Seismic Sounding Profiles Research Articles

Deep structural insights into the origin of the Heyuan ms 4.5 revealed by deep seismic sounding profiles in southeast China

This study presents an interpretation of a deep seismic sounding (DSS) profile that carried out along the Cathaysia Block in southeast china, aiming to explore the crustal velocity structure. Data used in the survey were obtained from three controlled-source explosions conducted along the 320 km long Lianping-Heyuan-Shanwei profile. The modeling was based on ray tracing, using the extrapolation of seismic wave arrival times with the help of travel times predicted from a one-dimensional velocity model. The average velocity structure of the middle crust is 6.0–6.4 km/s, while a low velocity anomaly of approximately 0.1–0.2 km/s in the vicinity of the Heyuan-Shaowu fault zone. The resulting 2D velocity model indicates that steeply dipping low-velocity zones that correlate with the projection of two major fault zones. These zones, together with a flat LVZ at a depth of 12 km, define a triangular region that correlates with numerous hypocenters. This tectonic setting is favorable for the accumulation and release of strain in high-velocity media within the triangular region. The unique triangular structure in the upper crust provides necessary shallow medium conditions for seismic activity. This indicates that increased seismicity within this area is partially attributed to heightened stress within higher-velocity material. The triangular annular low-velocity body, situated in the upper crust, is influenced by dynamic environmental factors caused by deep thermal disturbances. The deep-seated fault serves as a conduit for the historical migration of thermal material, likely contributing to the seismogenic conditions for earthquakes in Heyuan’s region through deep-seated thermal disturbances. These findings provide a novel geophysical reference model for the regional seismicity near the Xinfengjang reservoir and significantly contribute to understanding the causal relationship between tectonic setting and seismicity. In comparison with previous studies, our research is dedicated to investigating the causes of shallow earthquakes in the region and exploring the relationship between deep and shallow structures.

Anomalous geoobjects in the Lithosphere of Uzbekistan and their interrelations with ore deposits

One of the priority tasks of the Geological Service of the Republic of Uzbekistan is the study of the paleozoic foundation under the cover of Meso-Cenozoic sediments, mainly using deep seismic sounding profiles, in order to trace known ore fields and mineralized zones in the overlying part of the basement. In recent years, in the laboratory “Structure of the Lithosphere” of the Institute of Geology and Geophysics named after Kh.M. Abdullayev, new data have been obtained as a result of reinterpretation of a complex of geophysical data. In the present work examines the role of anomalous geoobjects with high and low seismic density parameters along deep seismic sounding (DSS) profiles in the Lithosphere of Uzbekistan from the point of view of their spatial correlation with explored deposits of ore minerals. New regional features of the interrelationships between the geological structure of the lithospheric deep structures of Uzbekistan and the spatial distribution of deposits have been identified.

СТРУКТУРНО-ПЛОТНОСТНАЯ МОДЕЛЬ ЗЕМНОЙ КОРЫ ЗАПАДНОГО ШЕЛЬФА О-ВА САХАЛИН И ЕЕ ГЕОЛОГИЧЕСКАЯ ИНТЕРПРЕТАЦИЯ

Structural and density modeling of the Earth's crust carried out using a deep seismic sounding profile previously tested along the western shelf of Sakhalin Island offered a clearer view of the layered block structure of the main tectonic faults and their newtwork within the crust of the region, where a large number of strong crustal earthquakes are concentrated. The density structures of the resulting model were compared with geological data on the adjacent territory of Sakhalin Island. Our structural and density model made it possible to separate volcanic blocks and blocks of sialic crust and to trace the submerged prolongation of the largest geological complexes of the western edge of Sakhalin Island on the shelf. There are signs of spatial correlation between seismic events and some tectonic faults.

