Crustal P-wave Velocity Structure Research Articles

Integrating Multimodal Deep Learning with Multipoint Statistics for 3D Crustal Modeling: A Case Study of the South China Sea

Constructing an accurate three-dimensional (3D) geological model is crucial for advancing our understanding of subsurface structures and their evolution, particularly in complex regions such as the South China Sea (SCS). This study introduces a novel approach that integrates multimodal deep learning with multipoint statistics (MPS) to develop a high-resolution 3D crustal P-wave velocity structure model of the SCS. Our method addresses the limitations of traditional algorithms in capturing non-stationary geological features and effectively incorporates heterogeneous data from multiple geophysical sources, including 44 wide-angle seismic crustal structure profiles obtained by ocean bottom seismometers (OBSs), gravity anomalies, magnetic anomalies, and topographic data. The proposed model is rigorously validated against existing methods such as Kriging interpolation and MPS alone, demonstrating superior performance in reconstructing both global and local spatial features of the crustal structure. The integration of diverse datasets significantly enhances the model’s accuracy, reducing errors and improving the alignment with known geological information. The resulting 3D model provides a detailed and reliable representation of the SCS crust, offering critical insights for studies on tectonic evolution, resource exploration, and geodynamic processes. This work highlights the potential of combining deep learning with geostatistical methods for geological modeling, providing a robust framework for future applications in geosciences. The flexibility of our approach also suggests its applicability to other regions and geological attributes, paving the way for more comprehensive and data-driven investigations of Earth’s subsurface.

Three-dimensional crustal P-wave velocity structure in the Yangbi and Eryuan earthquake regions, Yunnan, China

A magnitude 5.5 earthquakes occurred in Eryuan County, Dali Bai Autonomous Prefecture, Yunnan Province, China, on March 3. And a magnitude 5.0 earthquake occurred in the same place on April 17, 2013, i.e., 45 days later. Then, on May 21, 2021, multiple earthquakes, one with magnitude 6.4 and several at 5.0 or above, occurred in Yangbi County, Dali Bai Autonomous Prefecture, Yunnan Province, China. All of these occurred in the Weixi-Qiaohou-Weishan fault zone. In this study, 1,874 seismic events in Yangbi and Eryuan counties were identified by automatic micro-seismic identification technology and the first arrivals were picked up manually. Following this, a total of 11,968 direct P-wave absolute arrivals and 73,987 high-quality P-wave relative arrivals were collected for joint inversion via the double difference tomography method. This was done to obtain the regional three-dimensional fine crustal P-wave velocity structure. The results show that the travel time residuals before and after inversion decreased from the initial –0.1–0.1 s to –0.06–0.06 s. The upper crust in the study area, which exhibited a low-velocity anomaly, corresponded to the basin region; this indicated that the low-velocity anomaly in the shallow part of the study area was affected by the basin. Results also showed some correlation between the distribution of the earthquakes and velocity structure, as there was a low-velocity body Lv1 with a wide distribution at depths ranging from 15–20 km in the Yangbi and Eryuan earthquake regions. In addition, earthquakes occurred predominantly in the high-low velocity abnormal transition zone. The low-velocity body in the middle and lower crust may be prone to concentrating upper crustal stress, thus leading to the occurrence of earthquakes.

Crustal P-wave velocity structure and earthquake distribution in the Jiaodong Peninsula, China

