profiles, and combining gravity and magnetic data, we first identify 45 seamounts in the northern margin of the SCS and simulate their shape with elliptical cones. Results show that the total volume of these 45 seamounts above seafloor is estimated at about 1885–3078 km3 and the total volume of intrusive magma above Moho is about 0.15 Mkm3, which is close to the estimates for classic large igneous provinces across the world. These seamounts are mostly located on the continental slope with thin crust (approximately 12–18 km), which reduces the overlying pressure and shortens the magmatic conduits. The dominant azimuth of elliptical major axis in seamounts is consistent with the synrift and synspreading fault strikes (NE-NEE), indicating that these pre-existing faults provide magmatic conduits for the subsequent postrift intraplate seamounts. Based on three existing clues, i.e., (1) the intraplate seamounts, high velocity layer and Hainan mantle plume are contiguous in 3-D space, (2) the high velocity layer is thicker beneath the continental shelf but thinner beneath the slope and (3) the basalts dredged from certain seamounts show OIB-type geochemical features, we propose a magmatic upwelling pattern which contains Hainan mantle plume to explain the spatial and morphological characteristics of these intraplate seamounts.

We present a 2-D seismic tomographic image of the crustal structure along the OBS2012 profile, which delineates the Moho morphology and magmatic features of the northeastern South China Sea margin. The image was created by forward modeling (RayInvr) and traveltime tomographic inversion (Tomo2D). Overall, the continental crust thins seaward from ~27 km to ~21 km within the continental shelf across the Zhu I Depression and Dongsha Rise, with slight local thickening beneath the Dongsha Rise accompanying the increase in the Moho depth. The Dongsha Rise is also characterized by ~4–7 km thick high-velocity layer (HVL) (~7.0–7.6 km/s) in the lower crust and exhibits a relatively high velocity (~5.5–6.4 km/s) in the upper crust with a velocity gradient lower than those of the Zhu I Depression and Tainan Basin. Across the continental slope and continent-ocean transition (COT), which contain the Tainan Basin, the crust sharply thins from 20 km to 10 km seaward and a ~2–3 km thick HVL is imaged in the lower crust. We observed that volcanoes are located only within the COT, but none exist in the continental shelf; the Dongsha Rise exhibits a high magnetic anomaly zone and different geochemical characteristics from the COT. Based on those observations, we conclude that the HVL underlying the COT is probably extension related resulting from the decompression melting in the Cenozoic, whereas the HVL beneath the Dongsha Rise is probably arc related and associated with the subduction of the paleo-Pacific plate. These findings are inconsistent with those of some previous studies.

In this study, we investigated the extensional tectonics and post-rift magmatism in the southern South China Sea (SCS) using a newly obtained seismic reflection profile, which crosses the Southwest sector of the oceanic basin (SWSOB), West Liyue Trough and Liyue Basin along the southern margin of the SCS. A wide range of magmatic features was identified in the SWSOB, with a late stage marked by a broad Pleistocene unconformity separating horizontal sediment layers from strata deformed by igneous structures. Along the continental slope, most fault activity continued until Horizon T4 (15.5 Ma), implying that crustal extension continued during seafloor spreading in the SW oceanic sector of the SCS. A low-angle listric normal fault with a large offset, marking the southern boundary of the trough, controlled regional crustal extension. Thick syn-rift sedimentary strata and significant crustal thinning in the southern part of the trough reveal relatively strong extension. Post-rift magmatism occurred in this thinned crust and affected sedimentary strata up to the seafloor. Importantly, a series of high-angle normal faults rapidly deepened the top of the basement, forming a sharp contact between the continental slope and oceanic basin. The distal domain is very narrow (~20 km) with no known detachment faults, suggesting rapid continental breakup that may be related to the presence of rigid blocks near the study area.

As a typical rifted margin, the coastal region of the northern South China Sea has experienced many large crustal earthquakes in the past 400 years. For example, two destructive earthquakes with ~M7.0 in 1600 and ~M7.5 in 1918 occurred close to the NanAo Island. However, the mechanism of these earthquakes and seismotectonics of the region are still unclear. In this work we make the first study of the detailed three-dimensional crustal structure in the source zone of the 1918 NanAo earthquake (M7.5) using newly acquired wide-angle seismic reflection data. Our results show that current seismicity mostly occurred in a midcrustal low-velocity zone or its nearby high-to-low velocity transitional areas. We ascertain locations of the ENE-strike Littoral fault zone and the NW-strike Huanggangshui fault based on the seismic reflection profiles. These faults have caused a large-scale vertical dislocation of the sedimentary basement. The structure of the upper crust is very heterogeneous, containing many high or low velocity anomalies. However, the lower crustal structure is less heterogeneous, mainly characterized by low velocity mostly in the northern part of the study area and high velocity mainly in the southern part. The crust in the seismicity zone shows strong heterogeneity, reflecting highly cracked structural features possibly caused by destructive earthquakes. We infer that the midcrustal low-velocity zone and the intersection between the Littoral fault zone and the Huanggangshui fault could reflect weak structures in the crust and form a local stress buildup, which strongly affected or controlled the generation of intraplate seismicity. The 1918 NanAo earthquake very likely occurred within the current seismicity zone with a depth range of 15–25 km estimated from our tomographic images and previous geological and geophysical results.