Lithosphere Density Model of the Middle Urals Segment

For the Middle and North Urals segments and adjacent territories of the East European Platform and West Siberian Plate (within the graticular trapezoid with geographic coordinates 53–65° N, 48–72° E), ten deep seismic sounding (DSS) profiles were reprocessed using two-dimensional (2D) seismic tomography method, and gradient sections of crustal velocities were constructed in the form of grid functions. In the same format, density sections were constructed. The coefficients of density–velocity empirical correlation were calculated using the solution algorithm of 2D inverse problem of gravimetry. The method and calculation technology of a three-dimensional (3D) density distribution with tying to 2D data from reference seismic sections are embedded in the procedure of quantitative interpretation of potential fields with construction of 3D geophysical models. A stable solution of the 3D inverse problem of gravimetry is sought on the correctness set of the family of horizontal layers with 2D density distribution.

Characterizing the shallow structure with the multimodal dispersion curves and the body wave refraction traveltimes from deep seismic sounding data

Deep seismic sounding (DSS) profiles are one of the most powerful tools for detecting crustal structures, and they have been deployed worldwide. Generally, the analysis of DSS data mainly focuses on body waves, while the surface waves are considered noise. We suggest that the surface waves in DSS data can be used to constrain subsurface structures. In this study, we use a DSS profile in the Piedmont and Atlantic Coastal Plain as an example to present the usage of the DSS surface wave. Multimodal dispersion curves were extracted from the DSS data with the Frequency-Bessel transform method, and were used in Monte Carlo joint inversions with body wave refraction traveltimes to constrain the shallow structures. Through the inversion, a horizontal stratum on the surface was identified in the Piedmont, and a two-layer sedimentary structure was identified in the Atlantic Coastal Plain. Comparisons with existing studies verified the accuracy of the shallow structures obtained in this study, demonstrating that the shallow velocity structure could be well constrained with the additional constraints provided by the multimodal dispersion curves. Thus, we believe that further research on the surface waves recorded in DSS surveys is warranted.

Implications on crustal structure from the South Finland Coastal (SOFIC) deep seismic sounding profile

We present results from a deep seismic sounding (DSS) experiment carried out along the southern coast of Finland in summer 2015. Data used in the survey derived from industrial blasts recorded by temporary project stations and permanent network stations. The western 220 km part of the 450 km long Southern Finland Coastal (SOFIC) profile runs along the Uusimaa belt (UB) in the 1.7–1.9 Ga Southern Finland subprovince (SFS) of the Svecofennian domain, while the 170 km part in the east crosses the 1.62–1.65 Ga Wiborg rapakivi batholith (WRB). The farthest 60 km cross a geologically diverse area consisting of supracrustal rocks and granitoids of the Saimaa area (SA), an eastern extension of the SFS. Our results show that the Moho boundary depth varies significantly, from ca. 52–54 km below UB to 40–45 km below WRB. All three crustal layers (upper, middle, and lower) have their maximum depth in the contact zone between UB and the WRB. Below WRB, a lower crust with Vp ~6.7–6.9 km/s is observed. High velocity lower crust was observed below UB (Vp ~7.2 km/s) and possibly below SA (Vp ~7.35 km/s). The modelling was based on ray tracing, using the extrapolation of seismic wave arrival times with the help of travel times predicted from a one-dimensional velocity model. The resulting two-dimensional velocity model partly relies on data from the intersecting DSS profiles and supports previous observations of the lithospheric structure of southeastern Fennoscandia.

Crustal Structure of the Chuan‐Dian Block Revealed by Deep Seismic Sounding and its Implications for the Outward Expansion of the East Tibetan Plateau

