Northeastern South China Sea Research Articles

Intra-seasonal variation of the Kuroshio southeast of Taiwan and its possible forcing mechanism

The intra-seasonal variation of the Kuroshio southeast of Taiwan has been studied using satellite data and a numerical model. Superimposed with the main stream of the Kuroshio, two intra-seasonal signals are revealed in the study region. The fluctuation with a period of 1–6 months results from offshore eddies. The westward propagating cyclonic eddies can reduce or reverse the northward flow east of the Kuroshio between 121° and 123° E, but only slightly touch the core velocity of the Kuroshio. The fluctuation with a period of 2–4 weeks is only significant between Taiwan and the Lan-Yu Island (the low-velocity region). Different mechanisms are responsible for the fluctuation in the low-velocity region in different seasons. In winter, the change of negative wind stress curl in the northeastern South China Sea modulates the circulation southeast of Taiwan, while the typhoon-induced intense wind is responsible for the current fluctuation in summer.

The influence of coastal upwelling and a river plume on the subsurface chlorophyll maximum over the shelf of the northeastern South China Sea

Frequently observed subsurface chlorophyll maximum (SCM) contributes substantial biomass to the waters over the continental shelf of the northeastern South China Sea (NSCS) but it has not been sufficiently investigated. In this study, observations and a three-dimensional coupled physical–biological numerical model were utilized to investigate the characteristics of the SCM under strong controls of coastal upwelling circulation and the Pearl River plume in the NSCS. The model captures the observed characteristics of the SCM in the NSCS reasonably well. Both the depth and intensity of the SCM are spatially variable and regulated by the variable upwelling circulation and associated plume distribution over the complex shelf topography. In nearshore waters, the SCM shoals and weakens towards the coast as a result of the upwelling of high nutrient low chlorophyll deep water while surface productivity is enhanced. The intensity of the SCM weakens when the surface layer is covered by the river plume because of the substantial reduction of photosynthetic active radiation (PAR). Strengthening upwelling-favorable wind weakens the intensity of the SCM due to dilution by the enhanced mixing, but the SCM depth remains relatively stable in the offshore water as a result of no apparent shift of the nutricline. Changing incident PAR leads to an interactive response in chlorophyll concentration, nutricline, and the depth of the SCM.

Seismic Images of Ocean Meso‐Scale Eddies and Internal Waves

AbstractMarine reflection seismology is usually used for imaging the seafloor geological structure and exploring hydrocarbon and gas hydrate resources. Recent researches have shown that the multichannel seismic reflection method can also image the thermohaline fine structure in the water column. Meso‐scale eddies and internal waves are important phenomena in oceans. Because conventional physical oceanography measurements are carried out at discrete sites separated several kilometers with low lateral resolution, they cannot reveal the lateral distribution of Meso‐scale eddies and internal waves very well. This paper provides new evidence of clear seismic images of the Meso‐scale eddies in the eastern Atlantic Ocean and internal waves in the northeastern South China Sea. Compared with the traditional physical oceanographic methods, reflection seismology has several advantages, such as high lateral resolution and the ability of imaging a large ocean profile in short time. Reflection features change due to the Meso‐scale eddies and internal waves can be clearly imaged, which helps improve understanding of energy transfer among different scale processes of water motions in oceans.

Seismic structure in the northeastern South China Sea: S-wave velocity and Vp/ Vs ratios derived from three-component OBS data

A nearly 500-km-long seismic profile with reflective and refractive wide-angle Ocean Bottom Seismometer (OBS) data and Multi-Channel Seismic (MCS) data was acquired across the northeastern continental margin of the South China Sea (SCS). The S-wave crustal structure and Vp/ Vs ratios have been obtained based on a previously published P-wave model using the software RayInvr. Modeling of vertical- and horizontal-component OBS data yields information on the seismic crustal velocities, lithology, and geophysical properties along the OBS-2001 seismic profile. S-wave velocities in the model increase generally with depth but exhibit high spatial variability, particularly from the shelf to the upper slope of the northeastern SCS margin. Vp/ Vs ratios also reveal significant lithological heterogeneity. Dongsha–Penghu Uplift (DPU) is a tectonic zone with a thicker crust than adjacent areas and a high magnetic anomaly. With a Vp/ Vs of 1.74 and a P-wave velocity of 5.0–5.5 km/s, the DPU primarily consists of felsic volcanic rocks in the upper crust and is similar to the petrology of Zhejiang–Fujian volcanic provinces, which perhaps is associated with a Mesozoic volcanic arc. The ocean–continent transition (OCT) in the northeastern SCS is characterized by a thinning continental crust, volcanoes in the upper crust, and a high velocity layer (HVL) in the lower crust. The S-wave velocity and Vp/ Vs ratio suggest that the HVL has a mafic composition that may originate from underplating of the igneous rocks beneath the passive rifted crust after the cessation of seafloor spreading.

