沂沭断裂带及周边区域Pn波速度结构与各向异性

Pn velocity and anisotropy beneath the Yishu fault zone and surrounding areas

沂沭断裂带位于鲁西隆起、胶辽隆起与苏鲁褶皱带之间,是郯庐断裂带地震活动最为强烈的部分,上地幔顶部Pn波速度结构与各向异性研究对于认识沂沭断裂带对地震活动的控制作用具有重要意义.本研究通过手工拾取与观测报告相结合的方法,最终挑选出研究区域2008—2019年期间290个台站记录到的1665个地震事件的26598条Pn走时数据,通过反演获得一个新的沂沭断裂带及周边区域上地幔顶部Pn波速度结构及各向异性结构模型.结果显示,研究区域上地幔顶部Pn波速度结构存在显著的横向变化,华北盆地南端、太行山造山带、鲁西隆起、胶辽隆起表现为低速异常,华北盆地内部速度结构存在显著的横向不均匀性,其强高速异常表现出明显的分块现象.沂沭断裂带呈现明显的分段特征,其两侧速度结构存在明显差异,表明沂沭断裂带可能延深至上地幔顶部.强震多数发生在上地幔顶部具有高速和低速异常变化特征区域上方的地壳内,说明强震的孕育发生与上地幔顶部结构有关.太行山造山带、胶辽隆起区Pn波各向异性快波方向与地质构造的伸展方向趋于一致,但与SKS波分裂结果不同,说明在岩石圈形变一定深度范围可能存在局部解耦现象.南黄海盆地Pn波各向异性快波方向与断裂走向基本一致,暗示扬子块体北向挤压在南黄海地区形成的部分断裂的深度已达上地幔成为岩石圈尺度的断裂.

The Yishu fault zone is located among the Luxi uplift,Jiaoliao uplift and Sulu fold belt,which is the most seismically active part of the Tanlu fault zone.The investigation of uppermost mantle velocity and anisotropy is of great significance for understanding the control of the Yishu fault zone on seismic activity.In this study,by integrating manually picked Pn arrival times with those in seismic observational bulletins,we obtain 26598 Pn travel-time data of 1665 earthquakes recorded at 290 seismic stations during 2008 to 2019,and invert these data for the velocity and anisotropy of the uppermost mantle beneath the Yishu fault zone and its surrounding areas.Our results show that there are significant lateral variations in velocity and anisotropy.The southern North China basin,Taihang orogenic belt,Luxi uplift and Jiaoliao uplift exhibit low-velocity anomalies.The North China basin shows significant lateral heterogeneities,and the strong high-velocity anomalies in the basin show obvious blocks.The Yishu fault zone shows obvious segmentation characteristics,and there are obvious differences in the velocity structure on both sides,indicating that the Yishu fault zone may extend down to the uppermost mantle.Most strong earthquakes occurred in the crust above the transition zones from high-velocity to low-velocity anomalies in the uppermost mantle,indicating that the occurrence and generation of these earthquakes could be related to lateral heterogeneities in the mantle.The fast direction of Pn propagation beneath the Taihang orogenic belt and Jiaoliao uplift area coincides with the extension direction of the geological structure,but it is different from the SKS splitting results,indicating that there may be local decoupling at certain depths of the lithosphere.The fast direction of Pn anisotropy in the South Yellow Sea basin is basically the same as the fault strike,suggesting that the partial faults in the South Yellow Sea formed by the northward extrusion of the Yangtze block reach the uppermost mantle and become lithospheric-scale faults.