AbstractThe Chuan‐Dian Block (CDB) is located in the southeastern margin of the Tibetan Plateau, with a complex geological structure and active regional faults. The present tectonic condition with strong crustal deformation is closely related to the ongoing collision of the India and Eurasia plates since 65 Ma. The study of the crustal structure of this area is key to revealing the evolution and deep geodynamics of the lateral collision zone of the Tibetan Plateau. Deep seismic sounding is the most efficient method with which to unravel the velocity structure of the whole crust. Since the 1980s, 19 deep seismic sounding profiles have been captured within the CDB area. In this study, we systematically integrate the research results of the 19 profiles in this area, then image the 3D crustal velocity, by sampling with a 5 km spacing and 2D/3D Kriging interpolation. The results show the following. (1) The Moho depth in the study area deepens from 30 km in the south to 66 km in the north, whereas there is no apparent variation from west to east. The Pn wave velocity is higher in stable tectonic units, such as 7.95 km/s in the Lanping‐Simao block and 7.94 km/s in the western margin of the Yangtze block, than in active or mobile tectonic units, such as 7.81 km/s in the Baoshan block, 7.72 km/s in the Tengchong block and 7.82 km/s in the Zhongdian block. (2) The crustal nature of the Tengchong block, the northern Lanping‐Simao block and the Zhongdian block reflects a type of orogenic belt, having relatively strong tectonic activities, whereas the crustal nature of the central Lanping‐Simao block and the western margin of the Yangtze block represents a type of platform. The different features of the upper‐middle crust velocity, Moho depth and Pn wave velocity to both sides of the Red River fault zone and the Xianshuihe fault zone, reflect that they are clearly ultra‐crustal. (3) Based on the distribution of the low velocity zones in the crust, the crustal material of the Tibetan Plateau is flowing in a NW–SE direction to the north of 26°N and to the west of 101°E, then diverting to flowing eastwards to the east of 101°E.

Formation mechanism of the North–South Gravity Lineament in eastern China

Gravity lineaments are well recognized along plate boundaries and in the interiors of oceanic plates. However, the formation mechanism of intracontinental gravity lineaments remains poorly understood. The North–South Gravity Lineament (NSGL), which extends >4000 km from Russia to South China, is characterized by an abrupt change in the observed Bouguer gravity anomaly, ranging from −100 mGal in the west to −40 mGal in the east over a distance of <100 km. Here we employ gravity modeling along a deep seismic sounding profile that transects the NSGL to delineate the relative contributions of the thickness and composition of the crustal and upper mantle and the stagnant slab in the mantle transition zone to the observed gravity anomaly, and to ultimately constrain the formation mechanism of the NSGL. Our results indicate that crustal thickness is the dominant factor in the formation of the NSGL, with lithospheric composition also playing a role. Combining our results with those of previous studies, lateral variations in the lithospheric mantle composition and crustal/lithospheric thickness across the NSGL can be attributed to lithospheric thinning and mantle replacement in the eastern part of the North China Craton during the late Mesozoic. This was triggered by the subduction of the Paleo-Pacific plate and subsequently led to the persistence of a stagnant slab and big mantle wedge. This study highlights the fundamental need for integrated geophysical–petrological analyses to better understand the surface response to deep tectonic processes.

Seismic Density Model of the White Sea’s Crust

Study of the deep structure of the White Sea region is relevant to active geodynamics, manifestations of kimberlite magmatism, and the prospects of oil and gas searches. The aim of this work was to model the velocity and density structure of the earth’s crust in the White Sea region. Modelling was carried out using the known data of instrumental observations and the software complex “Integro”. With the help of 2D models based on deep seismic sounding (DSS) profiles and a digital map of the anomalous gravity field, the density structures of local areas of the region’s crust were refined. A 3D density model was built. Within the framework of this model, the positions of the density layers were determined. The relief of the Mohorovichich (Moho or M) discontinuity reflects the anomalies of the gravity field. Depression of the Moho boundary in the bottleneck of the White Sea indicates the vertical structure of the earth’s crust associated with manifestations of kimberlite magmatism.