Observation and modeling of sound propagation through a dynamic ocean on the continental shelf of the northeastern South China Sea.

In April 2005, a 3-day acoustic transmission experiment was carried out on the shelf of the northeastern South China Sea. A 400-Hz signal was transmitted every 5 min from a moored source to a moored vertical hydrophone array 17 km away. Additionally, a series of oceanographic moorings was deployed along the transmission path to sample the water-column variability that was dominated by the evolution of nonlinear internal tides and high-frequency nonlinear internal waves. Applying time-series filtering, principal component analysis, and a feature tracking technique to the oceanographic data, a continuous space-time empirical model for the sound-speed field was developed. Using a coupled-mode sound propagation model, interfacing with the sound-speed model, the temporal variations of the vertical distribution of signal intensity at the hydrophone array were computed. Analyzing the model results and comparing them to the measured signal intensities have allowed for quantification and comparison of the effects of the nonlinear internal tides, depression waves, and elevation waves on the sound transmission. The modeled sound-speed and sound-intensity fields and their comparisons to the measured data are discussed in this paper. [This research is jointly sponsored by the U.S. Office of Naval Research and the Taiwan National Research Council.]

Intensified upwelling over a widened shelf in the northeastern South China Sea

Observational and three‐dimensional modeling studies reveal that the intensified upwelling in the northeastern South China Sea (NSCS) is formed as a result of intensified upslope advection of dense deep waters that cross the middle shelf toward the inner shelf over a distinctly eastward widened shelf. The strongest advection occurs over the converging isobaths near the head of the widened shelf. As these dense deep waters advance shoreward, they are advected downstream by the quickly developed upwelling current over the inner shelf and eventually outcropped at the lee of a coastal cape. Dynamically, the shoreward cross‐isobath transport over the widened shelf is geostrophically enhanced by a quasi‐barotropic negative (westward) along‐isobath pressure gradient force as a result of the net rate of the momentum influx and by an intensified bottom frictional transport owing to the flow confluence near the head of the widened shelf. A negative pressure gradient also exists at the lee of the coastal cape over the inner shelf and locally amplifies shoreward motion. Induced by the respective widened shelf and the coastal promontory, the along‐isobath variations of cross‐isobath transport in the water column over the middle and inner shelves interactively characterize intensified upwelling in the NSCS.

P wave Velocity Structure in the Crust and Upper Mantle Beneath Northeastern South China Sea and Surrounding Regions

With the local and teleseismic P wave arrival times from Chinese and International Seismological Centre (ISC) stations, the tomographic inversion was conducted for the three-dimensional velocity structure of the crust and upper mantle beneath the northeastern South China Sea (SCS) and surrounding regions. The tectonic discrepancies and deep geodynamic characters were analyzed based on the tomographic image. High-velocity anomalies at the uppermost mantle beneath the northeastern SCS reveal the extension of the lithospheric mantle from the South China continent to the marginal sea. The cold, thick lithosphere beneath the northeastern SCS inferred from high-velocity anomalies suggests that no magma was intruded from the upper mantle. Therefore, it can be inferred that the rifting of South China continent may only occur inside the crust rather than extend down to the upper mantle. The continental margin of SCS indicates the tectonic nature of passive non-volcanic continental margin. Low-velocity anomalies at the central sub-basin suggest the upwelling of hot materials from the mantle. The high-velocity anomalies in the South China continent and Taiwan region and the low-velocity anomalies at Okinawa Trough delineate the different tectonics of the crust. The former ones belong to the normal crust of Eurasia continents or the thickened crust induced by the collision between the Philippine Sea plate and Eurasian plate, whereas the latter one belongs to the thinned crust induced by back-arc spreading. The Binhai Fault belt represents the suture zone of South China continent and SCS, which separates the high velocities located in its two sides. The low velocities at the upper mantle beneath the marine of South China continent reveal the deep-driven magmatism. Velocity anomalies at Luzon, Manila Trench and eastern Lunzon Trench are all related to their tectonic natures. Low velocities at the upper mantle from southern Taiwan to northern Luzon depict two volcanic chains: western volcanic chain and eastern volcanic chain, whereas the high velocity belt near the north Luzon ridge suggests the subducted continental slab.