The Jiaodong Peninsula of eastern China is densely populated and prone to frequent moderate-strong earthquakes that are mostly distributed along a group of northeastern oriented Mesozoic-Cenozoic faults. An improved understanding of the seismic velocity structure in the area is of great significance for the roles that the faults played in the formation and evolution of the various tectonic features and for the assessment of future earthquake risks. By utilizing earthquake records of the Shandong Provincial Seismic Network for the period from January 2013 to January 2020, this study simultaneously re-locates earthquakes and determines the three-dimensional P-wave velocity structure beneath the Jiaodong Peninsula using regional P-wave travel times by applying the double-difference tomography method. After the relocation, the travel time residuals are reduced by an order of magnitude, and the epicenters are more concentrated and demonstrate a closer spatial relationship with known active faults. The resulting P-wave velocity structure at different horizon depths indicates lateral heterogeneities in the study area. Clear differences in the characteristics of velocity anomalies are observed between the Jiaobei Uplift, Jiaolai Basin and Sulu ultra-high pressure metamorphic belt. The anomalies are mostly NE oriented, which is consistent with the strike of the regional faults and may suggest structural control of the faults to the geological configurations. Earthquakes generally occurred along the edges of high or low velocity regions, and areas accommodating the intersections of the Penglai-Weihai, Penglai-Qixia and Mouping-Jimo fault zones have the greatest potential for future damaging earthquakes in the area.

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.

Upper crustal P-wave velocity structure beneath two volcanic areas in northern Iran

Detailed information about the crustal structure is essential for better understanding the occurrence and mechanisms of earthquakes and volcanoes. Here we present a study of the upper crustal P-wave velocity structure of two seismically and volcanically active areas in northern Iran using the two-dimensional Pg travel time tomography method. The imaging results suggest low velocities in the upper crust beneath the Damavand and Sahand-Sabalan volcanic areas in the central and western parts of northern Iran, respectively. The upper crustal low velocities in these two areas roughly coincide with previously imaged low Pn velocity anomalies, suggesting that the Late Cenozoic volcanic activity was probably caused by the upwelling of hot materials from the mantle. The image feature of the Pg velocity structure beneath the Sahand-Sabalan volcanic area further indicates that the hot materials stored in the upper crust beneath Sahand may be larger in size than those stored beneath Sabalan. Comparison of the Pg velocity images with the earthquake distribution in north Iran suggests that earthquakes mainly occur at moderately low velocity or low to high velocity boundary areas instead of significantly low or high velocity regions. The anisotropy results show that the Pg wave fast direction is consistent with the GPS direction at high Pg velocity areas and the fast direction is inconsistent with the GPS direction but consistent with the strike direction of faults at low velocity areas. Our new upper crustal structural images provide the basic observation for better understanding of the regional seismicity and volcanism, and link the surface geological phenomena to deep crustal and mantle processes associated with the active tectonics in northern Iran.

Tomography of the Chukou Fault Zone, Southwest Taiwan: Insights from Microearthquake Data

The vigorous collision between the Eurasian plate and Philippine Sea plate in Taiwan causes a series of imbricate fold and thrust belts to develop at the deformation front. The Chukou Fault (CKF), characterized by a thrust type fault, located in Chiayi County, southwest (SW) Taiwan, is a prominent boundary between the fold-thrust belts and the Western Coastal Plain. Most of the seismicity in SW Taiwan is associated with this fault and its neighboring fault systems. The seismotectonic structures in the CKF zone, especially in the east, are complex due to the interactions among fault systems with distinct slip motions. To gain better insights into the seismogenic characteristics in the CKF zone, we used 1661 microearthquakes recorded by a temporary dense broadband seismic network and the Central Weather Bureau Seismic Network (CWBSN) between 2003 and 2004 to investigate the physical properties of the crust in the CKF zone. A waveform cross-correlation technique was applied to 143086 pairs of waveform data to determine the relative differential travel time between the P- and S-waves. By combining both the absolute and relative differential travel time data, we were able to obtain a new 3-D crustal P-wave velocity structure and Vp/Vs ratios. This study suggests that by using both absolute and relative differential travel time data in tomographic inversion can obtain precise 3-D velocity images and also gain better correlation between seismic events and fault structures, which is crucial for understanding the seismogenic process in our study area.