Discontinuous Fault Structures in the Junction Zone of the East European Platform and the Crimean Segment of the Scythian Plate along the DOBRE-5 DSS Profile

The geological interpretation of the velocity model for the DOBRE-5 deep seismic sounding profile, which runs across Fore-Dobrogea, the northwestern shelf of the Black Sea, and the Crimean Peninsula, resulted in the recognition of several major fault zones. The first zone consists of a system of closely spaced listric normal faults that gently dip S–SE and are interpreted as the structural expression of the suture between the East European Platform and Crimean segment of the Scythian Plate. The second and third zones, which were established for the first time, are thrust-type structures—the Upper and Lower Central Crimean thrusts. The admissible variants of the attitudes of these faults were analyzed and the kinematic settings of their formation were reconstructed by tectonophysical analysis on stereographic nets. It was demonstrated that the inception and subsequent activation events of the junction zone of platforms of different age and the Central Crimean thrust fault system took place in alternating (inverse) kinematic settings of ~N–S longitudinal compression and crustal extension. The chronological order of changes of the kinematic settings and periods, in which a certain setting was predominant, is governed by crustal oscillation cyclicity, expressed in the composition and areal extent of the platform sequences of the Crimean Plain and northern Black Sea coast. The crustal oscillation cyclicity, based on a comparison with paleogeodynamic reconstructions of the Mediterranean Belt, is governed by trends in the evolution of back-arc basins of Neo-Tethys extending along the southern margin of Eurasia in front of subduction zones that existed during the initial periods of the Alpine stage at various distances from the study area.

Anatomy of a crustal-scale accretionary complex: Insights from deep seismic sounding of the onshore western Makran subduction zone, Iran

Abstract The Makran subduction zone has produced M 8+ earthquakes and subsequent tsunamis in historic times, hence indicating high risk for the coastal regions of southern Iran, Pakistan, and neighboring countries. Besides this, the Makran subduction zone is an end-member subduction zone featuring extreme properties, with one of the largest sediment inputs and the widest accretionary wedge on Earth. While surface geology and shallow structure of the offshore wedge have been relatively well studied, primary information on the deeper structure of the onshore part is largely absent. We present three crustal-scale, trench-perpendicular, deep seismic sounding profiles crossing the subaerial part of the accretionary wedge of the western Makran subduction zone in Iran. P-wave travel-time tomography based on a Monte Carlo Markov chain algorithm as well as the migration of automatic line drawings of wide-angle reflections reveal the crustal structure of the wedge and geometry of the subducting oceanic plate at high resolution. The images shed light on the accretionary processes, in particular the generation of continental crust by basal accretion, and provide vital basic information for hazard assessment and tsunami modeling.

The crustal structure of the transition from the East Black Sea Basin to the Shatsky Ridge from the reinterpretation of deep seismic sounding data on profiles 14-15-16

The paper represents the results of velocity modeling (ray-tracing modeling) performed for three deep seismic sounding (DSS) profiles 14, 15 and 16 acquired in the eastern part of the Black Sea more than 40 years ago. These profiles represent a system of radial profiles diverging from one common shot point in the East Black Sea Basin (EBSB) and crossing the Shatsky Ridge. The performed modeling showed that thin (~10 km) crystalline EBSB crust, with velocities increasing from 6,5 km/s in the basement to 7,0 km/s on the Moho (20—22 km), is overlain by sediments as thick as ~10 km. The continental crust of the Shatsky Ridge of ~30 km thickness comprises two layers — the upper crust (Vp=6,0÷6,5 km/s) as thick as 15 km and the 10 km thick lower crust (Vp =6,5÷7,0 km/s). The transition from thin EBSB suboceanic crust to the Shatsky Ridge continental crust occurs rather sharply, over an interval of ~25 km, where changes are observed in all crustal layers — from sediments to the Moho. The transition of the two types of the crust is ascribed by a lineament, parallel to the coastline of the eastern part of the Black Sea, and is associated with a collinear Alushta-Batumi magnetic anomaly of the same (NW) strike. These features may testify in favor of the tectonic nature of the transition zone, the formation and activation of which took place during the main stages of evolution of the study region — at the closure of Mesozoic Tethys ocean, during the riftogenic opening of the EBSB in Cretaceous, and during the Alpine orogeny in the compression setting. The wedge-like shape of the EBSB, expanding southeastward up to 160—180 km width, is consistent with the concept of riftogenic opening of the EBSB in the Early Cretaceous as a result of clockwise rotation of the Mid Black Sea Ridge.