Observed temporal statistics of acoustic travel time and intensity in the South China Sea.

The coupled ocean and acoustic variability in the Northeastern South China Sea basin was monitored using moored oceanographic and acoustic sensors during the 2005‐2006 Windy Island Soliton Experiment (WISE). The temperature and current records captured prominent ocean variability at multiple scales, induced by mesoscale eddies, internal tides, and large amplitude internal waves. The latter two were transbasin, propagating from near the Luzon Strait, through the deep basin, onto the northeastern shelf. The concurrent acoustic measurements were attained from the transmission of a 400 Hz, phase‐modulated signal along a 166 km path every 15 min. The receptions were processed to give the arrival structure of a pulse and its temporal change over a nine‐month period. The observed temporal variability in the statistics of the acoustic travel time and intensity were analyzed using time‐series techniques and models with emphasis to elucidate the connection and sensitivity to the observed ocean variability. Results from the analysis are discussed, as they pertain to the ocean processes and those parameters that can potentially be inferred from these types of acoustic transmissions. [The research is sponsored by the Office of Naval Research.]

Interaction of a river plume with coastal upwelling in the northeastern South China Sea

Observational and modeling studies were conducted to investigate the Pearl River plume and its interaction with the southwesterly driven upwelling circulation in the northern South China Sea during the summer. After exiting the Pearl River Estuary, the discharged freshwater generates a nearly stationary bulge of freshwater near the entrance of the estuary. Forced by the wind-driven coastal upwelling current, the freshwater in the outer part of the bulge flows downstream at the speed of the current and forms a widening and deepening buoyant plume over the shelf. The plume axis gradually shifts offshore of the current maximum as a result of currents induced by the contrasting density at the nose of plume and by the intensified Ekman drift in the plume. In this plume–current system, the fraction of the discharged freshwater volume accumulated in the bulge reaches a steady state and the volume of newly discharged freshwater is transported downstream by the upwelling current. Enhancement of stratification by the plume thins the surface frictional layer and enhances the cross-shelf circulation in the upper water column such that the surface Ekman current and compensating flow beneath the plume are amplified while the shoaling of the deeper dense water in the upwelling region changes minimally. The pressure gradient generated between the buoyant plume and ambient seawater accelerates the wind-driven current along the inshore edge of the plume but retards it along the offshore edge. Along the plume, downward momentum advection is strong near the highly nonlinear source region and a weaker upward momentum advection occurs in the far field over the shelf. Typically, the plume is shaped by the current over the shelf while the current itself is adjusting to a new dynamic balance invoked by the plume-induced changes of vertical viscosity and the horizontal pressure gradient. The spatial variation of this new balance leads to a coherent change in the cross-isobath transport in the upper water column during upwelling.

Anticyclonic eddies in the northeastern South China Sea during winter 2003/2004

The origins and evolutions of two anticyclonic eddies in the northeastern South China Sea (SCS) were examined using multi-satellite remote sensing data, trajectory data of surface drifting buoys, and in-situ hydrographic data during winter 2003/2004. The results showed that buoy 22918 tracked an anti-cyclonic warm-core eddy (AE1) for about 20 days (December 4–23, 2003) in the northeastern SCS, and then escaped from AE1 eventually. Subsequently to that, buoy 22517 remained within a different anti-cyclonic warm-core eddy (AE2) for about 78 days (from January 28 to April 14, 2004) in the same area. It drifted southwestward for about 540 km, and finally entered into the so-called “Luzon Gyre”. Using inference from sea level anomaly (SLA), sea surface temperature (SST), geostrophic currents and the buoys’ trajectories, it is shown that both eddies propagated southwestward along the continental slope of the northern SCS. The mean speeds of AE1 and AE2 movements were 9.7 cm/s and 10.5 cm/s, respectively, which are similar to the phase speed of Rossby waves in the northern SCS. The variation of instantaneous speeds of the eddy movement and intensity of anticyclonic eddy may suggest complex interactions between an anticyclonic eddy and its ambient fluids in the northern SCS, where the eddy propagated southwestward with Rossby waves. Furthermore, SLA and SST images in combination with the temperature and salinity profiles obtained during a cruise suggested that AE1 was generated in the interior SCS and AE2 was shed from the “Kuroshio meander”.

