Deep Seismic Reflection Profiles Research Articles

Evidence for normal and deep-buried features of the Longquan Shan fault zone at the eastern margin of the Tibetan plateau

The Longquan Shan fault zone (LQSF) locates in the Sichuan Basin of China, parallel to the well-known Longmen Shan fault zone. Different models for the LQSF indicate different understanding of the geological evolution of the Tibetan Plateau. In this paper, we provide evidences which demonstrate that the LQSF is a normal and deep-buried fault zone. Firstly, we find distinct extensional strainsaround the LQSF based on Global Positioning System (GPS) observations. We also find that the downward GPS velocities across the LQSF decrease suddenly from 4 to 5 mm/yr at the west to roughly zero at the east. Secondly, we collate focal mechanism solutions to earthquakes of M ≥ 3.0 in the Sichuan Basin and find that 12 out of 21 earthquakes are normal earthquakes. The surrounding extensional strains and the high proportion of normal earthquakes indicate that the shallow part of the LQSF is normal at present. Thirdly, based on the migration section obtained from deep seismic reflection profiling, we outline the geometric structure of the LQSF. It is a deep-buried fault zone that reaches the Moho, with two branches at the top. Such a fault model corresponds with the realities of the low seismic activities around the Longquan Shan anticline, and the scattered earthquakes beneath the Chengdu plain. Finally, we propose a three-stage evolutional model about the Longquan Shan fault system, which includes the formation of anticline in Late Triassic, the uplift of the Mountain in Oligence, and the stage of normal characteristics of the fault zone from Late Miocene.

Predictive models for the deep geometry of a thick-skinned thrust matched to crustal structure: Wind River Range, western USA

AbstractThe ability of models designed to use near-surface structural information to predict the deep geometry of a faulted block is tested for a thick-skinned thrust by matching the surface geometry to the crustal structure beneath the Wind River Range, Wyoming, USA. The Wind River Range is an ∼100-km-wide, thick-skinned rotated basement block bounded on one side by a high-angle reverse fault. The availability of a deep seismic-reflection profile and a detailed crustal impedance profile based on teleseismic receiver-function analysis makes this location ideal for testing techniques used to predict the deep fault geometry from shallow data. The techniques applied are the kinematic models for a circular-arc fault, oblique simple-shear fault, shear fault-bend fold, and model-independent excess area balancing. All the kinematic models imply that the deformation cannot be exclusively rigid-body rotation but rather require distributed deformation throughout some or all of the basement. Both the circular-arc model and the oblique-shear models give nearly the same best fit to the master fault geometry. The predicted lower detachment matches a potential crustal detachment zone at 31 km subsea. The thrust ramp is located close to where this zone dies out to the southwest. The circular-arc model implies that the penetrative deformation could be focused at the trailing edge of the basement block rather than being distributed uniformly throughout and thus helps to explain the line of second-order anticlines along the trailing edge of the Wind River block.Key points: (1) The circular-arc fault model and the oblique-shear model predict a lower detachment for the Wind River rotated block to be ∼31 km subsea, consistent with the crustal structure as defined by teleseismic receiver-function analysis. The thrust ramp starts where this zone dies out. (2) The kinematic models require distributed internal deformation within the basement block, probably concentrated at the trailing edge. (3) The uplift at the trailing edge of the rotated block is explained by the circular-arc kinematic model as a requirement to maintain area balance of a mostly rigid block above a horizontal detachment; the oblique-shear model can explain the uplift as caused by displacement on a dipping detachment.

Acquisition and Processing of Wider Bandwidth Seismic Data in Crystalline Crust: Progress with the Metal Earth Project

The Metal Earth project acquired 927 km of deep seismic reflection profiles from August to November of 2017. Seismic data acquired in this early stage of the Metal Earth project benefited greatly from recent advances in the petroleum sector as well as those in mineral exploration. Vibroseis acquisition with receivers having a 5 Hz response (10 dB down) generated records from a sweep signal starting at 2 Hz, sweeping up to 150 Hz or 200 Hz. Not only does this broadband signal enhance reflections from the deepest to the shallowest crust, but it also helps the use of full waveform inversion (e.g., to mitigate cycle-skipping) and related techniques. Metal Earth regional-scale transects using over 5000 active sensors target mineralizing fluid pathways throughout the crust, whereas higher spatial-resolution reflection and full-waveform surveys target structures at mine camp scales. Because Metal Earth was proposed to map and compare entire Archean ore and geologically similar non-ore systems, regional sections cover the entire crust to the Moho in the Abitibi and Wabigoon greenstone belts of the Superior craton in central Canada. Where the new sections overlap with previous Lithoprobe surveys, a clear improvement in reflector detection and definition is observed. Improvements are here attributed to the increased bandwidth of the signal, better estimates of refraction and reflection velocities used in processing, and especially the pre-stack time migration of the data.