Full-Scale Crustal Interpretation of Kokkola\u2013Kymi (KOKKY) Seismic Profile, Fennoscandian Shield

The Kokkola–Kymi Deep Seismic Sounding profile crosses the Fennoscandian Shield in northwest-southeast (NW–SE) direction from Bothnian belt to Wiborg rapakivi batholith through Central Finland granitoid complex (CFGC). The 490-km refraction seismic line is perpendicular to the orogenic strike in Central Finland and entirely based on data from quarry blasts and road construction sites in years 2012 and 2013. The campaign resulted in 63 usable seismic record sections. The average perpendicular distance between these and the profile was 14 km. Tomographic velocity models were computed with JIVE3D program. The velocity fields of the tomographic models were used as starting points in the ray tracing modelling. Based on collected seismic sections a layer-cake model was prepared with the ray tracing package SEIS83. Along the profile, upper crust has an average thickness of 22 km average, and P-wave velocities (Vp) of 5.9–6.2 km/s near the surface, increasing downward to 6.25–6.40 km/s. The thickness of middle crust is 14 km below CFGC, 20 km in SE and 25 km in NW, but Vp ranges from 6.6 to 6.9 km/s in all parts. Lower crust has Vp values of 7.35–7.4 km/s and lithospheric mantle 8.2–8.25 km/s. Moho depth is 54 km in NW part, 63 km in the middle and 43 km in SW, yet a 55-km long section in the middle does not reveal an obvious Moho reflection. S-wave velocities vary from 3.4 km/s near the surface to 4.85 km/s in upper mantle, consistently with P-wave velocity variations. Results confirm the previously assumed high-velocity lower crust and depression of Moho in central Finland.

Multiple phases analysis of deep seismic sounding records in North China Craton*

The North China Craton (NCC) is a key region to study the destruction of the ancient craton. Two groups of phases (denoted as “Pw1” and “Pw2”), which are parallel to the PmP phase reflected from the Moho discontinuity and the PLP phase reflected from the Lithosphere and Asthenosphere Boundary (LAB) respectively, are found on the record section of the Rongcheng-Xinzhou-Alxa long-range deep seismic sounding profile. The nature of the two phases is still unclear, although they are clearly observable and reverberant. In this paper, we use travel time inversion and amplitude forward modelling to fit the reflected and refracted phases in the lithosphere. The results show: (1) the Pw1 is a multiple reflected phase which is successively reflected by the crystalline basement, the surface, the Moho and then finally received on the surface; (2) the Pw2 phase is also a multiple reflected phase successively reflected by the crystalline basement, the surface, the LAB interface and then received on the surface. We conclude that the significant velocity difference between the thick sedimentary cover and the crystalline basement in the North China rifted basin may be the main reason for generating the multiple reflections. Furthermore, the two multiple reflections provide potent constraints on the lithospheric velocity model, and constitute seismological evidence for the lithospheric thinning in the eastern NCC.

GIGJ: A Crustal Gravity Model of the Guangdong Province for Predicting the Geoneutrino Signal at the JUNO Experiment