Study on the velocity structure of the crust in Southwest Yunnan of the north-south seismic belt—Results from the Menghai-Gengma-Lushui deep seismic sounding profile

Southwest Yunnan, located in the southern segment of the north-south seismic belt, is one of the regions with strong tectonic movement and seismic activity in China. Study on the characteristics of tectonic setting and deep geophysical field in the region is an important issue in basic science. In 2013, we conducted a 600-km-long Menghai-Gengma-Lushui profile of deep seismic wide-angle reflection/refraction and high-resolution seismic refraction in Southwest Yunnan. In this paper, we use 6 groups of clear intracrustal P-wave phases picked from the seismic record sections of 11 shots to build a velocity structure model of basement and 2D crustal P-wave of the region by using finite difference inversion and ray travel time forward fitting technology. The results show that, from south to north, the crust gradually thickens along the profile and its basement shows a significant lateral heterogeneity. In the vicinity of the Nanting River fault, the basement structure shows the character of alternate depressions and uplifts, and the shallowest basement is about 1.0 km. In the vicinity of Tengchong and Lancang, the basement is about 5.0 km deep. The velocity of the middle and lower crust in the region generally increases with the increasing of depth. At the block boundary and beneath the fault tectonic belt, the velocity contours show apparent irregularity and the P-wave velocity changes sharply. In this region, the Moho gradually deepens from south to north with relatively large lateral undulations. The shallowest point of the Moho is located near Menghai at a depth of about 32.0 km. The deepest point of the Moho is located near Tengchong at a depth of about 40.5 km. Between Gengma and Yongde, the Moho shows significantly fast uplifting and depressing with an amplitude of about 4.0 km. Beneath the Nanting River fault, Longling-Ruili fault, Dayingjiang fault and Tengchong volcano, the basement velocity structure, 2D crustal P-wave velocity structure, distribution of average profile velocity and intracrustal interface spreading also show significant changes from the basement to the top of the Moho, indicating that the crustal velocity and medium physical properties beneath the fault tectonic belt are apparently different from the crustal materials on its both sides, which suggests that these faults should be in a certain scale and may extend to the lower crust or the top of the upper mantle. The earthquakes in the region mainly occurred at a depth of 10–20 km, and the seismic activity is related to the intracrustal medium velocity difference and fault belt distribution. The results can serve as the important data of the crust-mantle structure for the analysis of the deep tectonic setting, earthquake precise positioning, seismogenic structure modeling of the seismic activities in Southwest Yunnan, as well as the important reference for the evaluation of seismic hazard and the planning of earthquake disaster mitigation of this region.

Upper crustal structure under Jingtai–Hezuo profile in Northeastern Tibet from topography-dependent eikonal traveltime tomography

The Northeastern Tibetan plateau records Caledonian Qilian orogeny and Cenozoic reactivation by continental collision between the Indian and Asian plates. In order to provide the constraint on the Qilian orogenic mechanism and the expansion of the plateau, wide-angle seismic data was acquired along a 430 km-long profile between Jingtai and Hezuo. There is strong height variation along the profile, which is dealt by topography flattening scheme in our crustal velocity structure reconstruction. We herein present the upper crustal P-wave velocity structure model resulting from the interpretation of first arrival dataset from topography-dependent eikonal traveltime tomography. With topography flattening scheme to process real topography along the profile, the evenness of ray coverage times of the image area (upper crust) is improved, which provides upper crustal velocity model comparable to the classic traveltime tomography (with model expansion scheme to process irregular surface). The upper crustal velocity model shows zoning character which matches with the tectonic division of the Qaidam-Kunlun-West Qinling belt, the Central and Northern Qilian, and the Alax blocks along the profile. The resultant upper crustal P-wave velocity model is expected to provide important base for linkage between the mapped surface geology and deep structure or geodynamics in Northeastern Tibet.

Geophysical transect across the North China Craton: A perspective on the interaction between Tibetan eastward escape and Pacific westward flow

