Sediment Thickness At Seafloor Of South China Sea Basin Revealed By Teleseismic P-To-S Converted Phases

Seafloor sediments not only record the history of global climate change,but also reflect the regional tectonic evolution.As a marginal sea,the sea-land interaction in the South China Sea(SCS)is more intense than in open oceans,its sediments,therefore,are more sensitive to climate change and tectonic evolution.In addition,the seismic wave travel time delay caused by the relatively thick sedimentary layer of the South China Sea will also affect the accuracy of deep structure seismic imaging.Although the active-source seismic profiling and IODP have conducted extensive studies,on local scales,on the sedimentary layers of the SCS,there is still a lack of problem of insufficient breadth.The large-scale passive source ocean bottom seismograph(OBS)deployment in the SCS provides a new approach for regional 2D seafloor sediment characteristics analysis.In this paper,the converted P-to-S waves at the sedimentary base from earthquakes are used to obtain the thickness and lateral change of the sediment layer in the entire SCS basin,through which the sediment responses to the tectonic events are presented.When the direct P-waves of earthquakes propagate from the deep to the sediment-basaltic crust interface,part of the energy will be converted into Sp waves in the radial component.The time delay between them depends on the thickness of the sedimentary layer,so we derive the thickness from the time delay between the first arrival wave P of the vertical component and the converted wave of the radial component.In this study,we collected data from a total of 32 passive source OBS stations deployed in various basins in the South China Sea,and selected the three components of global teleseismic events with magnitude greater than 5.5 and epicenter distance from 24 ° to 96 °.After preprocessing the seismic waveform data,6 invalid stations were eliminated,and finally 329 three-component seismic records from 26 OBS stations were used.Because the passive source OBS records have much higher noise,identifying phases is more difficult.We applied several different methods to improve the signal-to-noise ratio and data utilization.First,to obtain accurate radial components,we use the Rayleigh wave polarization characteristics to determine the horizontal orientation of the OBS;second,some OBS seismometers have tilt due to imperfect levelling.By calculating the transfer function between the horizontal component and the vertical component,the tilt angle and direction can be obtained,and the original three-component records are corrected accordingly.329 P-to-S time delays are obtained by hand-picking on the processed waveform.Based on the existing empirical relationship of P and S velocities in the SCS,the time delays were converted into the thickness.The results show that there are obvious spatial changes in the thickness of the sediments in the basin: overall,the sediment layers are approximately symmetrically distributed along the mid-ocean ridge,reflecting that the lithosphere age controls the sediment thickness;in addition,at sites close to the seamounts and the post-spreading volcanic activity areas,the thicknesses are evidently decreased,suggesting the multi-phase rift,volcanism and other post-spreading tectonic events can locally change the distribution of sediments.This indicate that such post-spreading tectonic events are also factors affecting the distribution of sediments in the SCS.