应用远震有限频率层析成像反演中国东北地区上地幔P波三维速度结构

选取黑龙江、吉林、辽宁、内蒙古区域地震台网,以及NECESSArray流动台阵记录的223个远震事件的波形资料,采用多道互相关方法得到了22569个P波相对走时数据,并计算了相应的走时灵敏度核,应用有限频率层析成像反演得到中国东北地区上地幔600 km以上的P波三维速度结构模型,利用检测板评估了反演结果的分辨率。结果表明,松辽盆地下方80～200 km的深度上呈主体的低速异常,与这一地区上地幔浅部的高地温值和低密度的特征相互对应,可能暗示了部分熔融的地幔。南北重力梯度带两侧的速度结构明显不同,这一差异可以延伸到200 km以下,表明在中国东北地区南北重力梯度带有可能是一条上地幔内部结构的变化带,或是深部结构的分界线。长白山火山区下呈大范围的低速异常,并可从上地幔浅部延伸到地幔转换带中,推测此低速异常可能反映了地幔转换带内上涌的热物质,上涌的原因则主要是受到太平洋板块俯冲运动的作用。

3D P-wave Velocity Structure of the Upper Mantle beneath Northeast China from Teleseismic Finite-frequency Tomography

We selected waveform data from 223 teleseismic events recorded by Heilongjiang, Jilin, Liaoning, Inner Mongolia regional permanent networks and NECESSArray temporary network, obtained 22569 P-wave relative travel-times by multi-channel cross correlation measurement, calculated travel-time sensitivity kernels, and applied the finite-frequency tomography to determine three-dimensional P-wave velocity structure model of the mantle down to the depth of 600 km beneath Northeast China. We evaluated the resolution of inversion results by using checkerboard test. Our results reveal a low-velocity anomaly extending from 80 to at least 200 km depth beneath the Songliao basin, which corresponds to the characteristics of high temperature and low density in this depth range, so this low-velocity anomaly may imply partial melting of the upper mantle. The velocity structures on the both sides of the North-South Gravity Lineament (NSGL) are obviously different, and this difference can extend below 200km depth, which is suggest that NSGL may be a variation belt or a boundary of internal structures in the upper mantle beneath Northeast China. There is a wide range of low-velocity anomalies beneath the Changbaishan volcanic region, which can extend from the shallow part of the upper mantle to the mantle transition zone. We speculate that these low-velocity anomalies may reflect the upwelling of hot materials in the mantle transition zone, and the upwelling is mainly caused by the subduction of the Pacific plate.