Using Ambient Noise Correlation Technique Based On The Time-Frequency Phase-Weighted Stacking To Study The Mantle Discontinuities’ Structures In Northeast China

The Northeast China is located between the Sino-Korean Block and the Siberian Block which lies to the south of the North China Craton and at the leading edge of the subducting Pacific slab.Affected by the subducting western Pacific slab,the geological structures of this area are complex,and not only deep-earthquakes occur frequently but also volcanic activities are strong.It is considered to be an ideal place to study the intraplate volcano and the subducting slab.Previously,the traditional seismological methods such as tomography,receiver function,and triplication phases have been used to study mantle structure of the Northeast China and have achieved a lot of results.In recent years,research on ambient noise has attracted much more attention.Our research selects the continuous waveform data from 124 temporary stations arranged by multinational cooperation in the northeastern China from September 2009 to August 2011,and 148 permanent stations of the China Digital Seismic Network in the same time period.The continuous waveforms are processed to obtain ambient noise.We calculate the ambient noise cross-correlation and select cross-correlation waveforms with inter-station distance less than 2.0° to obtain approximately 3600 pieces.Using the time–frequency phase-weighted stacking method based on the S-transform to stack the cross-correlation waveforms,we have extracted the P-wave signals reflected on the 410 km and 660 km discontinuities,and studied the lateral variations of mantle discontinuities under Northeast China,and then explored the effect of the subducting Pacific slab and the origin of the Changbaishan volcano.The results show that under the Changbaishan volcano,the 410 km discontinuity is depressed by as much as about 8 km-17 km,and find the thickness of the 410 km discontinuity is sharpened about 8 km in the same area by calculating the synthetic seismogram.The 410 km discontinuity structures indicate that there is an abnormally high temperature zone above it,which possibly reflect the Changbaishan volcano is formed by the upwelling of the melt that’s produced by buoyancy and convective circulation in the mantle wedge.This result indicates that the molten materials which form the Changbaishan volcano may originate from the upper mantle and may be related to the dehydration of the subducting Pacific slab.In the southeastern margin of the study area,the 660 km discontinuity is depressed by as much as about 12 km-18 km,indicating that there is an abnormally low temperature zone above it,which indicates that the Pacific slab may have subducted below the northeastern China and stagnated in the mantle transition zone.To the west of the stagnant slab,the 660 km discontinuity is slightly uplift in local area,which may be caused by the upwelling of the molten materials that formed by the vertically tearing of the stagnant slab.In addition,we preliminarily explore the mantle discontinuities in the northern part of the study area and find that the 410 km discontinuity is significantly depressed below the northeastern margin.We speculate that the depressed discontinuity may be caused by the reflected P wave signal on the lower boundary of the hydrous wadsleyite.At the same time,our research also observes that there is a large-scale depression of the 660 km discontinuity in the northern part of the study area,which may be related to the stagnant slab.