Crustal velocity structure imaging and tectonic characterisation in Liaoning Province, China

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.

Seismic techniques provide the highest‐resolution measurements of the structure of the crust and have been conducted on a worldwide basis. We summarize the structure of the continental crust based on the results of seismic refraction profiles and infer crustal composition as a function of depth by comparing these results with high‐pressure laboratory measurements of seismic velocity for a wide range of rocks that are commonly found in the crust. The thickness and velocity structure of the crust are well correlated with tectonic province, with extended crust showing an average thickness of 30.5 km and orogens an average of 46.3 km. Shields and platforms have an average crustal thickness nearly equal to the global average. We have corrected for the nonuniform geographical distribution of seismic refraction profiles by estimating the global area of each major crustal type. The weighted average crustal thickness based on these values is 41.1 km. This value is 10% to 20% greater than previous estimates which underrepresented shields, platforms, and orogens. The average compressional wave velocity of the crust is 6.45 km/s, and the average velocity of the uppermost mantle (Pn velocity) is 8.09 km/s. We summarize the velocity structure of the crust at 5‐km depth intervals, both in the form of histograms and as an average velocity‐depth curve, and compare these determinations with new measurements of compressional wave velocities and densities of over 3000 igneous and metamorphic rock cores made to confining pressures of 1 GPa. On the basis of petrographic studies and chemical analyses, the rocks have been classified into 29 groups. Average velocities, densities, and standard deviations are presented for each group at 5‐km depth intervals to crustal depths of 50 km along three different geotherms. This allows us to develop a model for the composition of the continental crust. Velocities in the upper continental crust are matched by velocities of a large number of lithologies, including many low‐grade metamorphic rocks and relatively silicic gneisses of amphibolite facies grade. In midcrustal regions, velocity gradients appear to originate from an increase in metamorphic grade, as well as a decrease in silica content. Tonalitic gneiss, granitic gneiss, and amphibolite are abundant midcrustal lithologies. Anisotropy due to preferred mineral orientation is likely to be significant in upper and midcrustal regions. The bulk of the lower continental crust is chemically equivalent to gabbro, with velocities in agreement with laboratory measurements of mafic granulite. Garnet becomes increasingly abundant with depth, and mafic garnet granulite is the dominant rock type immediately above the Mohorovicic discontinuity. Average compressional wave velocities of common crustal rock types show excellent correlations with density. The mean crustal density calculated from our model is 2830 kg/m3, and the average SiO2 content is 61.8%.

A multi-frequency receiver function inversion approach for crustal velocity structure

The fault belts in Sanjiang mainly include Jinshajiang-Honghe fault, Lancangjiang fault and Nujiang fault (called Sanjiang faults) in western Yunnan Province, China. By interpreting the wide-angle seismic reflection/refraction profile between Zhefang and Binchuan, which crosses Tengchong and Baoshan blocks in Dianxi (western Yunnan) tectonic zone, we reconstruct the crustal structure with seismic traveltime tomography for crustal P-wave velocity and the seismic scattering image for crustal seismic reflection structure. In this paper, we firstly present the crustal structure images of P-wave velocity and seismic reflection under the wide-angle seismic profile. These results demonstrate that, the crustal velocity structure and seismic reflection structure along the profile can be divided into 3 segments, and there is an obvious difference of crustal structure among the eastern, the western and the middle segment. Generally, crustal P-wave velocities in the Baoshan segment are 0.1―0.2 km/s slower and seismic reflection amplitudes from Moho discontinuity are stronger than the other 2 segments. In the studied area, crustal thickness is about 40 km, and shows the thickening tendency from west to east along the profile. Additionally, it can be seen that there is one strong-amplitude seismic reflection event as bright points at the depths of 8―10 km, along the segment of 80―115 km of the profile (southward of Tengchong); and seismic reflection wave-field from Moho discontinuity varies obviously along the lateral direction. Finally, we make some discussions on the crustal thickening pattern in the Sanjiang fault belt, structural environment of earthquake development and the contact rela-tionship between the Tengchong block, Banshan block and Luxi trough.

