| East Asia is a hotspot in seismology research,containing several continental basins and marginal seas.Since Mesozoic,east Asia has experienced complex tectonic evolution.Under the interaction of the Eurasian plate,the Pacific plate,the Philippine sea plate and the Indian plate,this region gradually evolved into the present tectonic form.In our study,ambient noise tomography was conducted in east Asia.This research was aimed at velocity structure of crust and upper mantle and the thinning of the whole lithosphere in east Asia.The entire research area includes eastern coastal provinces of China,Korean Peninsula,Japan island,Taiwan island and marginal seas.The noise data we used was continuous waveform data from 573 broadband stations of different networks in 12 month.Rayleigh Green's functions were retrieved from inter-station cross-correlations.Millions of high-quality dispersion curves were selected for inversion.Rayleigh wave group velocity maps in 10?70s are obtained by surface wave tomography from dispersion curves.High resolution three-dimensional shear wave velocity model is obtained by further inversion.In order to test the reliability of data,checker board test was made based on the ray coverage in research area.In addition,Moho and lithosphere thickness were extracted from the final three-dimensional shear wave velocity model.From surface wave velocity distribution maps and three-dimensional shear wave velocity maps,we can observe the distribution of crust and upper mantle velocity structures.The short period surface wave is sensitive to shallow structure,and the velocity distribution is similar to the shear wave velocity map in 9km,which shows close correlation with the surface tectonic unit.Low velocity anomalies display in several sedimentary basins,high velocity anomalies display in mountains and orogenic belts.Surface wave in medium period corresponds to shear wave in depth of 20?30km,and the velocity distribution pattern mainly displays difference between continent and ocean.The Japan Sea area has entered the top of the upper mantle,showing high velocity anomalies,while other areas are still in the middle and lower crust,showing low velocity anomalies.Long period surface wave is sensitive to deep structure,which basically reflects the upper mantle structure in the region.Three distinct low velocity zones can be observed in velocity distribution maps,and lithosphere in upper mantle is significantly thinned beneath these areas.Two of low velocity zones are located in the accretionary wedges above the Pacific and Philippine subduction slabs.It is presumably associated with the dehydration of the subduction plate.The other low velocity zone is under the Changbai Mountains and the Korean Peninsula area.The formation mechanism may be associate with the stagnation of the Pacific subduction slab.The crust and lithosphere are generally thin in research region.The crust thickness is 23?40km in the continent and 10?30km in the ocean.The lithosphere thickness of east Asia is predominantly less than 100km.The crust thickness and lithosphere thickness both decrease progressively from continent to ocean.In addition,from the extracted lithosphere thickness map,the regional lithosphere anomalies display as stripes in NNE-SSW direction.In profiles of shear wave velocity,the lithosphere shows a striking "sausage"-type structure with alternating thickness.These features coincide with the distribution pattern of episodic magmatism in southeastern China.We propose that the crustal extension and lithosphere thinning in east Asia range from eastern China to marginal seas.The evolution is not only affected by the subduction of the Pacific plate and the Philippine sea plate,but also by the collision between the Indian plate and the Eurasian plate.The stripe distribution pattern of the lithosphere thickness is mainly related to the subduction angle change and the asynchronizing rollback of the Pacific plate. |
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