Crustal structure beneath NE China from ambient noise tomography of NECESSArray

From September 2009 to August 2011 the NECESSArray, an array of 127 temporary broadband seismic stations, was operated in NE China by an international collaboration of scientists from China, Japan, and US. This large 2D array was designed not only to study the crustal and upper mantle structure of NE China for the understanding of the extensive magmatism and extension during the Cenozoic but also was specially targeted to the deep earthquakes and structures of the mantle transition zone. We report here the results of the detailed crustal structure of NE China from ambient noise tomography using data collected by the NECESSArray during the two years of deployment.We have measured Rayleigh-wave phase velocity dispersion curves and obtained 2D phase velocity maps for the periods of 6~40 seconds and then obtained the 3D S-wave velocity structure of NE China. Preliminary results show good agreement between crustal velocity structures at shallow depths and surface geology. For example, the strong low velocity anomalies beneath Songliao Basin correspond well to a rather thick sediment in the basin and the southern part of the Songliao Basin is separated into two small sub-basins. The Cenozoic rift along the Yilan-Yitong fault does not have low S-wave velocities. There is a low-velocity anomaly extending from mid-crust to uppermost mantle beneath the Changbaishan volcano, which is consistent with the P- and S-wave tomography obtained from the NECESSarray data. One significant finding is that the S-wave velocity in the mid and lower crust is much lower beneath the Daxing'an mountain range than that beneath the Changbaishan mountain range, which was not expected since the Changbaishan mountain range has experienced extensive volcanic activities in the Cenozoic, including the active Changbaishan volcano. One possible scenario is that the lower crustal velocity anomaly beneath the Daxing'an mountain range was the consequence of an on-going lithospheric convective removal process.