AbstractGravimetric methods are expected to play a decisive role in geophysical modeling of the regional crustal structure applied to geoneutrino studies. GIGJ (GOCE Inversion for Geoneutrinos at JUNO) is a 3‐D numerical model constituted by ~46 × 103 voxels of 50 × 50 × 0.1 km, built by inverting GOCE (Gravity field and steady‐state Ocean Circulation Explorer) gravimetric data over the 6° × 4° area centered at the JUNO (Jiangmen Underground Neutrino Observatory) experiment, currently under construction in the Guangdong Province (China). The a priori modeling is based on the adoption of deep seismic sounding profiles, receiver functions, teleseismic P wave velocity models, and Moho depth maps, according to their own accuracy and spatial resolution. The inversion method allowed for integrating GOCE data with the a priori information and some regularization conditions through a Bayesian approach and a stochastic optimization. GIGJ fits the highly accurate and homogeneously distributed GOCE gravity data with a ~1 mGal standard deviation of the residuals, compatible with the observation accuracy. GIGJ provides a site‐specific subdivision of the crustal layers masses, of which uncertainties include estimation errors, associated to the gravimetric solution, and systematic uncertainties, related to the adoption of a fixed sedimentary layer. A consequence of this local rearrangement of the crustal layer thicknesses is a ~21% reduction and a ~24% increase of the middle and lower crust geoneutrino signal, respectively. The geophysical uncertainties of geoneutrino signals at JUNO produced by unitary uranium and thorium abundances distributed in the upper, middle, and lower crust are reduced by 77%, 55%, and 78%, respectively. The numerical model is available at this site (http://www.fe.infn.it/radioactivity/GIGJ).

Crustal P-wave velocity structure in the northeastern margin of the Qinghai-Tibetan Plateau and insights into crustal deformation

The transitional area between the northeastern margin of the Qinghai-Tibetan Plateau, Ordos Block and Alxa Block, also being the northern segment of the North-South Seismic Belt, is characterized by considerably high seismicity level and high risk of strong earthquakes. In view of the special tectonic environment and deep tectonic setting in this area, this study used two seismic wide-angle reflection/refraction cross profiles for double constraining, so as to more reliably obtain the fine-scale velocity structure characteristics in both the shallow and deep crust of individual blocks and their boundaries in the study area, and further discuss the seismogenic environment in seismic zones with strong historical earthquakes. In this paper, the P-wave data from the two profiles are processed and interpreted, and two-dimensional crustal velocity structure models along the two profiles are constructed by travel time forward modeling. The results show that there are great differences in velocity structure, shape of intra-crustal interfaces and crustal thickness among different blocks sampled by the two seismic profiles. The crustal thickness along the Lanzhou-Huianbu-Yulin seismic sounding profile (L1) increases from ~43 km in the western margin of Ordos Block to ~56 km in the Qilian Block to the west. In the Ordos Block, the velocity contours vary gently, and the average velocity of the crust is about 6.30 km s−1; On the other hand, the velocity structures in the crust of the Qilian Block and the arc-like tectonic zone vary dramatically, and the average crustal velocities in these areas are about 0.10 km s−1 lower than that of the Ordos Block. In addition, discontinuous low-velocity bodies (LVZ1 and LVZ2) are identified in the crust of the Qilian Block and the arc-like tectonic zone, the velocity of which is 0.10–0.20 km s−1 lower than that of the surroundings. The average crustal thickness of the Ordos Block is consistently estimated to be around 43 km along both Profile L2 (Tongchuan-Huianbu-Alashan left banner seismic sounding profile) and Profile L1. In contrast to the gently varying intra-crustal interfaces and velocity contours in the Ordos Block along Profile L1, which is a typical structure characteristic of stable cratons, the crustal structure in the Ordos Block along Profile L2 exhibits rather complex variations. This indicates the presence of significant structural differences in the crust within the Ordos Block. The crustal structure of the Helan Mountain Qilian Block and the Yinchuan Basin is featured by “uplift and depression” undulations, showing the characteristics of localized compressional deformation. Moreover, there are low-velocity zones with alternative high and low velocities in the middle and lower crust beneath the Helan Mountain, where the velocity is about 0.15–0.25 km s−1 lower than that of the surrounding areas. The crustal thickness of the Alxa Block is about 49 km, and the velocity contours in the upper and middle-lower crust of the block vary significantly. The complex crustal velocity structure images along the two seismic sounding profiles L1 and L2 reveal considerable structural differences among different tectonic blocks, their coupling relationships and velocity structural features in the seismic zones where strong historical earthquakes occurred. The imaging result of this study provides fine-scale crustal structure information for further understanding the seismogenic environment and mechanism in the study area.