A Fine Velocity and Strain Rate Field of Present-Day Crustal Motion of the Northeastern Tibetan Plateau Inverted Jointly by InSAR and GPS

Interferometric synthetic aperture radar (InSAR) data from 6 Envisat ASAR descending tracks; spanning the 2003–2010 period; was used to measure interseismic strain accumulation across the Northeastern Tibetan Plateau. Mean line-of-sight (LOS) ratemaps are computed by stacking atmospheric-corrected and orbital-corrected interferograms. The ratemaps from one track with different atmospheric-corrected results or two parallel; partially overlapping tracks; show a consistent pattern of left-lateral motion across the fault; which demonstrates the MERIS and ECMWF atmospheric correction works satisfactorily for small stain measurement of this region; even with a limited number of interferograms. By combining the measurements of InSAR and GPS; a fine crustal deformation velocity and strain rate field was estimated on discrete points with irregular density depending on the fault location; which revealed that the present-day slip rate on the Haiyuan fault system varies little from west to east. A change (2–3 mm/year) in line-of-sight (LOS) deformation rate across the fault is observed from the Jinqianghe segment to its eastern end. Inversion from the cross-fault InSAR profiles gave a shallow locking depth of 3–6 km on the main rupture of the 1920 earthquake. We therefore infer that the middle-lower part of the seismogenic layer on the 1920 rupture is not yet fully locked since the 1920 large earthquake. Benefit from high spatial resolution InSAR data; a low strain accumulation zone with high strain rates on its two ends was detected; which corresponds to the creeping segment; i.e., the Laohushan fault segment. Contrary to the previous knowledge of squeezing structure; an abnormal tension zone is disclosed from the direction map of principal stress; which is consistent with the recent geological study. The distribution of principal stress also showed that the expanding frontier of the northeastern plateau has crossed the Liupan Shan fault zone; even arrived at the northeast area of the Xiaoguan Shan. This result agrees with the deep seismic reflection profile.

Geological interpretation of the 2008 seismic reflection, refraction and magnetotelluric data from the northern Eyre Peninsula, Gawler Craton

A deep seismic reflection profile and coincident refraction and magnetotelluric data were acquired across the northern Eyre Peninsula, Gawler Craton in 2008 to enhance the prospectivity of this region for uranium and geothermal energy, by establishing the crustal architecture and regional geodynamic framework. The seismic line crossed several tectonic domain boundaries, including the Kalinjala Mylonite Zone, which is a key structure in the southern and central Gawler Craton. In the vicinity of the seismic section, this mylonite zone dips moderately to the east and appears to be a major crustal-scale fault which cuts through to the Moho; it separates middle to lower crust of differing seismic reflectivity. The changes in seismic character correspond with the boundary between the Olympic Fe-oxide Cu- Au-U Province (host of Olympic Dam and Prominent Hill deposits) in the east, and the Central Gawler Au Province in the west, and also are suggested to limit the extent of the South Australian Heat Flow Anomaly.

Geology, geophysics, geochemistry of a hidden Palaeoproterozoic ocean-continent transition in the northern Gawler Craton

Craton margins are known to host many major deposit styles across the globe, and constraining the spatial and temporal relation between permissive geometries and thermal drivers for alteration processes are key for identifying prospective terranes. Orthogonal deep crustal reflection seismic profiles provide insight into the three-dimensional crustal architecture of the north-western Gawler Craton, South Australia. Correlating between north-south seismic line 08GA-OM1 and east-west seismic line 13GA-EG1, has enabled the interpretation of a major crustal boundary separating the core of the Gawler Craton from re-worked crustal provinces to the west and north. We use seismic character, potential fields and magnetotellurics to locate and constrain the geometry of this major boundary, and isotopic signatures from sparse drillholes to characterize the crustal age and composition either side of the interpreted boundary.In recent years, isotopic evidence has been used to infer the presence of early Palaeoproterozoic oceanic crust having existed between the Gawler and Yilgarn Cratons. We present a new model for the north-western Gawler Craton, locating a transitional region between a cratonic core and this oceanic crust, and suggest that the craton margin was ~100 km inboard of current interpretations.

