Pn Anisotropic Tomography Of The Uppermost Mantle Beneath Northwestern Pacific Subduction Zone

Uppermost mantle velocity tomography(Pn tomography)is an important tool for understanding the plate movements,the occurrence of earthquakes,the origin of volcanoes and exploring the nature of disasters like earthquakes and volcanoes.The Pn phase which critically refracts at the Moho discontinuity and propagates below the Moho along the uppermost mantle is the pivotal data for studying the velocity and anisotropy structure of uppermost mantle.With the development of dense broad-band digital seismographs worldwide,we can collect a large number of data recorded at permanent and portable stations,which provides a great opportunity to obtain the high resolution uppermost velocity structure.Due to the effect of deep subduction of the western Pacific plate,the complicated geological structural features of basins and ranges with the NE-NNE direction extension have formed in Northeast China,leading to intense seismic and volcanic activities in the region,The subductions of Pacific plate and Philippine Sea plate formed a series of trench-arc-backarc systems as well,including Japan Islands,Japan Sea and Okinawa trough.In the present study,we investigate the velocity and anisotropy structure in the uppermost mantle beneath northwestern Pacific subduction zone to further understand the plate subduction,the lithospheric defamation and intraplate volcanism.Firstly,we picked 9197 high-quality Pn arrival-time data from seismograms recorded at 319 provincial seismic stations and 127 NECESSArray portable seismic stations recording the earthquakes with magnitude greater than 2 in Northeast(NE)China.Using these Pn arrivals,having much better crisscrossing ray-paths in the uppermost mantle,we determine a new high-resolution anisotropic tomographic model in NE China.Our new model shows obvious lateral velocity heterogeneities and anisotropy,which are closely related with surface geology.Obvious high Pn velocities are imaged under the Songliao,Hailar and Erlian stable basins.Prominent low Pn velocities are observed under the Changbaishan,Jingpohu,Longgang,Wudalianchi,and Datong volcanic areas as well as the Halaha and Abaga volcanic groups.Furthermore,some low-velocity anomalies are well connected or they are separated by high-velocity anomalies but connected by the fast directions of Pn propagation.These results suggest the existence of some correlations among these volcanoes in the present day or the traces preserved in the past tectonic evolution history.Hence,we propose a possible dynamic model illustrating the material and energy exchange among the volcanoes,which could be caused by a complicated rather than simple E-W horizontal flow of mantle convection in the "Big Mantle Wedge" due to the stagnancy and dehydration of the western Pacific slab subducted down to the mantle transition zone.These results provide new insights into the mechanism of active volcanoes and mantle dynamics of NE China.For better understanding the volcanism and deep dynamics of NE China,we picked 21321 high-quality Pn arrival-time data from 233 events recorded by 879 Japanese Hi-net(High-sensitivity seismic network)stations.Using the dense Pn rays which have covered the whole Japan Islands,we present a new anisotropic tomography of the uppermost mantle beneath the Japan Islands.Our results exhibit significant low velocity anomalies on the mainland of Japan,which well correspond with the distribution of the volcanoes,suggesting the formation of volcanoes in Japan may be related to the hot materials upwelling due to the quick dehydration of serpentinite during the western Pacific plate subduction under Japan Islands.Some high-velocity anomalies are found beneath the region to the east of Honshu,which may represent the subducting western Pacific plate.We extend our study area to the entire northwestern Pacific subduction zone and collect the seismograms data recorded by Japanese Hi-net stations during 2008 to 2009,five portable seismic networks during 2008 to 2011 published on the IRIS website and provincial seismic stations during 2008 to 2016,finally we pick 48772 Pn arrival-time data totally.After the inversion,a high resolution Pn anisotropic velocity structure of the entire northwestern Pacific subduction zone is obtained.The velocity and anisotropy structures of NE China and Japan Islands are almost the same as those inferred from NE China and Japan Islands data individually.It is worth noting that there exist prominent high Pn velocities under Japan Sea,suggesting that the lithosphere under Japan Sea is relatively less affected by the subducted western Pacific plate,while the lithosphere under NE China and Japan Islands are more influenced by the "Big Mantle Wedge" structure and dynamics which was caused by the deep subducted western Pacific plate.