Researches And Applications Of Receiver Function Method For Passive Ocean-bottom-seismometer

As one effective method to estimate the velocity structure and discontinuity depth beneath stations,receiver function has been widely used on the basis of land-based station data inversion.However,when the receiver function is applied in passive oceanbottom-seismometer(OBS)data,ideal results are not obtained,because the quality of the passive OBS data are not very good and the recording period is not very long.In this paper,the receiver function method is introduced into orientation correction of passive OBSs to obtain high quality waveform data.After that,the receiver function method is conducted to study the crustal structures beneath the Yap subduction zone and the Eastern Sub-basin of the South China Sea,and more accurate crustal structures are obtained.These are of great significance to make full use of passive OBS data for further seismological researches and understanding regional tectonic evolution.The OBS orientation is a critical parameter for analyzing three-component seismograms,but it is difficult to estimate accurate orientation because of the uncontrollable OBS posture after its deployment.In this paper,a new method is developed to estimate the OBS orientation by fitting the amplitude of direct P-wave of teleseismic receiver functions.Firstly,the feasibility and stability of this method are verified using synthetic data and observed waveforms recorded at land-based seismic stations in Shandong Province,China.Then,the orientations of OBSs deployed in the Yap subduction zone are estimated and corrected using our new method.After the correction,the direct P waves of the teleseismic receiver functions show very good consistency,which indicates that our method is reliable.Moreover,the effects of white and coloured noise in different levels,epicentral distance,backazimuth coverage,sensor tilting,dip and anisotropy of underground structures are investigated.The comparision with conventional Rayleigh wave polarization and P-wave partical motion methods is also conducted.Both demonstrated that our new method has significant advantages in anti-noise,stability and controlling of multi-solution.Based on the corrected seismic waveform data,the crustal structures beneath the Yap subduction zone and the Eastern Sub-basin of the South China Sea are obtained using waveform matching method with the receiver function forward modeling.The results show that the northern and southern sections of the Yap subduction zone divided by the Yap Island have different crustal structures: a double-layer crust is revealed beneath the island arc and the backarc area south of the Yap Island,whereas similar features are only revealed beneath the island arc north of the Yap Island.The upperlayer and lower-layer crust may be the overlying crust and the subducting crust,respectively.Flat and normal subduction may be occurring along the souther and northern Yap Trench.The tomographic results reveal a horizontal high velocity anomaly zone above ～100 km depth south of the Yap Island,and a near vertical or even overturned high velocity anomaly zone north of the Yap Island.Based on the receiver function and tomographic results,it is inferred that the South Caroline Plateau south of the Sorol Trough may have partially subducted and break off,and the remnants of the plateau rebounded due to eclogitization.North of the Sorol Trough,the subducted Pacific plate may overturn because of its own gravity and the eastward horizontal mantle flow due to ultra-slow convergence rate,which may lead to stretching and thinning of the unsubducted North Caroline Plateau.The different slab morphologies could be responsible for the short-lived arc volcanism,horst-graben structures,different Bouguer gravity anomalies and stress regimes between the north and south sections of the Yap subduction zone.The thickness of crust beneath the Eastern Sub-basin of the South China Sea are distributed unevenly.The lower crust near the fossil ridge is thickened obviously,whereas the upper crust is thickened slightly only near the seamount.The tomographic results show that widespread low-velocity anomalies exist under the basin.There may be a dynamic mechanism transformation for the magma upwelling in the Eastern Subbasin of the South China Sea.After the sea floor spreading,the passive mantle upwelling caused by the sea floor spreading ceased,and then a new mantle upwelling formed caused by the retreat of the subducted South China Sea slab,which resulted in a small amount of magma eruptions in structurally weak zone.The thickening of the lower crust is the the main mechanism for the seamount chain formation near the fossil ridge.