Investigation of the Yellow River buried fault in the Wuhai basin, northwestern Ordos Block, China, using deep/shallow seismic reflection and drilling techniques

Studying the faults in Wuhai Basin, which is located in the northwestern corner of the Ordos Block in China, can not only guide earthquake-safe construction in the region, but also provide a greater understanding of the structure of the Wuhai Basin and the dynamic environment of Northwestern Ordos Block. In this study, we effectively detected the Yellow River buried Fault (YRF), which we believe currently presents a risk to human life in the area. First, we laid out a deep seismic reflection profile (DSRP) in the north of the Wuhai Basin and found that the YRF is composed of two subparallel faults, the east branch of the Yellow River buried Fault (EYF) and the west branch of the Yellow River buried Fault (WYF), which formed a Y-type graben in the section. We then used 25 shallow seismic exploration profiles to find out how the faults are distributed. Finally, we used drillings and Quaternary dating methods to certify their existence and obtain their activity parameters. We used the sequence stratigraphy method to compare sediment strata and identified paleo-earthquake events by finding the unequal thickness layers (UTLs) on both sides of the fault. The drillings revealed that four earthquakes occurred on the YRF around 25.6 ± 0.11 ka BP, 39.5 ± 0.45–41.7 ± 0.57 ka BP, 58.25 ± 7.13 ka BP, and 111 ± 1.21 ka BP. Since the last activity on the YRF occurred much longer ago than its estimated recurrence cycle, we believe that the fault YRF currently presents a dangerous risk. Our study confirms that the YRF is a normal fault graben and that the Wuhai Bain is an extensional basin formed by the relative movement of the Tibetan Plateau, the Alashan Block, and the Ordos Block.

Shallow structure of the Tangshan fault zone unveiled by dense seismic array and horizontal-to-vertical spectral ratio method

The Tangshan fault zone has been considered to be a potential seismogenic fault from the devastating 1976 Ms 7.8 Tangshan earthquake. Based on the dense seismic array with interstation distance of ∼1 km and length of ∼40 km, HVSR (horizontal-to-vertical spectral ratio) curves at 37 stations are obtained with the micro-tremor seismic waveforms, and the two-dimensional Quaternary sedimentary structures cross the Tangshan fault zone are imaged by frequency-to-depth conversion. Results suggest that two significant seismic impedance interfaces at ∼100 m and 300–800 m depth are clearly revealed, respectively. The interface at ∼100 m depth can be attributed to the strong contrasts between the unconsolidated sand-clay and semi-consolidated silt, which suggests very small variations and is well consistent with the seismic reflection interface from shallow seismic reflection exploration. While the deeper interface at 300–800 m depth can be attributed to the Quaternary sedimentary basement, which increases from 300 to 800 m from the west to the east, and is also consistent with the results in previous studies. Rapid variations (∼200 m) of the Quaternary sedimentary basement depth just beneath the fault zone well agrees with spatial characteristics of the fault revealed by deep seismic reflection profiling. Our work suggests that the Tangshan fault zone has been significantly ruptured and modified by strong earthquake activities since Quaternary. This study also suggests that the HVSR method with dense seismic array is an efficient, effective, non-invasive and low-cost method for investigating active faults in shallow sediments. It is useful for probing the active faults buried by thick sediments in the densely populated urban settings where seismic investigation with explosive sources are too hazardous to carry out.

Reappraisal of the magma-rich versus magma-poor rifted margin archetypes

Abstract Rifted margins are commonly defined as magma-poor or magma-rich archetypes based on their morphology. We re-examine the prevailing model inferred from this classification that magma-rich margins have excess decompression melting at lithospheric breakup compared with steady-state seafloor spreading, while magma-poor margins have inhibited melting. We investigate the magmatic budget related to lithospheric breakup along two high-resolution long-offset deep reflection seismic profiles across the SE Indian (magma-poor) and Uruguayan (magma-rich) rifted margins. Resolving the magmatic budget is difficult and several interpretations can explain our seismic observations, implying different mechanisms to achieve lithospheric breakup and melt production for each archetype. We show that the Uruguayan and other magma-rich margins may indeed involve excess decompression melting compared with steady-state seafloor spreading but could also be explained by a gradual increase with an early onset relative to crustal breakup. A late onset of decompression melting relative to crustal breakup enables mantle exhumation characteristic of magma-poor margin archetypes (e.g. SE India). Despite different volumes of magmatism, the mechanisms suggested at lithospheric breakup are comparable between both archetypes. Considerations on the timing of decompression melting onset relative to crustal thinning may be more important than the magmatic budget to understand the evolution and variability of rifted margins.

