The construction of the Xiong’an New Area is a national strategy and a long-term plan outlined by the Chinese government. To support the urban planning and development of this area, many scholars have conducted a series of geophysical surveys aimed at understanding the detailed subsurface structure. The Horizontal-to-Vertical Spectral Ratio(HVSR)method, first introduced by Nakamura, has recently gained widespread use for investigating shallow subsurface structures, site response, and microzonation.

In this study, we utilized a large seismic array with an interstation distance ranging from 500 to 1000 meters, deployed across the Xiong’an New Area. The array consisted of over 900 short-period seismographs, covering most of the area. Using ambient-noise recordings, we removed nonrandom transient signals from the waveform data with a short-term-average over long-term-average detector automatic picking algorithm, and applied the Konno-Ohmachi algorithm to smooth the HVSR curves. For each site, we analyzed the amplitude of the peak value of the HVSR curve(A)and the corresponding frequency(f₀). Both parameters were further elaborated through the creation of contour maps using the Kriging interpolation method. Additionally, the peak frequencies from the HVSR curves were used to calculate the sedimentary thickness, based on an average shear-wave velocity and the frequency-depth formula.

The frequency map shows that the peak frequencies range between 0.6 and 1.1Hz, with an overall peak frequency of about 0.7 to 1.0Hz. The lowest frequencies were found predominantly in the vast eastern area of the study region, corresponding to geological features such as the Niubei Slope, Niutuozhen High, and Baxian Sag. According to the frequency-depth formula, a lower peak frequency indicates greater sediment depth. The variation in peak frequencies across stations highlights changes in the bedrock interface, which correspond to fault structures depicted on the geological map. Furthermore, high-amplitude areas were mainly located between the Rongxi fault and Rongdong fault, suggesting an impedance contrast between shallow and deeper layers. Stratigraphic profiles reveal that Quaternary and Tertiary sedimentary layers directly overlie the crystalline basement composed of Proterozoic metamorphic rocks. Combined analysis of peak frequency and amplitude aligns well with the available geological data. Our analysis produced 3D depth images of the Quaternary sedimentary layer interface across the study area, clearly imaging a significant seismic impedance interface at depths of 100-220m. This shallow interface corresponds to the contrast between the Tertiary rocks and the overlying Quaternary sedimentary layers. The sediment thickness progressively increases from east to west across the study area. Interfaces derived from the HVSR profiles display similar characteristics to those on the geological map and are consistent with borehole data and results from the high-density resistivity method. Moreover, we established a power-law relationship correlating the fundamental site resonance frequencies with sedimentary cover thickness obtained from borehole data in the Xiong’an New Area. The undulating characteristics of the sedimentary layers correspond closely to fault locations and geological tectonic units, confirming that faults such as the Rongxi, Rongdong, Niuxi, Niudong, and Xushui-Dacheng faults serve as boundaries for secondary geological tectonic units, influencing the structure of the near-surface sedimentary layers.

We developed a 3D shallow subsurface sedimentary model for the Xiong’an New Area and created contour maps of amplitude(A)and peak frequency(f₀). The results both support and extend previous understandings of the region’s structure. This study demonstrates that the HVSR method, in conjunction with a large seismic array, is a rapid and effective technique for investigating shallow subsurface structures and seismic site responses. The exploration of sedimentary structures and seismic site response characteristics, which are closely related to earthquake hazards, provides a critical foundation for seismic fortification and urban planning in the Xiong’an New Area.

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.

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.

The research area involved in this paper is the middle-southern segment of Liaocheng-Lankao fault zone(Lanliao fault zone)and its adjacent area. In order to study the fine crustal structure image and the tectonic features of the faults in this tectonic zone, we conducted a 70km-long deep seismic reflection profile along EW direction in Puyang City, Henan Province and got clearer lithospheric structure image along the profile.

As regards data acquisition, we applied the geometry with 30m group interval, 1 160 recording channels and more than 90 folds. Seismic wave exploding applies the 30kg shots of dynamite source with the hole depth of 40~50m. In addition, in order to ensure the signal-to-noise ratio of the deep reflector, explosive quantity of dynamite source is increased to 96kg every 1 000m interval. In data processing, the most important thing is to improve the signal-to-noise ratio. Data processing methods mainly include one-dimensional time-varying filtering combined with two-dimensional filtering, tomographic static correction, residual static correction, deconvolution, normal moveout correction(NMO), dip moveout correction, common mid-point(CMP)stack and post-stack denoising, post-stack migration, etc.

