On April 20, 2013, a strong earthquake with a magnitude M_s7.0 broke out in Lushan County, Ya’an city, Sichuan province. The main shock and aftershocks were located accurately and the epicenter was about 13km underground(Zhao et al., 2013; Lu et al., 2013). The epicenter was located in the southern segment ofLongmenshan fault zone, where Wenchuan M_s8.0 earthquake also broke out on May 12, 2008. The distancebetween two epicenters is only about 80 kilometers(Chen et al., 2013; Xu et al., 2013; Gao et al., 2013). Thescholars at home and abroad have carried out researches to study the location, the earthquake focal mechanism and the earthquake rupture process after Lushan earthquake(Wang et al., 2013; Liu et al., 2013; Zhang, 2013).The results show that Lushan earthquake took place in the southern segment of Longmenshan fault zone and was a thrust tectonic quake which is same in nature as Wenchuan earthquake in 2008(Zhang et al., 2009a).Since these two earthquakes occurred successively in the Longmenshan nappe tectonic zone, there must be acertain correlation between them in terms of the temporal-spacial distribution, the seismogenic structure, focalmechanism and so on. Therefore, a heated debate on whether Lushan earthquake is one large aftershock ofWenchuan earthquake was aroused(Xu et al., 2013; et al., 2013; Du et al., 2013; Yan et al., 2014).

As time goes by, new data is keeping on being obtained, especially the new research findings by the activefaults study, the geophysical exploration and GPS observation, etc. which have an important influence on thefurther researches of tectonic deformation and seismic activity background of this region. However, based onthe existing researches which are influenced by the precision of the data and restricted by the technologicalmeans, the crust-mantle velocity structure model of this region is difficult to be figured out accurately(Wang et al., 2003a; Xu et al., 2014). Therefore, a Jinchuan-Lushan-Leshan artificial seismic profile which crossesthrough the southeast margin of Qinghai-Tibet plateau and the Sichuan Basin was completed. Rebuilding ahigh accuracy crust-mantle velocity structure model by means of multiple calculation methods as well as thecomprehensive researches is not only helpful to make clear the latest process and the mechanism of the crustaldeformation and scientifically evaluate the tendency of the coming strong earthquakes, as well as to plan and develop the regional territory reasonably, but will also offer credible basic data of crust stability for the majorconstruction projects(Wang et al., 2013).

After the Lushan earthquake, it is urgent to fully underst and the neotectonics, the active tectonics settings and the genetic problems of the strong earthquakes in the southeast margin of Qinghai-Tibet plateau(Chen et al., 2013). Under this situation, the scientific investigations headquarter was quickly established by ChinaEarthquake Administration, and many fieldwork groups with specific tasks led by the Institute of Geophysicscarried out investigations on the background of Lushan earthquake by a rounded analysis. The seismic explorationgroup completed an about 410-km-long wide-angle reflection/refraction profile through Lushan seismicregion to work out a high-resolution two-dimensional crustal velocity structure and the fine structure of thisregion, to make clear the seismogenic structure, seismogenic environment and the relation between the deepshallowstructures, to build a seismogenic structure model, to investigate the background of the deep dynamics, to compare and analyze the deep structure and the characteristics of tectonic differences in different segments ofLongmenshan fault zone, to provide scientific basis of the deep-shallow structures for tendency of the earthquakeoccurrence and the seismic danger predication, to bring the other geophysical approaches(Lou et al., 2008)amore accurate crustal velocity structure model as a reference to validate and complement each other(Ding et al., 2008; Bai et al., 2011; Robert et al., 2010). This study has not only contributed to deepen the underst and ingof the fracture mechanisms but is also of great scientific significance in analyzing the trigger factors and thespatial migration of earthquakes and predicting the development tendency of the future earthquakes.