This thesis presents a study of the lateral variations of the crustal structure in Southern California from the waveform data recorded by the Southern California Seismic Network (SCSN) stations, the LARSE I and II surveys. The Receiver Function method is used to process the teleseismic waveform data to map the lateral variations of the crustal structure. A first arrival analysis of the LARSE II refraction data is used to determine the upper crustal velocity structure (less than 4 km). A 2-D upper crustal structure with seven steeply dipping faults was constructed from the LARSE II seismic refraction survey. The regional upper plate complexes of the granitic and gneissic crystalline rocks are observed to yield consistently anomalous low velocities of 3.7-4.3 km/s as compared with the expected velocities for the constituent rock types, which is inferred to result from the extreme shearing, brittle fracturing and related retrogressive hydration reactions. The faults are identified from the distinctive features in the first arrivals and they correlate very well with the geologically mapped faults. Lateral variations of the crustal structure in Southern California are imaged from the back-azimuthal-grouped receiver functions (RFs) of the SCSN stations and the LARSE I, II passive stations. Large variations in the crustal structure are commonly observed beneath the San Gabriel Mountains (SGM), the western Peninsula Ranges, and the eastern Mojave Desert, such as the transition from 2-layered crustal structure to 3-layered from the eastern to the western SGM and the large offsets on the Moho among the different RF station groups. Deep Moho of 34-39 km is observed beneath the western Peninsula Ranges (WPR), Sierra Nevada and the San Bernardino Mountains and no regional root is observed beneath the San Gabriel Mountains. Synthetic waveform modeling of the anomalous features in the RFs for two stations in the eastern San Gabriel Mountains indicates the existence of a flat-topped notch structure on the Moho. Moho is inferred to get shallowed from 37-39 km north of the San Andreas Fault, 33-35 km south of the San Gabriel Fault to a depth of ~29 km beneath the Mt. Baldy block.

The crustal velocity structure in the South Yellow Sea (SYS) Basin is crucial for understanding the basin’s geological structure and evolution. OBS (ocean-bottom station) data from the OBS2013 line have been used to determine the crustal velocity structure in the SYS. The velocity model of the upper crust in the northern SYS was determined using first-arrival traveltime tomography. The model showed a higher resolution shallow crustal velocity structure but a lower resolution middle-lower crustal velocity structure. The crustal velocity structure, together with the Moho discontinuity in the SYS Basin, was also constructed using a human–computer interactive traveltime simulation, and the result was highly dependent on the prior knowledge of the operator. In this study, we reconstructed a crustal velocity model in the SYS Basin using a joint tomographic inversion of the traveltime and its gradient data of the reflected and refracted waves picked from the OBS data. The resolution of the inverted velocity structure from shallow-to-deep crust was improved. The results revealed that the massive high-velocity body below the Haiyang Sag of the Jiaolai Basin extends to the Qianliyan Uplift in the SYS; the low-velocity Cretaceous strata directly cover the pre-Sinitic metamorphic rock basement of the Sulu orogenic belt; and the thick Meso-Paleozoic marine strata are retained beneath the Meso–Cenozoic continental strata in the northern depression. The Moho depth in the SYS Basin ranges from 28 to 32 km.

SUMMARY We present a new approach to extract deep crustal velocity structure from short-offset seismic refraction sections acquired over sedimentary basins. A coincident deep seismic near-vertical (NV) reflection stack section is used to constrain the derived crustal structure. The high-amplitude free-surface multiples, often found on refraction sections due to high velocity gradients in shallow sedimentary layers, are routinely modelled for velocity and Q structure of the sedimentary strata. These multiples almost completely mask most of the arrivals, including reflected phases from crustal interfaces. By application of velocity filtering with a rejection band that includes the apparent velocity of the free-surface multiples, they can, however, be significantly attenuated. The relatively weak signals, notably the deep crustal reflections in the subcritical (SC) range, can thus be well developed. This approach is demonstrated here by application to a short-offset refraction section in two steps: initially, the free-surface multiples are modelled for obtaining the sedimentary basin velocity structure, later they are substantially attenuated by velocity filtering to enhance the weak SC reflections, further modelled for the velocity structure of the deep crust underlying the west Bengal sedimentary basin, India. The stack section obtained by processing the deep seismic NV reflection data set, coincident with the short-offset refraction section, is consistent with and well substantiates the derived model of the crustal velocity structure in the region.