The section with high signal-to-noise ratio has been obtained. There are obvious characteristics of reflection wave groups in the crust, which reflects abundant information about geological structure. On this section, according to this study, the characteristics of deep and shallow structure and crustal reflection structures on both sides of the Lanliao fault zone are obviously different. The crust in this area is composed of brittle upper crust and ductile lower crust. There are rich reflective layers and clear tectonic framework in the upper crust. In the western area of Lanliao fault zone, there is a set of dense reflectors with strong energy, which reflects the sedimentary interface of different times since Mesozoic in the basin. The basement slope with gentle dip to the east is the bottom boundary of the “dustpan-shaped” sedimentary depression. The reflected wave of the crystalline basement presents a group of strong reflection wave groups with good continuity in the eastern area of Lanliao fault zone, which are parallel unconformities on the Ordovician strata of Paleozoic or older strata. There are some secondary faults in the hanging wall of Lanliao Fault, which together with the Lanliao fault zone control the tectonic framework of “dustpan-shaped” sedimentary depression, the Dongpu sag. The reflection structure of the lower crust is relatively simple. On the whole, it is mainly arc reflection with strong energy and short duration.

The depth of Moho surface beneath the central-southern Lanliao fault zone in this area is 31.7~34.8km, where the fault is characterized by a strong reflection band with piecewise continuous distribution in horizontal direction and a duration of about 0.3~0.8s in vertical direction. And it is a transition zone with a certain thickness after geological deformation, rather than a sharp first-order discontinuity, which is consistent with the research results of Li Songlin et al.(2011). This profile reveals 2 deep faults(F_D1 and F_D2)that offset the Moho surface, extend down to the top of the upper mantle and create conditions for the upwelling of hot materials from asthenosphere and the energy exchange in this area. It may also be the cause of arc reflection in the lower crust.

The deep seismic reflection profile shows that faults in the upper crust are well developed. Lanliao Fault is the largest boundary fault in this area, which controls the formation and evolution of the “dustpan-shaped” sedimentary depression and plays an important role in the filling of Paleogene strata in the sag. Pucheng Fault F_P1 and Weixi Fault F_P3 are developed in the hanging wall of Lanliao Fault, which are basement normal faults in the same direction as Lanliao Fault and control the structural framework of the depression. Pucheng Fault, Weixi Fault and Lanliao Fault constitute a domino fault system, which makes the basement of the depression incline to the SEE direction. In addition, a reverse secondary normal fault(Changyuan Fault F_P2)is developed in the hanging wall of Lanliao Fault, which intersects with Weixi Fault F_P3 at TWT 3.0s. These faults and Lanliao faults jointly control the basic structural pattern of the sedimentary sag.

The deep and shallow tectonic framework in this area is controlled by the shallow faults in the upper crust and the deep faults in the lower crust. Deep faults(F_D1 and F_D2)create conditions for the upwelling of hot materials from asthenosphere, while shallow faults play an important role in the formation and evolution of basin structures.

The North China Craton is the oldest craton in China and also the main tectonic unit of the Chinese mainland. The geological marks from Archean to Mesozoic era are complete and have attracted scientists all over the world. It has been the natural experimental site for the study of continental formation and evolution. A series of complex tectonic movement and evolution processes occurred in the North China Craton since Mesozoic. A series of rift basins were formed due to the thinning of lithosphere in its eastern part, so its crust structre is complicated. But the lithosphere is thick in its western part, so the crust structure of the Ordos block is simple. Shanxi rift zone is located between the eastern block of North China Craton and the western Ordos block. The crust and lithosphere structure of Shanxi rift zone is changed from stable craton structure in the west to severely damaged craton structure in the east, showing obvious transition characteristics. Therefore, it is of great significance to study the structural characteristics of the Shanxi rift zone and its two sides so as to reveal the failure dynamics of the North China Craton. Based on the teleseismic waveform data recorded by 150 mobile seismic stations in the central and western part of the North China Craton(107°E~117°E; 34°N~41°N)in the recent three years, the crustal velocity structure images of the study area are obtained by using the H-κ stacking method of P-wave receiver function and the common conversion point(CCP)superposition method. Our research results show that the crustal thickness in the Ordos block is between 37km and 47km, the Moho surface is relatively flat. The crust thickness of Shanxi rift zone is between 34km and 46km. Under the depression of Linfen Basin, Moho surface shows obvious uplift, and the uplift amount is between 4km and 10km. It is inferred that the formation of Shanxi rift zone is closely related to the movement of mantle materials. Compared with the existing Bouguer gravity anomaly data in this area, the distribution characteristics of crustal thickness in the study area are consistent with the distribution characteristics of positive and negative Bouguer gravity anomalies in the eastern and western Taihang uplift, respectively. The calculation results of crustal thickness and wave velocity ratio in different tectonic units in this region show that the wave velocity ratio in the three tectonic units decreases with the increase of crustal thickness. On the whole, the study area is divided into east and west areas with 111.5°E as the boundary. The Poisson's ratio of Ordos area to the west is lower than that of Shanxi rift zone to the east of 111.5°E, which reflects that the eastern part of Ordos block has the characteristics of stable ancient block and the crustal structure is relatively simple; however, the upwelling of upper mantle material under the Shanxi rift zone leads to higher Poisson's ratio than the mountainous areas on both sides. As far as the Shanxi rift zone is concerned, it is divided into north and south regions with 38°N as the boundary. The crust to the north of 38°N is characterized by low velocity due to partial melting, while the area south of 38°N still maintains a relatively stable crust and presents high-velocity characteristics. The difference of crustal structure and material composition between the north and the south of Shanxi rift zone may be related to the uneven subsidence of Shanxi rift zone, and more data are needed for further comprehensive study on the related dynamic process.