The study area is located in the southeast margin of Qinghai-Tibet plateau. So far, it is one of theareas where the most intense crustal deformation and earthquake activities have ever occurred in Qinghai-Tibetplateau. Geologically, it is a region where the Yangtze block and Songpan-Garze block converge. Many fracturestructures with distinct properties in different scales had been formed by the influence of multiphase magmatism and fierce tectonic deformation during the Cenozoic era(Bai et al., 2010; Wang et al., 2009). In the new tectonicperiod, especially the Quaternary or the late Quaternary, the crustal movement in this region was quite active and a number of active fractures which differed greatly in scales, types and activeness were developed underthe dynamic background that India plate and Eurasian plate continuously collided(Lei and Zhao, 2009; Xu et al., 2013; Liu et al., 2009; Zhang et al., 2010). Longmenshan fault zone is located in the mid-segment of thenorth-south seismic belt in central China. On the southeastern side is the Yangtze paraplatform and on the westside is the Songpan-Garze block. Since the Indosinian movement, especially the Himalayan movement, with thefierce uplift of Qinghai-Tibet plateau, continental crust collisions and extrusions to varying degrees have taken place. A large-scale fault zone, mostly thrust with some strike-slip, was formed in front of Longmenshan(Shi et al., 2009; Sunet al., 2011; Lei, 2009; Chen et al., 2013). The general strike of Longmenshan fault zone isN45°E, dipping to NW. From northwest to southeast, Longmenshan thrust and nappe tectonic zone consistsof three main west-dipping fault zones, namely, the back range fault, the central range fault and front-rangefault. Each of them is made up of several diverse sections respectively. Geological investigation results showthat(Shi et al., 2009; Sun et al., 2011)since late Quaternary, the thrust movements have been seen in thethree main fault zones from northwest to southeast, with an apparent dextral slip component. In the southernsegment of Longmenshan fault zone where the Lushan earthquake broke out, developed the imbricate subprimethrust fault. Several NE fractures are arranged in order from west to east, namely Yanjing-Wulong, Shuangshi-Dachuan, Xinkaidian, Dayi and Pujiang-Xinjin, etc. Yanjing-Wulong fracture is located in the southern segmentof the central range fault, and Shuangshi-Dachuan is located in the southern segment of the front-range fault, both of which are the boundary fault zone on the southeast side of Longmenshan tectonic zone. Dayi fractureis a blind fault with a steep dip angle in the Sichuan Basin. The deformation process of neotectonics and itsdynamic mechanism, the current active tectonic deformation patterns and the relation with strong earthquakesare the issues much concerned by the researchers of geophysics at home and abroad(Lei et al., 2008; Zhang et al., 2010). Therefore, building a high-precision crust-mantle velocity structure model in this region is of greatsignificance in underst and ing the current crustal deformation mode, the dynamic mechanics and the seismicsettings in the southeastern margin of the Tibetan Plateau(Xu et al, 2010; Wang et al, 2007). All aroundthese issues, the former scholars have done a lot of researches on seismology and geology condition, physicalgeography and some other problems in this region(Wang et al., 2003a; Royden et al., 1997; Unsworth et al., 2005; Wei et al., 2001; Zheng et al., 2010; Zhang et al., 2009b; Yao et al., 2009). These former researches haveprovided important references for this study.