SUMMARY The crustal structure beneath a seismic station has a large influence on the P-wave coda recorded by that station. In this study we employ the vertical and radial component of the crustal receiver response to determine the most important features of the crustal velocity structure beneath stations of the NARS array. The receiver response is estimated from the P-wave coda of teleseismic events by deconvolution with a source wavelet and by stacking responses of different events. The crustal velocity structure at the station is derived from these data by non-linear waveform inversion. The responses of some of the NARS stations show anomalous features such as an 'apparent delay' of the first arrival on the radial component relative to the onset on the vertical component. This appears to be a combined effect of very low velocities in the top layer of the model and a strong velocity discontinuity in the uppermost part of the crust. A high amplitude coda on the radial component is observed for stations on a structure with strong S-velocity gradients in the upper crust. The receiver responses of the NARS stations are generally well modelled by the synthetics of the final models of the inversions. The method provides an adequate procedure to estimate the dominant effects of the crustal structure at the site with the models representing the most significant velocity gradients of the crustal structure.

The Jiaonan uplift and its adjacent areas (JUAA) are the result of collision and amalgamation of the North China plate and Yangtze plate. In this area, the tectonic environment is complex and crustal deformation is strong. However, detailed and in-depth study of the upper crustal structure and medium properties in the JUAA has not previously been performed. The high-resolution three-dimensional crustal velocity structure of the JUAA is helpful to analyze the characteristics of the crustal structure in this area and is important for evaluating the tectonic environment and medium properties of the JUAA. We obtained the three-dimensional crustal velocity structure in the JUAA using the double-difference seismic tomography technique, and found that the Rizhao area and the sea areas to its southwest in the Jiaonan uplift are characterized by a high-velocity structure, and that most of the high-velocity anomaly area is located in the sea area. The crustal velocity values of the southern end of the Yishu fault zone are also high. The high crustal velocity anomaly areas in the JUAA are considered to be caused by the upwelling of mantle material. The velocity structure of the upper crust beneath the depression structures shows notable low-velocity anomalies, which are closely related to loose sediments in the depression structures. The existence of abnormally high-velocity and low-velocity structures in the Jiaonan uplift indicates that there are substantial differences in the properties of the crustal media in the Jiaonan uplift. Crust-mantle interaction in the Jiaonan uplift is mainly concentrated in the Rizhao area and sea areas to its southwest.

Lateral variation in crustal structure along the Lesser Antilles arc from petrology of crustal xenoliths and seismic receiver functions

The Gulf of Corinth (GOC), Greece is a continental rift with high rates of seismicity and extensional strain. How this strain is accommodated in the crust and whether there are variations in the mechanism along strike remain open questions, in part because of a lack of wide-angle reflection/refraction studies that constrain crustal velocity structure. In 2001, an extensive multichannel seismic survey was conducted within the GOC, one component of which included the wide-angle recording of sources from within the gulf at stations on land surrounding the gulf. In this paper we use wide-angle data in two separate, but allied, studies to constrain crustal velocities and depth to the Moho. A 2-D inversion of refraction and reflection traveltimes along an axial profile through the GOC constrains the shallow crustal velocity structure, images the Moho at 29 km depth in the east, dipping to 39 km in the west, and images the eastward subducting African slab beneath the western GOC at a depth of 74 km. The 1-D average of the 2-D velocity model was used in a tomographic inversion of PmP reflection times to solve for depth to the Moho throughout the Corinth region. This model shows generally thick, isostatically compensated crust (≥37 km) beneath the Hellenide Mountains, except immediately south of the GOC, and a singular region of thin crust (<30 km) beneath the Perahora Peninsula at the eastern end of the gulf. A comparison with Moho depths derived from gravity inversion shows a general agreement with crust thickening from east to west, but a number of differences in detail. The 3-D crustal thickness variations are more complex than those predicted by either pure shear or simple shear models of continental extension and suggest significant pre-rift structural variability.