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.

Shallow seismic reflection method is a commonly used technique in urban active fault detection,however,special geotectonic environment may sometimes make reflection survey inapplicable.In such cases,high-resolution seismic refraction could be a feasible option.In this study,we use the finite difference method as the main technique and the conventional methods of refraction data interpretation as auxiliary means in the interpretation of high-resolution shallow refraction data for active fault detection in Lanzhou area.After a comprehensive analysis of first-break refraction travel-time characteristics,the velocity structure and interface structure along each profile have been obtained.A detailed description of the detection results from SS04-1 and SS11-2 seismic profiles is presented in this paper.The main stratigraphic interfaces and tectonic features identified by the two profiles are quite consistent with the results from drilling surveys along the profiles.Our results indicate that high-resolution seismic refraction is an effective replacement in areas where reflection seismic survey is hard to carry out.

The Jingpohu volcano cluster lies on the southwest of Ningan county seat,Mudanjiang city,Heilongjiang Province,China.There are 13 craters in this area and the volcano cluster formed in Holocene Epoch is concentrated in the two areas of Crater Forest and Hamatang.According to the recent studies,Jingpohu volcano cluster is located on the western side of the Dunhua-Mishan Fault,where the volcanic activity has been highly frequent and the volcanoes were active in Paleocene,Eocene and Holocene.These volcanisms were associated with strong earthquakes of magnitude 6～7.In order to understand the volcanic activities as well as the structure features of crust and upper mantle in this area,14 broadband seismic stations with 24 bit digital seismographs were installed around Crater Forest in Jingpohu volcano area and various seismic events including the volcanic tectonic earthquakes and the volcanic-like events were recorded.In this paper,based on the analysis of a great deal of data,the earthquake type classification,seismicity analysis and earthquake location were carried out.The classification of recorded event types indicate that the earthquakes observed in Jingpohu volcanic area are mainly of volcanic tectonic ones while the seismicity was not high during the recording periods,and at the same time,two types of earthquakes which are different from tectonic ones were recorded.Among these events,the waveform features of one type of events are similar to the volcanic events with long period(LP),however,compared with standard volcanic events with long period(LP),their frequencies are higher;the other type of events have some similar features with volcanic tremors.It could be seen from earthquake location results that the most focus depths range from 10 to 30km and their epicenters are mainly concentrated on the southwestern side of craters.There are few earthquakes in the interior of craters and their magnitudes are mostly less than 2.0.It is suggested that the occurrence of these earthquakes is possibly related with the activities of Dunhua-Mishan fault because the volcanic and seismic activities during observation periods in Jingpohu volcano area are not too obvious and the epicenters are mainly distributed near Crater Forest and Dunhua-Mishan fault.Close attention should be paid to the volcanic-like events with long period and the tremors recorded around Crater Forest.However,it needs further research to make sure that whether the two types of events recorded in this area are related to the magmatic activities because of shorter observation time and a few recorded events available in this study.

This paper introduced the basic principles of various seismic prospecting methods and working methods briefly according to the nationwide practices of seismic prospecting for active faults beneath big cities in recent years.Furthermore,it mainly analyzed the available range of different seismic prospecting methods,main achievements and solutions,and discussed the best combination of seismic exploration methods for detecting crustal structure and locating the faults used in the present stage,that is,to trace the faults which are at the depths of hundred meters underground using shallow seismic investigation,to detect the basement faults which are above basement(at depth of kilo～meters)using high resolution refraction sounding,and the deep crustal faults using combined seismic prospecting method of reflection seismic sounding and wide-angle reflection/refraction sounding,and furthermore,to adopt 3-D deep seismic sounding method to get 3-D velocity structure beneath city area.Thus,we can get information about fault attitude and distribution at different depths and a complete image of fault from shallow part to deep part using the combined seismic exploration method.Some application examples are presented in the article.