Crustal structure along the Zhenkang–Luxi deep seismic sounding profile in Yunnan derived from receiver functions

The crustal thicknesses and the Poisson's ratios under the seismic stations can be calculated by receiver function method with H-ҡ stacking effectively. But the stacking results are affected to some extent by the average crustal P-wave velocity. To eliminate this effect and get more accurate crustal structure along the Zhenkang–Luxi deep seismic sounding profile which lies in Yunnan Province, we calculate the receiver functions from the teleseismic events recorded by 11 temporary stations as well as 5 permanent ones along the profile and carry out the stacking with Vp obtained from the profile in this study. Our study shows that the crustal thicknesses along the Zhenkang–Luxi profile range from 34.8 km to 41.8 km with an average of 39 km. The crust is thicker in the middle part of the profile and thinner in both sides in general. Dramatic changes of crustal thickness about 3 km are detected across both the Lancangjiang fault and the Xiaojiang fault, which implies that these faults cut through the Moho. The lowest Poisson's ratio under the stations is 0.22 and the highest is 0.27 with the mean of 0.25, which is lower than the global average value 0.27 in the continental crust. It suggests that most of the crust along the profile lacks mafic component, but contains more felsic substance. The low Poisson's ratio also indicates that there is no satisfying condition for partial melting. We deduce that the material flow in the middle-lower crust in the southeastern margin of the Tibetan plateau may occur only in the north region of 24°N.

Geophysical background field and deep dynamics of the Wenchuan-Yingxiu &amp;lt;italic&amp;gt;M&amp;lt;/italic&amp;gt;&amp;lt;sub&amp;gt;S&amp;lt;/sub&amp;gt;8.0 earthquake

The Wenchuan-Yingxiu M S8.0 earthquake occurred along the Longmenshan fault system where the tectonic and seismic activities are relatively less inactive than its surroundings. After the great earthquake, a dynamic failure corridor zone was formed at the surface, which was about 300 km-long and 80 km-wide and more than 70000 aftershocks occurred. The earthquake caused serious surface damages and large numbers of casualties, but no confirmed precursor phenomena were reported. In the northeastern Tibetan Plateau, geophysicists had applied many kinds of geophysical methods, including the artificial seismic probing, the natural seismic imaging, the gravity probing, and the electric and magnetic methods. Many researchers believed that the crust or mantle shortening is the main dynamic of the M S8.0 Wenchuan-Yingxiu earthquake. However, the evidences were not enough, due to the disadvantage of the geophysical methods, such as the dubious resolution problem and the ambiguous interpretation from colorful results. Based on the previous results and our extensively study, we made a further study on the Wenchuan-Yingxiu M S8.0 earthquake which mainly concentrating on the three issues, including the highly isostatic equilibrium of the Longmenshan fault system, the fine structure and abrupt deep dynamics of the Longmenshan fault zone, and the force source system, the development and occurrence of the Wenchuan-Yingxiu M S8.0 great earthquake. First, we reviewed the gravity filed and response of the boundary in the northeastern margin of Tibetan Plateau. The free-space and Bouguer gravity anomaly were given. The distribution of the theoretical isostatic crustal thickness and the isostatic thickness difference were analyzed. Considering the topography and gravity field of eastern Tibetan Plateau, the Longmenshan zone was far from gravity isostasy. Second, the electrical structures and the S-wave velocity structures were reviewed. We reanalyzed the four artificial seismic sounding profiles. Third, we showed a diagram of the 15±5 km deep convergence and the columnar force source of the seismogenic fault. Fourth, we summarized the three flow locus of deep matter in the eastern margin of Tibetan Plateau. Meanwhile, a model about the Wenchuan-Yingxiu M S8.0 strong earthquake and deep material motion was proposed, illustrating the opposite motions of crust and upper mantle materials, and the strong collision between the crust of the Songpan-Gantze block and the Sichuan Basin. We believe that the Wenchuan-Yingxiu M S8.0 earthquake was caused by the collision and extrusion of the Indian plate and Eurasian plate which the east tectonic knot was extruding north under the Tibetan Plateau and the large force system intensified the tectonic and seismic activities in the east margin. In summary, we illustrated the processes as follows. (1) The crust and mantle material in the central Tibetan Plateau was blocked in the east which resulted a large-scale stress concentration and strong exchange of deep materials and energy. The variation of the crustal thickness between the northeastern Tibetan Plateau and the Sichuan Basin was large as 15–20 km. (2) The structures and properties of the material in the deep source mutated and leaded to a highly disequilibrium of gravity field. (3) Blocked by the hard crust of the Sichuan Basin, the low velocity crust and mantle materials beneath the western Sichuan Plateau thrusted upward along the steep fault plane of the Longmengshan fault and collided at the convergence spot of the three west-dipped fault planes. (4) The three surficial faults in the Longmenshan fault zone were not the seismogenic faults, but the convergence fault at the depth of 15±5 km. The convergence fault was a columnar focal volume, which had a radius of 5 km and a length of 300 km. The convergence of the fault planes in the deep focus induced the seismogenic fracture of the M S8.0 Wenchuan-Yingxiu earthquake.