The North China Craton (NCC) is composed of the Eastern and Western Blocks amalgamated along the Central Orogenic Belt (Trans-North China Orogen, TNCO). Geological, geochemical, geochronological and geophysical studies have confirmed that the lithosphere beneath the Eastern Block has undergone significant destruction and modification during the Mesozoic, whereas the Western Block preserves the Archean lithosphere. The mechanisms and crustal response to the lithospheric disruption remain unclear. Here we construct a lithosphere-scale transect across the Western and Eastern Blocks and the intervening TNCO at the southern margin of the NCC. We interpret the lithosphere thickness, crustal P-wave velocity, density and thermal structure, and also seismic activity. Our results show that: (1) crustal thinning is consistent with lithospheric thinning from west to east of this transect; (2) the seismogenic structure is distinctly different among the Western Block, the TNCO and the Eastern Block; (3) a marked increase in crustal density and average P-wave velocity exists at the contacts between the Western Block and TNCO, and the TNCO and Eastern Block, as well as beneath the Tanlu fault at the eastern margin of the Eastern Block; and (4) a positive correlation is displayed between increasing P-wave velocity (density, temperature) and the occurrence of Mesozoic basalts and high-Mg andesites. Under the tectonic background of the amalgamation pattern between the Eastern and Western Blocks within the NCC, we attribute the lateral variation of crustal geophysical features to the lateral variation of crustal composition or the interaction of mantle flow from the eastward escape from Tibetan collision, and the westward mantle flow beneath the eastern NCC, without excluding the west–east variation of crustal response to lithospheric disruption.

Crustal structure across northeastern Tibet from wide-angle seismic profiling: Constraints on the Caledonian Qilian orogeny and its reactivation

The northeastern Tibetan plateau results from the superposition of the Caledonian Qilian orogenic belt and Cenozoic reactivation following continental collision between the Indian and Asian plates. In order to identify the constraints on the Qilian orogenic mechanism and the expansion of the plateau, we carried out a 430-km-long wide-angle seismic experiment between Jingtai and Hezuo. We herein present the crustal P-wave velocity structure model resulting from the interpretation of this wide-angle seismic dataset. The principal characteristics of the crustal velocity model include: (1) thinning of the crust from south to north within a range of about 48–54 km with some undulation of the Moho beneath the Northern Qilian block; (2) the thickness of the sedimentary layer and its average P-wave velocity exhibit the obvious tectonic division of the Qaidam–Kunlun–West Qinling belt, the central and Northern Qilian, and the Alax blocks; (3) the lower crustal layer has a P-wave velocity of 6.6–6.8 km/s beneath the Qaidam–Kunlun–West Qinling belt, Northern Qilian orogenic belt and Alax block, but has a very low value of 6.4–6.5 km/s beneath the Central Qilian block; and (4) the Central Qilian block shares a similar crustal P-wave velocity–depth relationship with the Sierra Nevada arc or the accretionary crust. The Qaidam–Kunlun–West Qinling and the Northern Qilian orogenic belts have a crustal P-wave velocity–depth relationship characteristic of the crust of the global average continent. The particular Vp–depth relationship beneath the Central Qilian orogenic belt may result from a Caledonian accretionary orogeny or from the delamination of the ecologized lower crust during the arc–continental collisional orogeny. The lower velocity layer (LVL) exists at the bottom of the upper crust, to the north of the middle of the central Qilian block. We infer that this LVL plays an important role as an intracrustal decollement to expand the plateau.

Imaging 3-D crustal P-wave velocity structure of western Yunnan with bulletin data

Western Yunnan is a region with intensive tectonic activity and serious earthquake risk. It is of significant importance to study three dimensional crustal structure of this region to understand the tectonic setting and disaster mechanism. Densification and digitalization of seismic networks in this region provides an opportunity to study the velocity structure with bulletin data. In this study, we collect P-wave data of 10 403 regional earthquakes recorded by 79 seismic stations from January 2008 to December 2010. In addition to first arrivals data (Pg with epicentral distance less than 200 km and Pn), the Pg (or \(\bar P\)) data with epicentral distance more than 200 km are also considered as later direct arrivals in the tomographic inversion. We also compare the quantity and the quality of the seismic data before 2010 and after 2010. The test results show that adding the follow-up Pg phase can effectively improve the inversion ability of crustal imaging, and quantity and the data quality are significantly improved since 2010. The tomographic results show that: (1) The Honghe fault zone, which is the major fault systems in this region, may cut through the entire crust, and the velocity contrasts between two sides at lower crust beneath the Honghe fault are estimated at higher than 10%, while the velocity difference below Nujiang fault zone extends only in the upper crust; (2) Most of the earthquakes in the region occurred at the interface of high-velocity media and low-velocity media, i.e., the areas with high velocity gradient, which has been validated in other areas.

