山西断陷带南部三维P波速度结构及地震分布特征

Three dimensional P-wave velocity structure and seismic distribution characteristics in southern Shanxi fault rift zone

本研究使用山西测震台网记录的2010年1月~2019年12月地震观测数据,使用TomoDD方法,反演得到了山西断陷带南部(110°~114°E,34.5°~38.5°N)分辨率为0.2°的三维P波速度结构以及该区域地震重定位结果。反演结果显示:研究区的地壳速度结构与该区域的地表地质构造和沉积作用有关,5~10 km太原盆地、临汾盆地显示明显V p低速分布,灵石隆起是以沉积作用为主导的地质活动,存在较大范围的沉积物,在5~10 km同样显示低速分布;峨眉山地台、吕梁山脉、太行山山区显示高速分布。而吕梁山脉在10 km以上为低速分布,可能与大同火山的上地幔岩浆构造活动有关;太原盆地自15 km深度不再延续5~10 km的低速分布而显示高速分布,说明太原盆地不受大同火山区上地幔构造活动影响,受青藏高原的推挤作用形成的可能性更大。重定位结果显示:地震丛集在断陷带内分布,震源深度集中在0~30 km。太原盆地内地震丛集事件发生在太原盆地北部,深度集中在20~25 km之间,速度剖面显示位于低速向高速转换区域内;交城断裂的应力集中以及介质结构的高低速变换是太原北部地震从集的主要原因。运城盆地内地震分布除盐湖序列外没有明显的丛集性。2016年3月12日发生的M_(L)4.8盐湖序列,主震发生在低速向高速过度区域内,其余震震源深度较主震浅,且基本发生在下方存在高速分布的低速区域内。盐湖序列M_(L)4.8主震的震源机制解与附近中条山北麓断裂的高角度正断层性质一致,说明主震受中条山北麓断裂活动影响。余震震源类型复杂,其中,逆断和逆断兼走滑机制与该地区区域背景应力场不符。综合机制解和速度结构的结果认为盐湖序列的发生机理较复杂,可能受该区域介质结构、隐伏断裂分布等综合作用,还需进一步研究。

In this paper,TomoDD method was used to invert the three-dimensional P-wave velocity structure and the seismic relocation results in the south of the Shanxi rift zone(110°~114°E,34.5°~38.5°N)by using the seismic observation data recorded by Shanxi seismograph network from 2010.01 to 2019.12.The resolution of the inversion result is 0.2°.The inversion result shows:The velocity structure reflects the surface geological structure and sedimentation in the study area.Taiyuan Basin and Linfen Basin show obvious low V p distribution between 0 and 5 km.The Lingshi uplift is a geological activity dominated by sedimentation and has a large range of sediments,which also show a low-velocity distribution within 5~10 km.The Emei Mountain platform,Lüliang Mountain,and Taihang Mountain show high-speed distribution.The low-velocity distribution of the Lüliang Mountain may be related to the magmatic activity of the upper mantle tectonic activity in the Datong volcanic area;since 15 km,Taiyuan Basin no longer continues the low-velocity distribution of 5~10 km and shows high-speed distribution,indicating that the Taiyuan Basin is not affected by the slow tectonic activity in the Datong volcanic area,and it is more likely to be formed by the push of the Qinghai-Tibet Plateau.The relocation results show that the seismic events distribute along the faults in the fault rift zone,and the focal depth is concentrated in 0~30 km.Most of the seismic events in Taiyuan Basin occur in the north of basin,and the depth is concentrated between 20 km and 25 km.The velocity profile shows that the seismic events are located in the low-speed to high-speed conversion region.The stress concentration of the Jiaocheng fault and the transformation of the medium structure are the main causes of the earthquake in northern basin.The seismic distribution in Yuncheng Basin has no obvious clustering except Salt Lake sequence.The main earthquake of Salt Lake M_(L)4.8 sequence occurred in the region of low-speed to high-speed transition,and its aftershock focal depth was shallower than the main earthquake,and basically occurred in the low-speed region with high-speed distribution below.The focal mechanism solution of the M_(L)4.8 major earthquake is consistent with the north foot fault of Zhongtiao Mountain near the main shock is a high angle normal fault,indicating that the main earthquake was affected by the activity of the north piedmont fault of Zhongtiaoshan.The source type of the aftershock is complex,among which the reverse fault and the simultaneous strike-slip fault do not correspond to the regional background stress field in this area.The results of comprehensive mechanism solution and velocity structure suggest that the mechanism of Salt Lake M_(L)4.8 sequence is complex and may be influenced by the medium structure and the distribution of concealed faults in the region,which needs further study.