The geological structure in the survey region is of great complexity, in which multi-groups of deep fracturesare crisscrossed. In recent years, a lot of researches on the moderate intensive earthquakes occurred in this regionshow that the occurrence of these earthquakes has a close relation to the giant deep faults, the complex crustmantletransition zone and the media environment with high-low velocity of mid-upper crust. In view of thespecific seismotectonic environment in this region, some researchers have carried out a lot of geophysical researchwork, which has offered important data of the deep seismogenic environment about the characteristics of deepphysical field and the seismogenic environment. According to wideb and teleseismic records from the stationsin and around Longmenshan, Wang Chunyong et al., (2010)worked out the crustal thickness and the wavevelocity by means of H-κ additive method. The results show that the overall variation of the crustal thicknessin the region is deepening from east to west. The crustal thickness in the fault zone across Longmenshanvaries greatly. The low Poisson’s ratio is measured both in the northern part of Songpan-Garze block and westQinling orogenic belt. The eastward movement of the southern Songpan-Garze block is blocked by the Yangtzeplatform with rigid strength, which leads to the strain accumulation along the Longmenshan fault zone. Thefault was weakened by crustal fluid, a rapid release of the accumulated strain energy results in Wenchuan M_s8.0earthquake. Zhao Bo et al., (2013)relocated the location of Lushan main shock and the aftershock sequencesby the double difference method. The results show that the depth of the earthquake was mainly in 10~20km. Aftershocks were distributed along NE direction approximately, about 35 km long. Along the NW profile, the aftershocks were mostly distributed in SE direction of Dachuan-Shuangshi fault. With the increase of thephase data and data of aftershocks collected by mobile stations, the distribution of aftershocks obtained byrelocation resembles the occurrence of the fault structures. Based on the continuous waveform data collectedby CENC, Zheng Yong et al.(2013)obtained the accurate S wave velocity structure, the crustal thickness and the distribution of Poisson’s ratio in and around the seismic region by using the background noise tomographymethods and teleseismic receiver function analysis. The researches show that the S wave velocity and the crustal thickness on both sides of the Longmenshan fault zone vary greatly. Lushan earthquake and Wenchuanearthquake were located in the regions where the crustal thickness and wave velocity structure vary intensely. The fault rupture surface and the distribution of the aftershocks were located in the regions where the transversevelocity gradient of both seismic waves and crustal thickness display a transition. The seismic depth is in thetransition region in which the uniform velocity structure changes into a non-uniform velocity structure. Tobetter underst and the characteristics of the deep geophysical field in the region, in 2013, a Jinchuan-Lusha-Leshan artificial source wide-angle reflection/refraction seismic profile was carried out across the focal area ofLushan M_s7.0 earthquake. A deep seismic velocity structure model was built up to figure out the accuratelocation of earthquakes and the mechanism of earthquake.

The profile of the observation starts from the Southeast around Wangchang town, Yibin city(Coordinates:104°39'15"E, 29°05'40"N, Stake number: 50 km). From southeast to northwest, the profile goes through Guwen and Liujia town of Zigong city, Tuzhu, Hanyang, Renmei and Zhangchang town of Leshan city, Cheling, Xindian and Longmen town, Baoxing, Yongfu, Xiaojin, north Mingshan county, Longmen, Baoxing, Longdong, Yongfu, Xiaojin and Jinchuan of Lushan county and other places and stopped around Qiesidu village, Taiyanghe town, Jinchuan county(Coordinates: 101°46'30"E, 31°41'25"N, Stake number: 460 km). The whole length of theprofile is 410 km(Fig. 1). From September to November in 2013, Geophysical Exploration Center fulfilled thefield work of drilling wells, figuring out shot positions, setting observation points and collecting data.

Fig. 1 Location of the profile and the tectonic map

The seismic profile goes through different regions with different terrain conditions and geographical environments, spanning a number of fault structure zones and different geological tectonic units. Along the profile, the terrain conditions and geographical environments are quite complicated and mainly in the mountainousterrain. The southeastern segment about 100 km of the profile is located in the Sichuan Basin. To the north of the stake number 280 km, the area is located in Western Sichuan Plateau with an average altitude of 2500~3500m. The altitude of some segments of the region is more than 4000 m. To the south of the stake number 280km, the area is located in Yangtze block with an altitude of 1000~2000 m. The observation points were mainlycarried out along the mountain road or county roads. The background noise level of the observation points isnot high.

Along the 410-km-long profile, 268 three-component PDS-2 seismographs were set up. The distancebetween observation points is designed to be alternating from 0.8~2.0 km. In and around fault zones and someparticular tectonic locations, the distance is up to 0.8 km to ensure the mapping of the shallow structuressound and clear. In the Western Sichuan Plateau and the internal Sichuan Basin, the distance is 2.0 km. Thedata of seismic wave field reflects the deep-shallow structures of the crust and upper mantle and the majorfault tectonic zones. Based on the acquired data, the shot locations are located and eight shots are set up.The specific parameters of each shot are shown in Table 1. The dosage of the single -shot is determined bythe research objectives, the environmental conditions of shots, the exciting conditions and the width of theobservation intervals, with the maximum of 3.5 ton and the minimum of 1.0 ton. A better observation system iscompleted according to the excitation condition of eight shots and the data collected by the 268 three-componentPDS-2 seismographs.