Crustal P-wave velocity structure of the Longmenshan region and its tectonic implications for the 2008 Wenchuan earthquake

The P-wave velocity structure of the crust in the Longmenshan region has been imaged by seismic travel time tomography using local and regional first P-wave arrivals recorded from 2000 to 2008. The tomographic model provides a way to analyze the deep tectonics of the Longmenshan fault belt and the tectonic implications for the 2008 Ms8.0 Wenchuan earthquake. The P-wave velocity images indicate that the initial rupture site and focal depth of the Wenchuan earthquake, together with the direction of rupture propagation, closely relate to the crustal structure of the Longmenshan region. The Pengguan massif to the west of the Longmenshan fault belt is characterized by high velocity anomalies, suggesting that the crust has a strong strain strength that can accumulate large stresses over a long period. The Ms8.0 Wenchuan earthquake is located at the southwestern end of the Pengguan massif and the western edge of the Sichuan Basin. The collision between the Pengguan massif and the Sichuan Basin becomes the primary reason for the occurrence of the Ms8.0 Wenchuan earthquake. To the north of Wenchuan, the occurrence and propagation of rupture benefit from low velocity anomalies along the Longmenshan fault belt; whereas to the south of Wenchuan, the brittle rupture can occur with more difficulty in relatively weak crust with low velocities. This may be one of the reasons for the absence of aftershocks to the south of Wenchuan, and the rupture induced by the Ms8.0 Wenchuan earthquake propagating from the north to the south along the Longmenshan fault belt. The deep geodynamics of the Ms8.0 Wenchuan earthquake may occur due to the discrepancy of crustal structures on the two sides of the Longmenshan fault belt. Ductile deformation and crustal flow can easily occur in the weak middle-lower crust beneath the Songpan-Garze orogenic belt. The eastward movement of the Tibetan Plateau is obstructed by the rigid lithosphere of the Sichuan Basin, and then the thickening of the middle-lower crust and vertical deformation occur in the crust of the Longmenshan fault belt. In addition, the down-warping of the Moho and the basement thrusting onto the range front induced crustal deformation and strain accumulation, which provided the potential energy to trigger the occurrence of the Ms8.0 Wenchuan earthquake.

Upper crustal P-wave velocity structure of Kii Peninsula, SW Japan by first-arrival traveltime inversion of the Kawachinagano-Kiwa refraction profile

SUMMARY We investigated the upper crustal P-wave velocity structure down to 5 km in the Kii Peninsula, SW Japan by a tomographic inversion of the first-arrival traveltimes from the seismic refraction experiment performed by the Research Group for Explosion Seismology (RGES) in 1988. The observations were carried out on a profile running in the N–S direction from Kawachinagano, Osaka Prefecture to Kiwa, Mie Prefecture. The profile extends for about 65 km across the major geological zones, which characterize the geological features of SW Japan. Six shots were fired and the generated seismic waves were recorded at 86 temporary observation sites. The inversion scheme was applied to 359 first-arrival times to delineate the velocity structure along the profile. In the tomographic scheme, velocity estimation was achieved by an iterative, linearized least-squares inversion. The Jacobian matrix was constructed via a finite-difference approximation by perturbing the slownesses of the cells instead of performing ray tracing. Traveltime calculations were carried out by using a fast finite-difference eikonal solver. Velocity updates were obtained by a matrix inversion algorithm using a conjugate gradient least-squares scheme. In addition, model covariance and model resolution matrices were obtained to assess the velocity image. Two low-velocity zones observed in the northern and southern parts of the profile are the most prominent features of the tomogram. The P-wave velocity structure is generally consistent with the surface geology, and the fault zones associated with the Median (MTL) and Gobo-Hagi (GHTL) tectonic lines across the peninsula.

