| With large numbers of digital seismic stations and high precision seismic data acquisition, seismic tomography has become an important tool to study the crust-mantle structure. In this paper,we carried out the crustal 3-D tomography in Yishu fault zone and adjacent area by using the latest high-precision seismic data of Shandong regional seismic network. According to the 3-D velocity structure,we mainly studied the geophysical characteristics of Yishu fault zone and detachment structure in Luxi area, and also analyzed the mid-deep tectonic environment and controlling factors of seismic activity. Using 102 seismic waveforms(Ms>6.0, epicenter distance between 30°-90°) which recorded by 28 seismic stations in the Luxi uplift and its surrounding areas from 2010 to 2013, we obtained the crustal thickness and Poisson’s ratio beneath each station from teleseismic receiver function study, and analyzed the crust structure characteristics and tectonic origin of Yishu fault zone and Luxi uplift.We have used a large number of high-quality arrival times to determine a detailed 3-D P-wave velocity model of the crust beneath the Yishu fault zone and adjacent area. Main findings of the present work are summarized as follows:(1) Tomographic images reveal that the P-wave velocity structure of the crust of Shandong area has significant heterougeneities. The blocks on both sides of the Yishu fault zone show different velocity structure, implying that the Yishu fault may be a boundary fault.(2) Our results also show that large scale low velocity anomaly related to the Luxi large-scale detachment structure in Eocene exists below 20 km depth in Luxi area, which possibly due to the westwards subduction of the Pacific Plate to induce the hot mantle material ascending along the Yishu fault zone.(3) The distribution of large earthquakes and moderate-strong earthquakes, most of which are strike-slip types, spatially coincides with the transition zone between low velocity zone and high velocity zone, accompanied by the deep fault zone. The focal depth distribution is in mid-upper crust. We think that the lower crust is easy to flow under the effect of the upper mantle heat and then it drives the movement and deformation of brittle upper crust block in where strain was accumulated, which leads to the shallow earthquakes. The regional tectonic stress can also causes the breakup of the crust vulnerable parts and leads to earthquakes. Focal mechanism solutions generally show strike-slip or near strike-slip types in Shandong area. Combing with the tomography results, we think that the large scale low-velocity anomaly in the depth of 20-30 km maybe the controlling factors of strike-slip or near strike-slip earthquakes.Using teleseismic receiver function, We obtained the crustal thickness and Poisson’s ratio beneath each station. The results showed that: The Crustal thickness and Poisson’s ratio of Yishu fault has obvious features segmented, low crustal thickness and high Poisson’s ratio be limited to Tancheng-Juxian section of Yishu fault, which is Consistent with the results of P wave imaging. It maybe mean crust-mantle interaction occurs mainly in the Tancheng-Juxian section. The crustal thickness of Luxi uplift is thinnest in the north, mainly changes in the north-south direction and increases toward-south; In contrast, Luxi uplift average Poisson’s ratio of crustal in northern area of Luxi uplift is largest, and gradually decreasing to south. According to the chatacteristics of the crustal structure and Poisson’s ratio beneath Luxi uplift area, We concluded that the characteristics of crustal structure of Luxi uplift area are caused by the mantle upwelling actively and the earth’s crust is been thermal erosion, especially the northern area of the Luxi uplift. The formation of Luxi uplift is the response of the lithospheric thinning of the North China in crust, and showed the lithospheric thinning process of the North China exists significant thermal erosion in Mesozoic. |
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