Distinct variations of crustal shear wave velocity structure and radial anisotropy beneath the North China Craton and tectonic implications

The Yishu segment of the Tanlu fault zone is the seismogenic structure of the 1668 Tancheng M8½ earthquake, and is also the research focus of the potential strong earthquake location in the future. Geological survey shows different activity degrees along the Shandong‐Jiangsu‐Anhui segment of the Tanlu fault zone, but few researches focus on the crustal velocity structure beneath this area, and the relationship of seismicity and the difference of crustal velocity structure. The 3D velocity structure beneath this segment of the Tanlu fault zone and adjacent areas (30°N–37°N, 113°E–122°E) was imaged by seismic tomography. Crustal velocity structure segmentation beneath the researched area, the velocity characters and its relationship with the geological structure segmentation and seismic levels are studied in this paper.We select the earthquakes occurred from 1980 to 2011 in the research area, relocate these events through a relative relocation technique. Based on the traveltime data of Pg, Sg, Pm, Sm, Pn, and Sn waves of these earthquakes, using GABWIT (Genetic Algorithms in Body Wave Inversion of Traveltime) inversion method, the 3D velocity structure (inversion grid: 30 km×30 km) beneath the research area is imaged by seismic tomography.The analysis of the velocity image of different depths for the research area suggests that the crustal velocity structure is segmented. For shallow layers, different velocity segments (north of 35.3°N, 34.5°N–35.3°N, and 33°N–34.5°N) are related to exposed strata, respectively corresponding to three rupture elements of the Tanlu fault zone. They are Anqiu segment, Juxian county‐Tancheng segment, and Xinyi‐Sihong segment, which have different earthquake activity patterns. It indicates that Xinyi‐Sihong segment is a locked segment of Tanlu fault. The crustal velocity structure beneath the Shandong‐Jiangsu‐Anhui segment of the Tanlu fault zone and adjacent areas can be roughly divided into three different segments from top to bottom. They include the south segment (south of 32.5°N–33°N), middle segment (from 32.5°N–33°N to 35°N–35.3°N) and north segment (north of 35°N–35.3°N). The segmentation of upper crust is related to insertion from Sulu UHPM belt, the segmentation of middle and lower crust is related to detention of volcanics. The velocities in the west of the Tanlu fault zone is higher than that in the east. The differences of velocity image at different depths show different tectonic blocks, which have different evolution histories and compositions, also meaning that this fault extends down to the Moho.Different velocity segments at different depths beneath the Shandong‐Jiangsu‐Anhui segment of the Tanlu fault zone are related to geology factors such as exposed strata, rupture units of the fault zone, Sulu UHPM belt, detention of volcanics or different tectonic blocks. Through analysis of the seismic activity of the different rupture units of the Tanlu fault zone in the research area, the consistency of velocity segments and rupture units indicate that Xinyi‐Sihong segment is a locked segment of the Tanlu fault zone, where strong earthquake may occure in the future.

A joint inversion technique of waveforms and travel times is applied to broadband seismic data from a local earthquake in order to estimate local crustal velocity structures of southern Korea. Combining the waveform and travel time inversion techniques, we can surmount the demerits of the techniques: the distortion of deep velocity structure in the waveform inversion caused by low signal-to-noise ratio (SNR), the velocity-depth trade-off, and errors in picking phases in the travel time inversion. The purpose of this study is not only for verifying whether the technique is performed well in the local areas where the number of stations is limited, but also for estimating the velocity structures of the areas that have been little investigated. We adopted the genetic algorithm (GA) as a search algorithm, since we could not expect appropriate initial models due to little a priori information about crustal velocity structure. Both broadband waveforms bandpassed between 0.05 Hz and 0.3 Hz and travel times of Pg, Pn, and PmP waves from the 26 April 2004 Daegu earthquake (ML=3.9) were used as input data. We performed the joint inversion ten times or more for each local area, and adopted the averaged model of optimal models to acquire credible crustal structure. Synthetic waveforms and travel time curves obtained from the estimated velocity models were generally agreed with observed seismograms, and the estimated source depths from the velocity models of the three local areas are similar to and consistent with each other. Therefore, we believe that the joint inversion technique is still applicable to local areas where the number of stations is limited.

Crustal velocity structure and seismotectonics of the Kinnaur region of northwest Himalaya: New constraints based on recent micro-earthquake data