The Longmenshan fault zone is located in the northeastern margin of the Qinghai-Tibet plateau, with an overall direction of NNE and a total length of about 500km. As we have known, the Longmenshan fault zone is the boundary fault between the Bayanqala block and Sichuan basin. Since the Cenozoic, the Longmenshan fault zone has experienced intense tectonic activity and multi-stage magmatic activity, forming a series of active faults with different scales and properties.

And Lushan M_S7.0 earthquake in 2013 and Lushan M_S6.1 earthquake in 2022 occurred in the southern section of Longmenshan fault zone, and the two earthquakes were only 10km far away apart. The generation of the two strong earthquakes is closely related to the seismic tectonic environment and crustal physical structure parameters. So to study the characteristics of shallow crustal physical structure and its relationship with deep dynamic processes, is good for us to understand the seismogenic environment of this area. The wide angle inverse/refraction detection method is an effective means to obtain the physical property parameters of the crust. In this paper we extracted the first arrival travel time data of P-wave and S-wave from Jinchuan-Lushan-Leshan deep seismic sounding(DSS)profile data. The 2D ray-tracing travel-time imaging method proposed by Zelt et al.(1998)was used to obtain the 2D P-wave, S-wave and Poisson’s ratio structure of the upper crust in the source area of the Lushan strong earthquake and its adjacent area. Then based on the results of deep crust exploration, seismic distribution characteristics and other geophysical and geological studies in this area, we focus on the response of shallow tectonic environment and deep dynamic processes in the upper crust, and analyze the seismogenic environment and seismogenic mechanism of M6-7 strong earthquakes in this area. The results show that: 1)The crustal velocity and Poisson’s ratio are significantly different at different positions of the profile. In the Songpan-Ganzi block, the velocities of P- and S-waves in the upper crust are relatively high and the Poisson’s ratio is relatively low. While in the Sichuan basin, the velocities of P- and S-waves in the upper crust are relatively low and the Poisson’s ratio is relatively high. In Longmenshan tectonic belt which between the Songpan-Garze block and the Sichuan basin, the velocities of P- and S-waves and Poisson’s ratio isolines of the upper crust are controlled by regional tectonic activities, which are basically consistent with the occurrence of the strata and show a near-vertical trend. The sedimentary basement below the tectonic transition zone shows obvious structural differences, and the velocity and Poisson’s ratio contour lines form “V” shape characteristics. 2)The characteristics of high crust velocity and low Poisson ratio(<0.26) in the Songpan-Ganzi block may be the direct reflection of the strong deformation of Sinian-Paleozoic strata caused by the orogenic activities in the northeastern margin of the Qinghai-Tibet plateau in the Indosinian period, and the bi-direction contraction of the strata in the Triassic Xikang Group, the obvious thickening of the crust, and the multi-stage magmatic activities. 3)The large lateral variation gradient of velocity and Poisson’s ratio in Longmenshan tectonic belt between Songpan-Ganzi block and Sichuan basin is the direct evidence of vertical crustal deformation caused by the compression of low Poisson’s ratio crust from the eastern margin of Qinghai-Tibet plateau to the hard Yangzi platform(high Poisson’s ratio)by the remote effect of the collision between the Indian plate and the Asian plate since late Quaternary. 4)The aftershocks of the M_S7.0 earthquake mainly occurred on the high-velocity and Low-Poisson’s ratio side of the velocity and Poisson’s ratio gradient belts in the crust. The seismicity in this area is not only controlled by the regional fault structure, but also closely related to the physical structure characteristics of the upper crust.