Table 1()

Table 1 Table of the shot point parameters

Table 1 Table of the shot point parameters

The acquired field detection data is reduced by the speed of 6.00 km·s^-1(Fig. 2–Fig. 3)(The verticalcoordinates of the profile in Fig. 2 and Fig. 3 show the reduced arrival time and the horizontal ordinates indicatethe offset, and the right side of the negative offset is corresponding to the southeastern survey line and thepositive offset is corresponding to the northwestern survey) and the profile is filtered at 2~12 Hz. The SNR ofvalid signal increases greatly after filtering. Each phase is clear and continuous and can be tracked reliably and compared. Based on the comprehensive analysis and study on the seismic phase characteristics of each seismicwave collected in the eight shots, seven phases are worked out by comparison. They are Pg, the turning wavein strong gradient belt above the crystalline basement, P₁, P₂, P₃, P₄, the crustal reflection waves, Pm, thereflection wave on the Moho surface and Pn, the turning wave at the top of upper-mantle.

Fig. 2 Converted time shot profile (SP1)

Fig. 3 Reduced time shot profile (SP7)

Each travel time of the seven phases is collected and calculated by means of WH and PLUC(Michel and Hirn, 1980; The State Seismological Bureau of Science and Technology Monitoring Department, 1988). Theapparent velocity, average velocity, the rough crustal thickness and the average depth of two different tectonicgeological units along the survey line are worked out. Being important reference and basis, these parametersare of great significance in establishing the two-dimensional crust-mantle velocity structure model.

Based on the construction of one-dimensional crust-mantle velocity structure model, the shallow velocitydistribution above the crystalline basement of profile, the fluctuation of each layer in the crust and the velocitychanges are determined according to the surface velocity around each shot which is calculated by means of bothfinite difference inversion(Duan et al., 2002; Xu et al., 2014) and WH inversion. Taking all results from othercalculation methods into consideration and exploring the existing drilling hole data in the region, the shallowseismic sounding results, the geological data, the other geophysical detection data(Xu et al., 2013; Li et al., 2013; Xie et al., 2013; Zheng et al., 2013; Fang et al., 2013) and the features of actual seismological observation, the initial two-dimensional crustal structure model of the profile is designed by reference to the parameters ofone-dimensional crustal velocity structure model(Wang et al., 2014; Wang et al., 2007). Using the improvedSeis software package for processing DSS data, the observational data of the eight shots is traced by meansof the two-dimensional inhomogeneous media dynamic ray tracing(Cerveny et al., 1997; Cerveny and Hron, 1980; Cerveny and Psenclk, 1984; Cerveny, 1984) and the travel-time fitting(Xu et al., 2004; Xu et al., 2006, 2010, 2014; Wang and Zhang, 2004). The theoretical travel-time, the amplitude characteristics of each wavegroup and recording features of measured data achieve the best fitting after repeated forward computations, as shown in Fig. 4. Meanwhile, the crustal P-wave velocity structure is worked out(Fig. 5). It turns out thatthe velocity structure features in Songpan-Garze block are different from that in Sichuan Basin. According tothe two-dimensional Jinchuan-Lushan-Leshan deep sounding profile, the crustal structure falls into upper crust, middle crust and lower crust, which are described in following one by one.

Fig. 4 Ray tracing and travel time fitting curves of SP1 and SP7

Fig. 5 2-D Crustal profile of Jinchuan-Lushan-Leshan F1: Longquanshan fault; F2: Jiangyou-Dujiangyan fault; F3: Maowen-Tianquan fault; F4: Xiaojin arcuate fault.