Crustal structure across the Three Gorges area of the Yangtze platform, central China, from seismic refraction/wide-angle reflection data

We present active-source seismic data recorded along a 300 km-long profile across the Three Gorges area of the western Yangtze platform, central China. From west to east, the profile crosses the Zigui basin, Huangling dome and Jianghan basin. The derived crustal P-wave velocity structure changes significantly across the Tongchenghe fault that lies at the transition from the Huangling dome to the Jianghan basin. West of the Tongchenghe fault, beneath the Zigui basin and the Huangling dome, we observe a ~ 42 km thick crust of relatively low average velocity (6.3–6.4 km/s). In contrast, east of the Tongchenghe fault, beneath the Jianghan basin, the crust is only 30 km thick and has a high average velocity (6.6–6.7 km/s). A west–east variation in crustal composition along the Tongchenghe fault is also inferred. West of the fault, P-wave velocities suggest a felsic composition with an intermediate layer at the base of the crust, whilst, east of the fault, felsic, intermediate, and mafic crustal layers are apparent. Our results suggest that the crust beneath the Jianghan basin has been thinned by rifting, accompanied by intrusion of the lower crust by mafic dikes and sills. The west-to-east division of the crust in the Three Gorges area coincides with first-order geophysical contrasts in gravity, topography, crustal and lithospheric thickness.

3-D P-wave velocity structure of the crust beneath the Loess Plateau and surrounding regions of China

The crustal P-wave velocity structure beneath the Loess Plateau and surrounding regions, which is a transition zone between the Tibetan Plateau and the Huabei and Huanan Blocks of China, was tomographically imaged for the first time down to the depth of 80 km. The seismic sources comprised both local and regional earthquakes and the reconstruction was accomplished using a newly developed simultaneous inversion procedure, which solves for both the earthquake hypocentres and the 3-D velocity field. Special features of the procedure include a modified shortest path algorithm for the bent ray tracing, an analytic Jacobian matrix for solution updating, and a damped, minimum norm, constrained non-linear solver based on a CG approach. The velocity structure obtained is more complex than previously thought. The lateral velocity variations are consistent with the U-shaped seismic ring structure but the vertical variations along the Fenwei seismic belt are consistent with the mechanism of the Mountain-Basin generalized system formation. The velocity images of Loess Plateau, the transition zone may help to build a geodynamic mechanism for inland China, to explain the formation of ground fissures so prevalent in the Loess Plateau.

Crustal P-wave velocity structure in Lower Yangtze region: Reinterpretation of Fuliji-Fengxian deep seismic sounding profile

The finite-difference inversion method and Raylnvr technique had been employed to interpret the wide-angle seismic reflection/refraction data of the Fuliji-Fengxian deep seismic sounding (DSS) profile in Lower Yangtze region, hence the velocity structure was acquired and conclusions were summarized as follows: (1) The velocity model along this profile can be divided into three large layers vertically (upper, middle and lower crusts) and six blocks laterally, and this velocity distribution agrees with the feature of stable platform. (2) The depth of Moho discontinuity is 30–36 km. The thickness of the upper crust is 10.5–13.0 km, where the lateral velocity varies strongly, and the velocity increases to 6.2 km/s−1 at bottom. Besides, the velocity distributions in the bottom layer of middle crust and lower crust have an apparent inhomogeneity. The velocity in upper layer of middle crust, lower layer of middle crust, lower crust and uppermost mantle is 5.9–6.2, 6.3–6.4, 6.6–7.0 and 8.06–8.29 km/s−1, respectively. (3) On two sides of the Tanlu fault belt (TFB), the mid-crustal velocity structure is quite different, nevertheless no apparent discrimination in velocity distribution and boundary topography exhibits in lower crust, hence it is inferred that the Jiashan segment of TFB had probably cut through whole crust in the Mesozoic, and the fault behaviour in lower crust had disappeared due to the low viscosity produced by the orogenic extension or crustal balance, while the fault features in the rigid middle-upper crust have been preserved up to the present. (4) The moderate earthquakes with Ms > 5.0 nearby Zhenjiang are related to the deep faults extending into the lower crust, and the earthquakes were probably induced by the energy been transferred from mantle lithosphere to upper-mid crust along the deep faults, and aggregated at some preferable tectonic positions.