The Yangtze fault zone is a typical tectonic regime transition zone of the eastern China. Tectonically, it is characterized by alternated rifts and uplifts and “several crystalline basements with one sediment cover”. Abundant metal metallogenic deposits are developed. Improvement of the velocity model and basement structure will benefit our understanding and knowledge about the regional tectonics. Large volume airgun sources have been broadly applied to seismic surveys due to significant advantages. For instance, they are environmentally friendly, use lower frequencies, and are repeatable. Several seismic and geological research institutions, such as China Earthquake Administration, carried out a three-dimensional comprehensive sounding using the large volume airgun as the seismic source which was fired at the channel of the Yangtze River in 2015. The source-receiver geometry of this seismic experiment covered the whole Anhui Province which locates at the Middle-Lower Yangtze River. The densest observational area is in the Middle-Lower Yangtze River Metallogenic Belt which is a narrow area along the Yangtze River and consists of the Luzong, Tongling, Ningwu, and Anqing-Guichi ore deposits. The Tanlu fault zone, a giant strike-slip fault of more than 2 000km long, passes through the northwestern margin of this area. Geophysical studies have demonstrated copious geological evidences for the Yangtze fault zone, which is approximately 450km long and crosses central China, extending to the eastern coastal area. The present fault and fold systems are the consequences of the repeated tectonic events since the Mesozoic. We collected and analyzed the seismic data of 20 fixed airgun shot points, then utilized tomography, time term method and head wave traveltime inversion based on ray tracing techniques to model the upper crustal velocity and crystalline basement structure of the Anqing-Maanshan segment beneath the Yangtze fault zone. The profile along the Yangtze River consists of 100 PDS-2 seismometers with a spacing of 2km. We applied the linear and phase weighted stack methods to improve the signal-to-noise ratio of the weak seismic phases from the airgun source. According to the comparison between the linear and phase weighted stack results, the phase weighted stack method significantly improves the quality of the stacked data. We applied the band-pass filter to the stacked data to improve the onset of the first arrival, then picked up the seismic phases and assessed the errors of the picked traveltime. The comprehensive results reveal that the upper crust velocity structure and crystalline basement images show a tectonic feature of alternating rifts and uplifts. The upper crust of the Huaining Basin is the thickest area along the Yangtze River. The basement of the Huaining Basin is around 4.5km and there are Mesozoic lacustrine sedimentary layers whose thickness is about 2km. The crystalline basement depth of the Luzong Basin is 4.1km and the consolidated basin shows clear depression basin shape. This feature of the Luzong Basin reveals that it experienced extensional depression. There is a high-velocity zone beneath the crystalline basement of the Luzong Basin, and the velocity is higher than other areas along the Yangtze River. This high velocity zone shows an arc shape, which agrees with the Paleozoic reflection images by the seismic reflection survey. The profile crosses the Yangtze River in Tongling area and there are obvious velocity differences between the two sides of the Yangtze River. The velocity differences show that the Yangtze faults cut the crystalline basement in Tongling. The upper crust velocity structure of the Tongling area shows clear uplift features and its crystalline basement depth is about 2.2km, which agrees with the arc-reflection structures of the upper crust from the seismic reflection data. This uplift image reveals that the upper crust of the Tongling area has experienced extrusion deformations. The consistency of the seismic reflection imaging results with the near surface geology demonstrates that the large volume air-gun source is applicable to land-based seismic survey.