Crustal structure revealed by a deep seismic sounding profile of Baijing-Gaoming-Jinwan in the Pearl River Delta

The Pearl River Estuary area, located in the middle part of the southern China coastal seismic belt, has long been considered a potential source of strong earthquakes above magnitude 7.0. To scientifically assess the potential strong earthquake risk in this area, a three-dimensional artificial seismic sounding experiment, consisting of a receiving array and seabed seismograph, was performed to reveal the deep crustal structure in this region. We used artificial ship-borne air-gun excitation shots as sources, and fixed and mobile stations as receivers to record seismic data from May to August 2015. This paper presents results along a line from the western side of the Pearl River Estuary to the western side of the Baijing-Gaoming-Jinwan profile. A two-dimensional velocity structure was constructed using seismic travel-time tomography. The inversion results show that the Moho depth is 27 km in the coastal area and 30 km in the northwest of the Pearl River Estuary area, indicating that the crust thins from land to sea. Two structural discontinuities and multiple low-velocity anomalies appear in the crustal section. Inside both discontinuity zones, a low-velocity layer, with a minimum velocity of 6.05 km s−1, exists at a depth of about 15 km, and another, with a minimum velocity of 6.37 km s−1, exists at a depth of about 21.5 km between the middle and lower crust. These low velocities suggest that the discontinuities may consist of partly molten material. Earthquakes with magnitudes higher than 5.0 occurred in the low-velocity layer along the profile. The deep Kaiping-Enping fault, rooted in the crust, may be one of the most important channels for deep material upwelling and is related to tectonic movement since the Cretaceous in the Pearl River Delta tectonic rift basin.

A seismic model for crustal structure in North China Craton

We present a digital crustal model in North China Craton (NCC). The construction of crustal model is based on digitization of original seismic sounding profiles, and new results of three-dimensional structure images of receiver functions. The crustal model includes seismic velocity and thickness of crustal layers. The depths to Moho indicate a thinning crust ~30 km in the east areas and a general westward deepening to more than 40 km in the west. The P wave velocity varies from 2.0 to 5.6 km/s in the sedimentary cover, from 5.8 to 6.4 km/s in the upper crust, and from 6.5 to 7.0 km/s in the lower crust. By analyzing regional trends in crustal structure and links to tectonic evolution illustrated by typical profiles, we conclude that: (1) The delimited area by the shallowing Moho in the eastern NCC represents the spatial range of the craton destruction. The present structure of the eastern NCC crust retains the tectonic information about craton destruction by extension and magmatism; (2) The tectonic activities of the craton destruction have modified the crustal structure of the convergence boundaries at the northern and southern margin of the NCC; (3) The Ordos terrene may represent a relatively stable tectonic feature in the NCC, but with the tectonic remnant of the continental collision during the assembly of the NCC in the north-east area and the response to the lateral expansion of the Tibetan Plateau during the Cenozoic in the south-west.