基于背景噪声研究华北克拉通中部Rayleigh波相速度和方位各向异性

Rayleigh wave phase velocity and azimuthal anisotropy of central North China Craton derived from ambient noise tomography

基于中国地震科学台阵探测项目在华北中部布设的306个台站记录的波形数据,利用背景噪声层析成像方法开展了Rayleigh波相速度和方位各向异性研究.结果显示,短周期的相速度异常及其方位各向异性主要与地表构造相关,盆地表现为低速异常,造山带表现为高速异常,快波方向主要表现为NE-SW向,与断层走向以及构造单元走向一致.在中长周期,山西地堑南北段的相速度异常以及方位各向异性都表现出显著差异.北段表现为大范围的低速异常,而南段则转变为高速异常,北段显著的低速异常可能与第四纪大同火山群的岩浆活动有关,其中心位置随深度的变化可能代表了地幔热物质在地壳内上涌的通道,相速度异常的南北差异可能代表着岩浆活动只在北段比较活跃.方位各向异性在北段较弱且主要表现为NE-SW向,南段的方位各向异性较强且逐渐向E-W向转变.中下地壳对应的面波快波方向与该区域最大压应力方向比较吻合,推测山西地堑的地壳方位各向异性主要受地壳应力场的影响,但北段还受到岩浆活动影响.与前人SKS分裂的结果对比发现,北段的地壳以及上地幔顶部的快波方向与地幔的快波方向不一致,考虑到该区域受岩浆活动影响,面波方位各向异性的来源比较复杂,其壳幔变形模式有待进一步研究.而中南段下地壳以及上地幔表现为稳定的高速异常,其面波快波方向逐渐转变为E-W向,与SKS分裂快波方向大致吻合,符合垂直连贯变形模式.

The phase velocity and azimuthal anisotropy of Rayleigh wave are obtained by ambient noise tomography based on the continuous seismic waveforms of 306 seismic stations deployed in the central North China Craton under the project ChinArray Phase III.The results show that the phase velocity anomaly and azimuthal anisotropy at short periods are mainly related to the surface structure.The basins exhibit low velocity anomalies and orogenic belts show high velocity anomalies.The fast wave direction is mainly along NE-SW direction,consistent with the fault strike and tectonic strike.At the middle and long periods,there are significant differences in phase velocity anomaly and azimuthal anisotropy in the northern and southern segments of Shanxi Rift.The northern segment shows a large-scale low velocity anomaly,while the southern segment turns into high velocity anomaly.The significant low velocity anomaly in the northern segment is related to the magmatism of the Quaternary Datong volcanic group.The change of its central position with depth may represent the crustal upwelling channel of mantle thermal material.The north-south difference of phase velocity anomaly may indicate that magmatism is only active in the northern segment.The azimuthal anisotropy is weaker in the northern segment and mainly along the NE-SW direction,while in the southern segment the azimuthal anisotropy is stronger and gradually rotates to the E-W direction.The fast wave direction of the surface wave corresponding to the middle and lower crust is consistent with the direction of the maximum compressive stress in this region.It is inferred that the crustal azimuthal anisotropy of the Shanxi Rift is mainly affected by the crustal stress field,but the northern segment is also affected by magmatism.Compared with the previous SKS splitting results,it is revealed that in the northern segment the fast wave direction of the crust and the uppermost mantle is not consistent with that of the mantle.Considering that this area is affected by magmatism,the source of surface wave azimuthal anisotropy is more complicated.The deformation pattern of crust and mantle needs to be further studied.On the other hand,the lower crust and uppermost mantle in the south-central segment show stable high velocity anomaly.The fast wave direction of surface wave gradually rotates to E-W direction,which is roughly consistent with the SKS splitting fast wave direction and accords with the vertical coherent deformation model.