Comprehensive Geophysical Research on the Deep Structure of Northeastern South China Sea

AbstractThe complex crustal structure of northeastern South China Sea and neighboring areas, especially the structure of the land‐sea transition zone, has been a hot subject of the lithospheric structure research of South China Sea. In this paper we construct the density model along two profiles based on gravity data and constraints from Pn wave tomography and deep seismic sounding. The result of 2.5D density modeling supports the point of view that a high velocity layer exists in the lower crust of the area from Qiongyue Uplift to Luzon island arc; and the fitting result to the Taixinan Basin indicates that the crust in the southern depressed region should belong to the transitional type. Although the tectonic feature of the northeastern shelf of South China Sea is generally of volcanic type, the structures of profile AA' and BB' are different, which indicates the lateral variation and the complexity of crustal structure in the northeastern margin of South China Sea.

Nd isotopic variations and its implications in the recent sediments from the northern South China Sea

The chondrite-normalized REE distribution patterns in recent sediments from the northern South China Sea and surrounding areas are similar, with the HREE values being almost equal to or slightly higher, and the LaN/LuN ratio being lower, than PAAS standards. However, samples from the Xijiang River, a major branch of the Pearl River system, show opposite trends, i.e., with slightly lower HREE values and higher LaN/LuN ratio than PAAS. The distribution of Nd isotopes in sediments from the northern South China Sea was controlled by the Pearl River and the inshore area of South China, respectively. The volume of eNd(0) from inshore areas of southern China is apparently higher than that from the Pearl River, and eNd(0) values in offshore sediments and Taiwan Island are between these values. The results clearly show that eNd(0) volume in the northeastern South China Sea is higher than in the southwest part of the northern South China Sea, indicating different source rock types. The main source rock of the Pearl River sediment is carbonate types and affects sediment deposition in the southwest of the northern South China Sea, while the source rock in the inshore area of southern China, mainly Mesozoic and Cenozoic granites, contributes to sediments in the northeastern area of the northern South China Sea. Due to different source rock types, the HREE values and the volume of eNd(0) in recent sediments supplied by the inshore area of southern China are higher than those from the Pearl River. Over 80% of the rapidly accumulated sediments on the northern slope of the South China Sea in the southeast of Dongsha Islands area were derived from Taiwan Island and from inshore areas of South China, with only less than 20% from the Pearl River.

Detrital fine-grained sediment contribution from Taiwan to the northern South China Sea and its relation to regional ocean circulation

Results of clay mineralogy in 140 samples collected in major rivers and lakes in southwestern Taiwan and on the seafloor off Taiwan in the northeastern South China Sea (SCS), combined with clay mineral records of the Pearl River drainage basin, rivers in Luzon, and the South China shelf and slope, are used to semi-quantitatively evaluate the detrital fine-grained sediment contribution of Taiwan to the northern SCS. The clay mineral assemblage of the Taiwan-sourced sediments consists dominantly of illite (average 56%) and chlorite (41%), with very scarce kaolinite and smectite. Their respective distribution from the rivers and lakes to the seafloor off Taiwan does not show obvious basin-wide differences. Linear correlations of illite chemistry index with illite crystallinity and of illite crystallinity with kaolinite (%) present two end-members of provenances, the Pearl River and Taiwan, for the South China shelf and slope. Assuming that kaolinite in the northern SCS is provided completely from the Pearl River, the contribution of Taiwan in clay minerals is evaluated as 29% to the South China shelf and 23% to the South China slope, respectively. Accordingly, the contribution of the Pearl River to the South China shelf and slope is 52% and 31%, respectively. The Luzon Arc accounts for the rest of clay mineral components for the northern SCS mainly by providing smectite. The Bashi Strait-crossed branches of the southward deep North Pacific Deep Water and the northward surface Kuroshio Current in the western Pacific may transport Taiwan-sourced suspended sediments westwards to the northern SCS.

Copepod community structure over a marine outfall area in the north-eastern South China Sea

This study focuses on the dynamics of copepod abundances and species composition in the upper water column of a marine outfall area Tso-Ying (T-Y) in the boundary waters of the north-eastern South China Sea and the Taiwan Strait as an example. Zooplankton samples were collected in March, June and September 2002. Mean copepod abundance at all stations ranged from a minimum of 9.4 (individuals m−3) in March to a maximum of 1685 (individuals m−3) in June. A total of 66 copepod species belonging to 31 genera and 19 families were identified during three cruises. Copepod assemblages were dominated byTemora turbinatawhich occurred in >97% samples with a relative abundance of 75.46% combining all three sampling cruises. The ordination diagram derived from non-metric multidimensional scaling separated samples on the basis of season and revealed that different sampling stations clustered differently during each cruise. The second and third most dominant species wereAcrocalanus gracilisandAcrocalanus gibber, representing 1.73% and 1.65% of the total copepod abundance respectively. The outfall area studied here correlates with lower copepod densities represented by a few species that show a higher relative abundance in comparison with non-affected areas. We provide here the first example where plankton assemblages indicate useful information about environmental changes in the course of sewage disposal at a stable outlet site.