Crustal P-Wave Velocity Structure from Altyn Tagh to Longmen Mountains along the Taiwan-Altay Geoscience Transect

Based upon the seismic experiments along Geoscience Transect from the Altyn Tagh to the Longmen Mountains, the crustal P-wave velocity structure was derived to outline the characteristics of the crust in the eastern margin of Tibetan Plateau. The section shows a few significant features. The crustal thickness varies dramatically, and is consistent with the tectonic settings. The Moho boundary abruptly drops to 73km depth beneath the southern Altyn Tagh from 50km in the Tarim basin, then rises again to about 58km depth beneath the Qaidam basin. The Moho drops again to about 70km underneath the Songpan-Garzê Terrane, then rises stepwise to 60km near Longmen Mountains. To further southeast, the crust thins to 52km beneath the Sichuan basin in the southeast of Longmen Mountains. In the north of the Kunlun Fault, a low-velocity zone,which may be a layer of melted rocks due to high temperature and pressure in the depth,exists in the the bottom of the middle crust.The two depressions of the Moho correlate with the Qilian and Songpan-Garzê Terranes, implying that these two mountains have deep roots. According to our results, it is deduced that the thick crust of the northeastern Tibetan Plateau probably is a result of east-west and northwest-southeast crustal shortening since the Mesozoic during the collision of the Asian and Indian plates.

Seismic Velocity Structure and Composition of the Continental Crust of Eastern China

Abstract On the basis of a one‐by‐one latitude‐longitude grid three‐dimensional seismic velocity model, the crustal P‐wave velocity structure in eastern China (105–125°E and 18–41°N) is obtained, and a set of geotherms for each grid is established for P‐T correction on P‐wave velocities. The average depths of sub‐crustal layers and their average P‐wave velocities of 18 tectonic units in eastern China are exhibited. Our result presents a 32–34 km thick crust beneath eastern China, which is thinner than previous studies, with an average velocity of 6.54 km/s, corresponding to a 5 kg/m3 variation in crustal mean density. The thicker upper but thinner middle and lower crust results in a lower average seismic velocity of eastern China. An intermediate crustal composition with a SiO2 content of 59.7 wt% has been estimated. However, there exists a significant lateral variation in the crustal structures among the tectonic units of eastern China. The structure and composition features of some regions in eastern China indicate that extension has played an important role in the continental crust evolution of eastern China.

Inversion of 3-D crustal P-wave velocity structure in Ningxia and its neighborhood by using direct, reflected and refracted waves

In this paper, simultaneous inversions for hypocentres and three-dimensional (3-D) crustral P-wave velocity structure of Ningxia and its neighborhood are performed, by using the data of direct P wave, reflected Pm and refracted Pn phases from the Moho discontinuity. The number of arrival-time applied to the inversion is 11 816 total, which are recorded by the Seismic Station Network of Ningxia from 1 107 local natural earthquakes happened since 1970. The LSQR algorithm with damping that has been developed in recent years to solve the large-scale sparse matrix is adopted. At different steps of iterative computing process, weighting is made on the actions of each earthquake at first and on each ray further, to aim at the inhomogeneous features of spatial distribution of earthquake and ray. Numerical analogue and computational results of actual multi-group data, and different iterative control processes, show that under a certain residual standard, there is obvious trade-off on the assignment of travel-time residuals between medium parameters and earthquakes parameters (especially the origin time and depth of earthquake), so the obtained solution is related to the selected initial values of medium parameters to some extent; but there are the basically same variation features on different results in the almost same area. That is the solution of surface layer has a close relation to the strata and landform. In Yinchuan basin there is obvious low velocity feature, and it coincides with the results of a man-made earthquake section crossing the region; tomographic patterns show that there are many low velocity regions in middle-lower crust, and it seems to have some connections between the positions of historical strong earthquakes and the low velocity region or the anomalous variations of velocity gradient.