The urban active fault survey is of great significance to improve the development and utilization of urban underground space, the urban resilience, the regional seismic reference modeling, and the natural hazard prevention. The Beijing-Tianjin metropolitan region with the densest population is one of the most developed and most important urban groups, located at the northeastern North China plain. There are several fault systems crossing and converging in this region, and most of the faults are buried. The tectonic setting of the faults is complex from shallow to deep. There are frequent historical earthquakes in this area, which results in higher earthquake risk and geological hazards. There are two seismicity active belts in this area. One is the NE directed earthquake belt located at the east part of the profile in northern Ninghai near the Tangshan earthquake region. The other is located in the Beijing plain in the northwest of the profile and near the southern end of Yanshan fold belt, where the 1679 M8.0 Sanhe-Pinggu earthquake occurred, the largest historical earthquake of this area. Besides, there are some small earthquake activities related to the Xiadian Fault and the Cangdong Fault at the central part of the profile.
    The seismic refraction experiment is an efficient approach for urban active fault survey, especially in large- and medium-size cities. This method was widely applied to the urban hazard assessment of Los Angeles. We applied a regularized tomography method to modeling the upper crustal velocity structure from the high-resolution seismic refraction profile data which is across the Beijing-Tianjin metropolitan region. This seismic refraction profile, with 185km in length, 18 chemical explosive shots and 500m observation space, is the profile with densest seismic acquisition in the Beijing-Tianjin metropolitan region up to now. We used the trial-error method to optimize the starting velocity model for the first-arrival traveltime inversion. The multiple scale checker board tests were applied to the tomographic result assessment, which is a non-linear method to quantitatively estimate the inversion results. The resolution of the tomographic model is 2km to 4km through the ray-path coverage when the threshold value is 0.5 and is 4km to 7km through the ray-path coverage when the threshold value is 0.7. The tomographic model reveals a very thick sediment cover on the crystalline basement beneath the Beijing-Tianjin metropolitan region. The P wave velocity of near surface is 1.6km/s. The thickest sediment cover area locates in the Huanghua sag and the Wuqing sag with a thickness of 8km, and the thinnest area is located at the Beijing sag with a thickness of 2km. The thickness of the sediment cover is 4km and 5km in the Cangxian uplift and the Dacang sag, respectively. The depth of crystalline basement and the tectonic features of the geological subunits are related to the extension and rift movement since the Cenozoic, which is the dynamics of formation of the giant basins.
    It is difficult to identify a buried fault system, for a tomographic regularization process includes velocity smoothing, and limited by the seismic reflection imaging method, it is more difficult to image the steep fault. Velocity and seismic phase variations usually provide important references that describe the geometry of the faults where there are velocity differences between the two sides of fault. In this paper, we analyzed the structural features of the faults with big velocity difference between the two sides of the fault system using the velocity difference revealed by tomography and the lateral seismic variations in seismograms, and constrained the geometry of the major faults in the study region from near surface to upper crust. Both the Baodi Fault and the Xiadian Fault are very steep with clear velocity difference between their two sides. The seismic refraction phases and the tomographic model indicate that they both cut the crystalline basement and extend to 12km deep. The Baodi Fault is the boundary between the Dachang sag and the Wuqing sag. The Xiadian Fault is a listric fault and a boundary between the Tongxian uplift and the Dachang sag. The tomographic model and the earthquake locations show that the near-vertical Shunyi-Liangxiang Fault, with a certain amount of velocity difference between its two sides, cuts the crystalline basement, and the seismicity on the fault is frequent since Cenozoic. The Shunyi-Liangxiang Fault can be identified deep to 20km according to the seismicity hypocenters.
    The dense acquisition seismic refraction is a good approach to construct velocity model of the upper crust and helpful to identify the buried faults where there are velocity differences between their two sides. Our results show that the seismic refraction survey is a useful implement which provides comprehensive references for imaging the fault geometry in urban active fault survey.

The Red River Fault in western Yunnan is one of the longest strike-slip faults in China and has a high seismic potential. To investigate its complicated structure, a near-NS directed 300km long wide-angle reflection/refraction seismic profile was laid out from Yunxian to Ninglang, across the Red River Fault. The 2-D velocity structure model along the profile was obtained through 1-D and 2-D analysis and fitting the observed data with combination of first-arrival traveltime tomography and forward modeling. The results indicate:In the crust, the average P-wave velocity is 6.2~6.3km/s and basically shows a positive gradient structure, but there are some low velocity anomalies at different area in upper and lower crust. Regarding the crust boundary, a relative large lateral variation exists in the depth of Moho, which goes deeper from south to north, ranging from 45km to as deep as 54km; compared to other typical continental crust, the study area demonstrates a striking thickening. It should be mentioned that the crustal thickening is mainly observed in the lower crust, while the upper and middle crust possess nearly constant thickness. We observed strong seismic velocity contrast across the Red River Fault, which emphasizes the role of the fault as an important tectonic boundary between Yangtze paraplatform and Sanjiang geosynclinal system. Along the profile, the Moho depth has no remarkable variation when crossing the Red River Fault. Combining with other study results on nearby area, it proves that there is notable heterogeneity between different parts of the Red River Fault.

In urban active fault prospecting, the shallow structures usually display strong lateral inhomogeneity, appearing as the heavy fluctuation of interfaces and considerable variation of layer velocities. In this case, the traditional refraction data processing and interpreting methods based on homogeneous layered structures with level interfaces can't be directly applied to the prospecting. It is very important, therefore, to study the seismic behaviors in these complex structures and to deve~lop a new technique that can be used to process and interpret seismic refraction data obtained from urban areas. In this paper, forward computing of wave field is carried out by using wavefront expanding method in terms of Huygens' principle. Furthermore, in the light of Hagedoorn wavefront refractor imaging principle a new processing method of seismic refraction data and the corresponding interpretation software are developed, in which Hole's original finite-difference codes were modified with Lecomte's five operators for computing seismic travel times. Applying this technique, we successfully process the data from two refraction profiles recently completed in Yixu, Fuzhou City during urban buried fault prospecting. The results show that the shallow structures in the investigation area display three layers, which are sedimentary cover, strongly weathered layer and bedrock, respectively. The buried depth of the upper surface of bedrock ranges from 52m to 58m or so. The variation of P wave velocity in sedimentary cover is considerable.