The upper crust: The upper crust refers to the section above the C₂ interface which consists of threesub-layers. Above G interface is the first layer. In Sichuan Basin, the sedimentary cover of G interface is ratherthick, from 3.0 to 7.8 km. In Songpan-Garze block, the cover is thinner, ranging from 1.0 to 2.5 km. Thevelocity increases rapidly with depth. It is 4.70 km·s_?1 near the surface and up to 5.75 km·s_?1 at the bottom ofthe crystalline basement. It is observed that there exists a strong velocity gradient layer above G interface. The second sub-layer refers to the layer between G interface and C₁ interface. Lacking of C₁ interface inthe Sichuan Basin makes the upper crustal structure possess a characteristic of bi-layer structure(Clark and Royden, 2000). C₁ interface can be gradually seen to the north of tectonic transfer zone. In Songpan-Garzeblock, the upper crustal structure is a structure of three layers. The depth of C₁ interface is about 10 km. AboveC₁ interface is a weak velocity gradient layer and beneath it exists a low velocity layer. The velocity rangesfrom 5.80~ 5.90 km·s^-1 with a drop of 0.1~0.20 km·s^-1. Below the coupling area of two blocks, the isolinesof velocity are disordered, which means that the structure of this region is complex. There exist both strong and weak velocity gradient layers. The lateral variation is intensive. The third sub-layer is the one between C₁ and C₂ layers. It can be seen that from south to north, C₂ interface is deepened in the profile. The interfaceis discontinuous near the boundary area of two blocks. The differences between the velocity structures in twoareas are striking. It is clear that C₂ interface is deepened sharply to the north, the depth varies from 16.0 to19.8 km.

The middle crust: The middle curst exists in between C₃ and C₂ interface. The undulation of C₃interface is similar with that of C₂ interface. The velocity ranges from 6.15 to 6.40 km·s^-1. The velocity inSichuan Basin is 0.05~0.10 km·s^-1 faster than that in Songpan-Garze block. With the increase of depth, thevelocity shows a positive gradient which is relatively moderate. The feature of disorder in velocity structureonly can be seen in the intersection of the two blocks. The cover depth of C₃ interface in Sichuan Basin rangesfrom 26 to 28 km, while in Songpan-Garzi block it varies from 29 to 34 km. C₃ interface is discontinuous nearthe coupling area of two blocks and is deepened sharply from south to north.

The lower crust: The lower crust lies in between C₃ interface and M interface. In the Songpan-Garzeblock, between C₃ and M interface, there exists C₄ interface. The depth of C₄ varies from 42 to 48 km. Thevelocity change in this layer is not great. Below C₃ interface, the velocity is 6.40 km·s^-1 and above C₄ interfaceit is 6.50 km·s^-1. Below the structural transfer zone, there are scattered low velocity blocks. The velocity is0.05~0.10 km·s^-1 slower than that of the surrounding media. In the structural transfer zone, this interfacealso presents the characteristics of discontinuity and abrupt changes in depth. Below Songpan-Garze block, thelower crust shows the characteristic of bilayer structure.

C₄ interface gradually disappears when coming into Sichuan Basin. Below Sichuan Basin, the lower crustshows the characteristic of single layer structure. According to the profile of the acquired eight shot records, both relatively strong energy and large amplitude PmP reflection seismic phases can be found. As can be seen inthe acquired two-dimensional profile, the variation of M interface is rather intense. In Sichuan Basin, the burialdepth of this interface is around 42 km, while in the deepest point of Qinghai-Tibet Plateau it is up to 62 km.In the coupling area of Yangtze and Songpan-Garze blocks, it can be seen that M interface is deepened by 8 kmabruptly. The range of variation is greater than those of other interfaces. Within the structural transfer zone, from top down, the reflected wave is weak in energy, disordered in waveform and the interface is discontinuous, which indicates that the region is an area of stress concentration as well as a weak zone between two collidingblocks. The seismic source of Lushan M_s7.0 earthquake is located in the structural transfer zone where thechanges of high and low-velocity are intense. Researches(Wang and Gao, 2014)show that strong earthquakesusually break out in such areas. The velocity above M interface is 6.60~6.70 km·s^-1 in Songpan-Garze blockwhile in Yangtze block it is 6.70~6.80 km·s^-1. The depth of M interface gradually increases from south tonorth. Here exists a mirror-image relation between the change feature of M interface and the basement.

Generally speaking, near the transition zone between Sichuan Basin and Songpan-Garze block, the crustmantlevelocity is characterized by the disordered velocity structure and the distinct interface variation. Suchcharacteristics may have something to do with the specific contact coupling relation and the complicated seismologicaltectonic background relation.(Teng et al., 2014; Gao et al., 2004; Wu et al., 2001).

(1)From the acquired two-dimensional crust-mantle velocity structure model, it can be seen that the crustalvelocity structure is characterized by the apparent transverse division. The low-velocity anomaly locates withinthe structural transform zone while the significant differences exist in both sides of the structural transformzone. The average velocity of Yangtze block is about 0.10 km·s^-1 higher than that of Songpan-Garze block.The depth of the crust increases from south to north. Near the structural transform zone there is a sharplychanging belt in which the interface is discontinuous.