Identification and analysis of shear waves recorded by three-component OBSs in northeastern South China Sea

Structure models associated with P- and S-wave velocities contain considerable amount of information on lithology and geophysical properties, which can be used to better understand the complexity of the deep crustal structure. However, records of converted shear waves are less due to the speciality of seismic survey at sea and the rigorous generated conditions. The study on shear waves has always been a weakness for studying the deep crustal structures of South China Sea (SCS). In this paper, eleven three-component OBSs were deployed along the Profile OBS-2001 in northeastern SCS. After the data processing of polarization and band-pass filter, converted S-wave phases were identified in the radical component records of nine OBSs. Taking the OBS7 as an example, identification and analysis of converted shear waves were presented and discussed in detail. A few phase groups, such as P wSc, PgSs, PnSc, PmS, and P wSn, were found coming from the deep crust or Moho interface by simple theoretical model calculation and ray-tracing simulation. The results not only provide the underlying basis for studies of S-wave velocity structure and Poisson’s ratio structure, but also reveal the relationship between crustal petrology and seismology, which will be of importance for making full use of S-wave information in the future.

Late Mesozoic tectonic structure and evolution along the present-day northeastern South China Sea continental margin

The northeastern South China Sea continental margin holds the key to understanding Late Mesozoic tectonics and evaluating hydrocarbon potentials in Mesozoic tectonic and stratigraphic structures offshore southeast China. With newly obtained and processed seismic data, and new drilling and logging data, we correlate regional Mesozoic stratigraphy and analyze major Mesozoic tectonic events and structures. In particular, we focus our study on the three major tectonic units in the area, the Chaoshan Depression, the Tainan Basin, and the Dongsha–Penghu Uplift, which are separated by basement high, thrust fold, and (or) faults. Stratigraphic correlations suggest a major phase of southeastward regression, spanning in time from the late Early Jurassic (∼180 Ma) to the Early Cretaceous (∼120 Ma). Seismic data reveal two major tectonic events, with the first one in the Late Jurassic to the Early Cretaceous, contemporary with the regression, and the second one in the Late Cretaceous. Regional magnetic anomaly map after the reduction to the pole clearly reveals the boundary between the Dongsha–Penghu Uplift and the Chaoshan–Tainan depositional system. The seismic and magnetic data also suggest that, while the Dongsha–Penghu Uplift has highly magnetized sources buried mostly in the upper crust at depths from about 2 km to about 20 km, the Chaoshan–Tainan depositional system has thick Mesozoic sediments of low magnetization.

The Characteristics of BSRs and Their Derived Heat Flow on the Profile 973 in the Northeastern South China Sea

AbstractProcessing of a recently acquired seismic line in the northeastern South China Sea by Project 973 has been conducted to study the character and the distribution of gas hydrate Bottom‐Simulating Reflectors (BSRs) in the Hengchun ridge. Analysis of different‐type seismic profiles shows that the distribution of BSRs can be revealed to some extents by single‐channel profile in this area, but seismic data processing plays an important role to resolve the full distribution of BSRs in this area. BSRs in the northeastern South China Sea have the typical characteristics of BSRs on worldwide continental margins: they cross sediment bed reflections, they are generally parallel to the seafloor and the associated reflections have strong amplitude and a negative polarity. The characteristics of BSRs in this area are obvious and the BSRs indicate the occurrence of gas hydratebearing sediments in the northeastern South China Sea. The depth of the base of the gas‐hydrate stability zone was calculated using the phase stability boundary curve of methane hydrate and gas hydrate with mixture gas composition and compared with the observed BSR depth. If a single gradient geothermal curve is used for the calculation, the base of the stability zone for methane hydrate or gas hydrate with a gas mixture composition does not correspond to the depth of the BSRs observed along the whole seismic profile. The geothermal gradient therefore changes significantly along the profile. The geothermal gradient and heat flow were estimated from the BSR data and the calculations show that the geothermal gradient and heat flow decrease from west to east, with the increase of the distance from the trench and the decrease of the distance to the island arc. The calculated heat flow changes from 28 to 64 mW/m2, which is basically consistent with the measured heat flow in southwestern offshore Taiwan.

