基于密集台阵的东濮凹陷中北段浅层速度结构

THE 3-D SHALLOW VELOCITY STRUCTURE OF THE MIDDLE-NORTH SECTION OF THE DONGPU DEPRESSION DERIVED FROM DENSE ARRAY OBSERVATIONS OF AMBIENT NOISE

文中利用布设在东濮凹陷中北段由412个台站组成的密集台阵记录的数据,基于噪声成像技术获得了研究区0~3.5km的三维S波速度模型,所得结果显示速度结构特征与断裂的形态展布特征具有较强的关联性。主要得到的认识包括:1)东濮凹陷中北段呈低速,两侧的内黄隆起和鲁西隆起为高速特征,隆起和凹陷的速度差异至少持续到3.5km深度处。2)东濮凹陷和鲁西隆起高、低速的分界位置与兰聊断裂一致。3)在1~3.5km深度处,东濮凹陷中北段表现出了明显的低速特征,说明古近纪兰聊断裂的活动强烈,影响了东濮凹陷中北段的沉积特征;在0~1km深度处,凹陷和隆起的速度差异减小,说明兰聊断裂在新近纪和第四纪的活动变弱,该时期的沉积构造受兰聊断裂的影响减小。

Dongpu depression is located at the junction of Henan and Shandong in the south of Bohai Bay Basin in eastern China.It is an early Tertiary faulted basin with NNE strike,with thick sedimentation.It is adjacent to Luxi uplift in the East and Luxi uplift in the West.There are mainly three major faults in the area:Lanliao fault,Changyuan fault,and Yellow River fault.Lanliao fault is a major fault that controls the boundary between the Dongpu depression and the Luxi uplift.Changyuan fault is the boundary between the Dongpu depression and the Neihuang uplift.Yellow River fault is a secondary fault in the Dongpu depression.Dongpu depression controlled by these three fault zones has formed a structural form of“two depressions and one uplift”.To understand better the distribution of faults and velocity structure in the Middle-North Section of the Dongpu depression,from March 26 to April 22,2018,the Geophysical Exploration Center,China Earthquake Administration set up a short-period dense seismic array consisting of 412 short-period seismometers in the middle-north section of the Dongpu depression,the Luxi Uplift the Neihuang Uplift.The array range is about 50km×45km,the station spacing is 1.3~2.5km,and the station spacing around the array is 4.5km.In the array,there is also a linear array with a length of about 50km,with a station spacing is about 500m,and 98 stations,which are distributed near vertical fractures.Based on noise cross-correlation technology,cross-correlations of vertical component ambient noise data of different station pairs are computed in 1-day segments and stacked.Clear fundamental-mode Rayleigh waves are observed from 0.5s to 5s period.Then we use the direct surface wave tomographic method with period-dependent ray tracing and a wavelet-based sparsity constrained to invert phase dispersion travel-time data simultaneously for 3-D shear-wave velocity structure.The shear-wave velocity model results from 0.5km to 3.5km depths are consistent with the known geologic features and reveal strong shallow crustal heterogeneity.The results follows:1)the velocity of the Middle-North Section of Dongpu depression in the study area is low,the velocity of the Neihuang uplift and Luxi uplift on both sides are high,and the shear velocity variation between uplift and depression continues to about 3.5km.2)The boundary between high and low velocity coincides with the boundary of depression and uplift,and is also consistent with Lanliao Fault and Changyuan Fault,indicating that the caprock deposition in the Dongpu depression is controlled by the Lanliao fault and Changyuan fault.3)The Cenozoic sedimentary structure of the Dongpu depression is mainly controlled by Lanliao fault.The 1~3.5km depression shows obvious low velocity characteristics,indicating that the Paleogene Lanliao fault activity has a strong impact on the sedimentary characteristics of the middle-north section of the Dongpu depression;the velocity difference between the depression and uplift of 0~1km decreases,the Neogene and quaternary Lanliao fault activities become weaker,and the sedimentary structures in this period are less affected by the Lanliao fault.Although the velocity of the Dongpu depression is generally low,the depression also shows some heterogeneity:the sedimentary structure of the northern section is not only controlled by the Lanliao fault,At the same time,it also received that the control of the secondary fault in the depression presents“W”shape,which disappears in the middle section,indicating that the Cenozoic sedimentary structure of Dongpu depression is mainly controlled by the Lanliao fault,and the Paleogene Lanliao fault activity has a strong impact,with obvious segmentation characteristics,resulting in the existence of multiple sedimentary centers in Dongpu depression,thus making the velocity structure in the Dongpu depression present non-uniformity.4)The characteristics of the Lanliao fault in the middle-north section of the Dongpu depression are shown as an SEE trend,and the dip angle of the Lanliao fault in the north section is significantly steeper,indicating that there are differences in the activity characteristics of Lanliao fault in the study area.The Shijiazhuang-Mazhai-Liuta fault is a branch fault of the Changyuan fault extending northward,with a strike of NNE and a dip of E or SEE.From the velocity distribution feature image,it can be seen that it is significantly different in the north-central section of the Dongpu depression.From the velocity distribution image,it can be seen that it is significantly different in the north-central section of the Dongpu depression,with a gradual steep dip from south to north,and then gradually slowing down.This feature is consistent with the different structural characteristics of each branch fault of the Changyuan fault at a different section.