(2)Along the basement of the profile, Songpan-Garze block has a thinner basement. The basement nearXiaojin is almost outcropped. Near Yongfu county, the thickness of the basement is about 2.5 km. SichuanBasin has a thicker basement. The thickest basement is about 7.8 km. Apparent velocity structure variations of the upper crust can be seen in the region between C₂ and C₁ interfaces beneath Songpan-Garze block. Hereexists a negative velocity gradient structure in which the velocity ranges from 5.80 to 5.90 km·s^-1, 0.10~0.20km·s^-1 lower than the surrounding average velocity. The depth ranges from 13 to 21 km and the thickness isabout 8 km. With the results from double-difference relocation of aftershocks, Zhao et al.(2013)present thatthe depth of Lushan aftershock sources is between 10 and 20 km. The focal depth lies in the junction areain our velocity profile where the high velocity meets the low-velocity. In this profile, the projections of 122magnitude 3.0 aftershocks in Lushan can be seen. It is observed that the main shock and aftershocks are mainlydistributed in the structural transform zone(Fig. 5). The study shows that the moderate earthquakes usuallybreak out in between the areas with low and high-velocity(Wang and Gao, 2014). According to the abnormalthickness of negative gradient velocity structure, this layer serves as a slip surface in the process in which thesubstance in Qinghai-Tibet plateau or stress migrates to the east. As a result, the stress of surrounding blockedges and active structures is steadily accumulated.

(3)In the whole profile, the Pn wave of upper-mantle cannot be recognized and therefore, cannot becompared(Liang et al., 2004). It can only be seen in the velocity structure of the upper mantle beneathYangtze block as shown in SP7. The seismic phase of Pn wave cannot be found in other shots. The variation inthe velocity structure of the upper mantle is apparent beneath the two blocks in the profile. Beneath Yangtzeblock, the velocity ranges from 8.00 to 8.05 km·s^-1 while beneath Songpan-Garze block it is 7.90~7.95 km·s^-1.In the upper mantle of Yangtze block, the velocity is higher than that of Songpan-Garze block. The result fromthe exploration profile of Jinshan-Lushan-Leshan shows that compared with Songpan-Garze block, SichuanBasin has stronger rigidity which at least extends to the top of the upper mantle. Beneath Songpan-Garzeblock, the activity of upper mantle substance is apparently more active than that of Yangtze block.

(4)The result of two-dimensional crust-mantle velocity structure in the study area shows that the velocitycontour lines are disordered nearby the stake numbers of 150, 260, 280 km and 300 km and the undulationof the crustal interface can be apparently observed, which correspond to the locations of Longquan fault, Jiangyou-Dujiangyan fault, Maowen-Tianquan fault, and the arc-shaped Xiaojin fault. The specific depths and dip angles of the faults need further researches. Studies show that this region is one of the areas where strongearthquakes are common. Seen from the seismological tectonic background of Lushan M_s7.0 earthquake, themodern seismo-tectonics in this region is dominated by the regional near south-to-north tectonic stress field and the local east-to-west stress field. The result of the two-dimensional crust-mantle velocity structure modelshows that near the epicenter Lushan about 13.0~14.0 km, the velocity structure undergoes a great changeoverwhich provides the condition for a strong earthquake. Therefore, the breakout of Lushan M_s7.0 earthquake isprobably an abrupt release of stress in the southwest segment of the Longmenshan fault belt.

This work was supported by the National Natural Science Foundation of China(41474076). High tributeshall be paid to the field staffs of geophysical prospecting center who have overcome oceans of difficulties tocollect the seismic data. The high signal and noise ratio is viewed as the basis and guarantee of the follow-upresearches. Special thanks should go to three anonymous experts who put forward constructive comments and suggestions for this essay. Grateful acknowledgment is made to chief editor and the other editors who offeredmeaningful suggestions for me to further refine the essay. Last but not least, I would like to dedicate thisessay to the Geophysical Exploration Center of China Earthquake Administration on the occasion of the 60thanniversary of its founding.