Seismic refraction method is tentatively used in the exploration of urban active faults where strong interference of surface waves exists and it is difficult to use seismic reflection survey. Original seismic refraction data are calculated and inversed by using time-terms, curve of differential time-distance and finite-difference tomography, to investigate the effect of the new technique in the exploration of urban active faults.In this paper, velocity structures and interface structures are obtained after the shallow seismic refraction data in the western segment of Xushui fault of western Zhengzhou are calculated and inversed by using the above-mentioned three calculation methods, and structural characteristics and depths of main geologic strata are got by integrating inversion results and features of seismic phases. The results of these three methods are similar, and they are confirmed by drilling data in the profile. The seismic refraction method can be used in the exploration of urban active faults.

Jiashi region, Xinjiang is a strong earthquake area in western China. In recent years, several great earthquake swarms have occurred in this region, causing tremendous hazards. In order to get an insight into the relation between the deep structures and the generation of great earthquake swarm in this region, a generalized inversion technique for determining probability distributions of spatial locations of earthquake events through the travel times of P and S waves in random and vertically inhomogeneous medium, is used in this paper. Seismic data recorded by temporal digital seismic network deposed in this region were located using this method. The located earthquakes show linear distribution in north-northwest and north-northeast directions, among which the former is more distinct than the later. Based on the obtained results, the deep structural background of seismic activities in Jiashi region is discussed. The results indicate also the close relation between the earthquake and fault structures in Jiashi region. It is clear that the generation of earthquake swarm might be related to the buried fault in the vicinity of seismic source area, and might be the result of violent crustal deformation on the northern margin of Tarim basin and the present tectonic movement. These complicated structural framework and peculiar environmental conditions might be responsible for the development and generation of the strong earthquake swarm in Jiashi region.

The crystalline basement velocity structure of Changbaishan Tianchi volcanic area is established by using Three-Dimensional Finite-Difference method and Pg data from four wide-angle reflection/refraction profiles and one three-dimensional array which was deployed in Changbaishan Mountain area. This paper presents the characteristics of the velocity structure of crystalline basement and the interface with 5.9～6.0km/s P-wave velocity in the crystalline basement. Inversion results reveal that the velocity of surface layer in the southern part is higher than that in the northern part of the studied area. In general, the depth of crystalline basement of the studied area is within the range of 2.0～3.0km, with the deepest of about 4.0km and the shallowest of about 1 5km near Songjiang. There are two low velocity areas in this region, one is beneath the Protection Station of Tianchi volcano, and the other is located beneath Erdaobaihe and Chixi Protection Station. In these two areas the depths of crystalline basement are about 4.0km. We found that the intense lateral variation of velocity and depth of crystalline basement in the seismic profile coincides well with the location of fault. Therefore, it is suggested that the intense lateral variation of velocity and the abrupt change of the depths of crystalline basement can be taken as an indication of the occurrence of 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.

Research on a large number of seismic events at home and abroad has indicated that tremendous earthquake hazards in urban areas are mostly attributed to earthquakes caused by active faults buried beneath the cities. The identification of urban buried active faults, therefore, is an important and urgent task. High-resolution seismic exploration is an effective geophysical technique that can be used to identify urban buried active fault at present. High-resolution seismic exploration for urban buried active faults is a sophisticated and systematic project, which involves excitation and receiving techniques, observational system, as well as seismic data processing and interpretation. The seismic source is of the first importance among the other problems that should be solved during the exploration. High-resolution seismic exploration for urban active fault calls for specific performance of the seismic source, because of peculiar environment in urban areas and particular characteristics of urban buried faults. For examples, relatively small offset of the fault requires a wider source spectrum, while strong disturbances in urban areas need a higher anti-jamming capability of the source. A comparative experiment on various types of sources, including vibroseis, vacuum accelerating weight drop, hammer-blow, air gun and explosive is carried out along the traverse across the Bayishuiku Fault. The features of various source spectrums are obtained by using spectrum analysis technique. The comparison of time-stacked sections obtained by using vibroseis, vacuum accelerating weight-drop and hammer blow from the traverse across the Bayishuiku Fault in Fuzhou City is presented in this paper. The effectiveness of various seismic sources in the exploration of urban buried active faults is discussed in detail.

In addition to a brief account of characteristics of disturbing waves in urban shallow seismic exploration, an exposition of technical facilities and seismic data acquisition techniques for anti-jamming and high-resolution shallow seismic exploration is given in this paper on the basis of shallow seismic experimental data of active fault surveying in Fuzhou City. The technical measures taken for anti-jamming, improving signal to noise ratio and resolution of seismic data are expounded as well. The experiment shows that the effective approach to accomplishing anti-jamming, high-resolution shallow seismic data acquisition is receiving with mini trace intervals, mini offsets, multi-channel and high-frequency Geophones by using mini-vibrator and the matched seismograph.