郯庐断裂带及其邻区上地幔顶部Pn波速度与各向异性层析成像

Uppermost-mantle Pn velocity and anisotropic tomography of the Tanlu fault zone and adjacent areas

本研究拾取了中国数字测震台网固定台站记录的2008—2016年2级以上地震事件中的27233条高质量Pn到时资料,反演得到了郯庐断裂带及其邻区上地幔顶部Pn波速度和各向异性结构模型.结果显示,研究区上地幔顶部Pn波速度结构存在强烈的横向不均匀性,速度异常形态与区域地质构造较为吻合.太行山造山带、鲁西隆起、大别造山带、苏鲁褶皱带、胶辽隆起和华北盆地南端等隆起区表现为低波速异常,而黄海北、南部盆地、渤海湾和华北盆地北部等凹陷区均为高波速异常.壳内强震主要发生在Pn低波速异常和高低波速异常的横向过渡地带,说明强震的发生与上地幔结构的横向变化之间存在有一定关联.郯庐断裂带两侧Pn波速度以郯城地震为界其东北侧和西南侧分别分布有与断裂带近平行的低波速异常条带,而西北侧和东南侧分别分布有高波速异常条带,各向异性快波方向近乎沿断裂带走向,可能由于上地幔热物质沿郯庐断裂带上涌形成低速异常后断裂带发生左旋平移运动所致.华北盆地内上地幔顶部Pn波速度结构和各向异性的明显变化,反映华北克拉通破坏过程中经历了地幔热物质上涌、莫霍面隆升以及岩石圈拆沉等复杂构造变形.

In this study, we manually pick27233.high-quality Pn arrival-time data from seismograms of events with magnitudes greater than2.0 recorded at the Chinese digital permanent seismic stations during 2008 to 2016. We establish a new Pn velocity and anisotropy model of the uppermost mantle beneath the Tanlu fault zone and adjacent areas by inverting these arrival times. Results show the existence of strong lateral heterogeneities in the study region, largely consistent with regional geological structures. Uplift areas such as the Taihangshan orogen belt, Luxi uplift, Dabie orogen, Sulu folded belt, Jiaoliao uplift and southern part of the North China Basin have low-velocity (low-V) anomalies, whereas the depressed regions such as Northern South Yellow Sea basin, Southern South Yellow Sea basin, Bohai Sea areas, and northern part of North China Basin, exhibit high-velocity (high-V) anomalies. Major earthquakes are mainly distributed along the transition zone of low- to high-V anomalies, suggesting a possible correlation between earthquakes and upper mantle structure. The Pn velocity structure shows obvious low-V zones on northeast and southwest sides and high-V anomalies on northwest and southeast sides, and the fast direction of Pn propagation is nearly along the strike of the fault zone. This is probably caused by the left-lateral translational movement after the hot materials upwelling from the upper mantle along the fault zone leading to the formation of low-V anomalies. Obvious variations in the Pn velocity and anisotropy in the uppermost mantle imply that the North China Craton has experienced complicated structural deformation such as the mantle hot materials upwelling, Moho uplift, and lithosphere delamination during the destruction of the North China Craton.