青藏高原东北缘远震P波走时层析成像研究

Traveltime tomography using teleseismic P wave in the northeastern Tibetan plateau

利用青藏高原东北缘地区固定地震台网2010年4月至2015年3月期间记录的远震事件,采用多道波形互相关方法(Multi-Channel Cross-Correlation)拾取了10697个有效P波相对走时残差数据,进而采用FMTT(Fast Marching Teleseismic Tomography)方法获得了青藏高原东北缘上地幔400km深度范围内的P波速度结构,结果显示:秦祁地块下面存在深达70km的高速异常,阻断了青藏高原块体中下地壳低速层向东北方向的延伸;40~140km深度范围内,四川西南部存在一个低速区,该低速区穿过龙门山断裂带进入到四川盆地内部;祁连山造山带东部低速异常区从地壳一直延伸到上地幔400km处,表明这里可能存在一个上地幔到地壳间的热流通道;松潘—甘孜地块分布大面积的低速异常区,而鄂尔多斯地块西南缘相对速度较高,这与青藏高原为软块体、介质密度低和鄂尔多斯块体为硬块体、介质密度高相吻合.

In this paper,the teleseismic waveforms recorded by permanent networks in northeastern Tibetan plateau from Apr.2010 to Mar.2015 are used for P wave tomography.We picked 10697 Pwave relative traveltime residuals by Multi-Channel Cross-Correlation method and then we get the P wave velocity structure of upper mantle beneath NE Tibet by Fast Marching Teleseismic Tomography（FMTT）.The results reveal that a high velocity anomaly extends to 70 km depth beneath the Qinling-Qilian Block.The result shows that the lower crustal channel flow model is not suitable for the whole NE Tibet;an obvious low-velocity is located at the region beneath Qilian Orogen from crust down to 400 km,which suggests that there may exist a hot flow channel in the upper mantle and the crust.A large area in Songpan-Garzêblock is low velocity anomaly and this is consistent with low density Tibetan plateau.The velocity of Ordos block is high,and it is consistent with high density Ordos block.