Pn Wave Velocity and Anisotropy in the Northeastern South China Sea and Adjacent Region

AbstractUsing seismic data from Chinese and ISC stations between 1980~2004, we inverted Pn wave velocity structure and anisotropy of the northeastern South China Sea and adjacent region. The velocity variations at uppermost mantle reflect the features of regional geology and tectonics. Southeastern China has fast velocities, correspondent to the lithospheric mantle in tectonically stable regions. Slow velocities appear near the Binhai fault zone along the coast of southeastern China, which indicates a possible penetration of the fault zone into the uppermost mantle. Similar to southeastern China, fast velocities throughout the northern South China Sea and Taiwan Strait reveal a property of the lithospheric mantle in continental margin, while fast velocities in the Xisha Trough reflect a southward extension of continental shelf and a mantle upwarp produced by Cenozoic rifting. There is no evidence of large‐scale mantle heat flows in the northern South China Sea. However, a very slow velocity anomaly is observed in the eastern sub‐basin of the South China Sea, where is an extinct mantle upwelling center with high heat flows, indicating a thinning or removal of lithospheric mantle. Along the eastern Taiwan, Luzon and northern Philippine, slow velocities are closely related to seismic and volcanic activity and magmatism in the arc zone, while fast velocities in the eastern South China Sea and western Philippine Sea reflect the character of oceanic lithospheric mantle. Pn velocity anisotropy also reveals the stress state in regional tectonics and the history of deformation of lithospheric mantle. Southeastern China has small anisotropy in response to less deformation there. Anisotropy is observed in the northern South China Sea, with fast directions of anisotropy aligned with shallow structures in crust, which reflect the evidence of the Mesozoic‐Cenozoic rifting and shear deformation in lithospheric mantle. Strong anisotropy is also found along the Ryukyu‐Taiwan‐Luzon arc zone, with fast directions parallel to trenches, which indicates that strong deformation of lithospheric mantle in the leading edge of the Philippine Sea plate and Eurasian continent. The change of the fast directions of anisotropy near the eastern Taiwan is probably caused by a conversion of collision mechanisms between the Eurasian continent and the Philippine Sea plate, and the tear of the lithosphere.

Anti‐cyclonic eddies northwest of Luzon in summer–fall observed by satellite altimeters

Anti‐cyclonic eddies northwest of Luzon of the Philippines in summer‐fall are identified in the merged data products of satellite altimeters of Topex/Poseidon, Jason‐1 and European Research Satellites. The generation and propagation of the anti‐cyclonic eddies, which are confirmed by satellite ocean color data, are found to be a seasonal phenomenon that is phase‐locked to the onset of the southwesterly monsoon and the relaxation of the cyclonic wind curl in the northeastern South China Sea. The eddies originate from northwest of Luzon in summer, move across the northeastern South China Sea to reach the China continental slope in fall, and propagate southwestward along the continental slope in fall–winter, inducing shelfbreak current variations in the western South China Sea in fall–winter. The anti‐cyclonic eddy discovered by Li et al. (1998) in the northern South China Sea is found to originate from northwest of Luzon and carry primarily the South China Sea waters. It does not appear to be an eddy shed from the Kuroshio in the Luzon Strait area as alluded by Li et al. (1998) and others.

Distribution and identification of the low-velocity layer in the northern South China Sea

The low-velocity layer (LVL), closely related with tectonic activities and dynamic settings, has always been a hot topic in the deep crustal structure studies. The deep seismic (OBS/OBH) and onshore-offshore experiments have been extensively implemented in the northern South China Sea (SCS) since the 1990s. Six seismic profiles were finished on the northern margin of SCS by domestic and international cooperations. The features of crustal structures were revealed and five velocity-inversion layers were discovered. Among them three LVLs with 3.0–3.5 km·s−1 velocity are located in the sedimentary structure (2.0–6.0km in depth and 2.0–4.6 km in thickness) of the Yinggehai Basin and Pearl River Mouth Basin. They were identified by the reflective and refractive phases for their shallow depth. The other two LVLs with 5.5–6.0 km·s−1 velocity generally existed in the middle crust (7.0–18.0 km in depth) with an about 2.5–6.0 km thickness in the transitional crustal structure of the northeastern and northwestern SCS. They were detected by the refractive phase from their overlain and underlying layers. We explored the possible tectonic formation mechanisms combining with previously reported results, which provided evidence for the formation and evolution of SCS.