An Andean-type retro-arc foreland system beneath northwest South China revealed by SINOPROBE profiling

In the Mesozoic, South China was situated along the convergent margin between the Asian and Pacific plates, providing an excellent laboratory to understand the interactions between deformation, sedimentation and magmatism in a retroarc environment. The crustal architecture of northwest South China is displayed along the ∼600-km-long SINOPROBE deep seismic reflection profiles and reveals from east to west: (1) highly folded and truncated reflectors in the upper crust of the Yangtze Fold Zone, which correspond to thin- and thick-skinned thrust systems, and document large-scale intraplate structural imbrication and shortening; (2) a crustal-scale flat-ramp-flat structure, termed the Main Yangtze decollement, which forms a weak, viscous layer to accommodate strain decoupling and material transport in the thin- and thick-skinned systems; and (3) nearly flat-lying reflectors in the Sichuan Basin, which support interpretation of the basin as a weakly deformed depocentre. The Yangtze Fold Zone and the Sichuan Basin represent a retro-arc foreland basin system that is >800 km away from the continental-margin magmatic arc. We suggest that tectonic processes across the arc and retro-arc systems, including arc magma flare-up, basin sedimentation, retroarc thrust propagation, lithosphere underthrusting, root foundering, and extension-related magmatism were interrelated and governed mass transfer. Age data and geological relations link the tectonic processes to evolving geodynamics of the subducting Paleo-Pacific plate.

Continentward-dipping detachment fault system and asymmetric rift structure of the Baiyun Sag, northern South China Sea

The rift architecture and deep crustal structure of the distal margin at the mid-northern margin of the South China Sea have been previously investigated by using deep seismic reflection profiles. However, one fundamental recurring problem in the debate is the extensional fault system and rift structure of the hyperextended rift basins (Baiyun Sag and Liwan Sag) within the distal margin because of the limited amount of seismic data. Based on new 3D seismic survey data and 2D seismic reflection profiles, we observe an array of fault blocks in the Baiyun Sag, which were tilted towards the ocean by extensional faulting. The extensional faults consistently dip towards the continent. Beneath the tilted fault blocks and extensional faults, a low-angle, high-amplitude and continuous reflection has been interpreted as the master detachment surface that controls the extension process. During rifting, the continentward-dipping normal faults evolved in a sequence from south to north, generating the asymmetric rift structure of the Baiyun Sag. The Baiyun Sag is separated from the oceanic domain by a series of structural highs that were uplifted by magmatic activity in response to the continental breakup at ~33 Ma and a ridge jump to the south at 26–24 Ma. Therefore, we propose that magmatism played a significant role in the continental extension and final breakup in the South China Sea.

Lithospheric heating by crustal thickening: a possible origin of the Parnaíba Basin

AbstractThe subsidence of basins formed by extension is now quite well understood. But that of cratonic basins is not, though a variety of models have been proposed. This volume is concerned with the structure and evolution of one such basin, the Parnaíba basin in NE Brazil. At the moment probably more is known about this basin than about any other cratonic basin. A long deep seismic reflection profile across the basin shows no evidence of extension, or of structural control by the underlying basement. Backstripping of two wells in the basin show that the time scales for their subsidence are 68 and 74 Ma. We show that these observations are consistent with a thermal model for the formation of the basin, where the thermal disturbance results from the emplacement and removal of a crustal layer whose thickness is 35–40 km, and not from extension. Such a layer is presently being emplaced as a gravity current on the thick lithosphere of northern India as Tibet flows southwards. Such currents and cratonic basins have similar circular shapes. This model is not incompatible with the relevant geological observations. It may be possible to test its predictions by careful study of the growth of metamorphic minerals.

The composition and structure of the deep crust of the Capricorn Orogen

ABSTRACTUnderstanding how the Australian continent came together requires an understanding of structure in all levels of the lithosphere. Deep seismic reflection profiles across several Proterozoic orogens have revealed entirely buried tectonic elements, termed seismic provinces. Although undoubtedly important, the nature of these seismic provinces is typically not well characterised. The Capricorn Orogen is one such region, where the upper crust is relatively well known from geological and geophysical studies, but much of the deep crust is buried beneath Proterozoic basins. Here we combine geophysical datasets, including active and passive source seismic data and gravity data, to image the density, seismic velocity and compositional structure of the deep crust of the Capricorn Orogen. Crustal structure interpreted from deep seismic reflection studies is re-scaled using velocity information from receiver function studies. This modified geometry is used to construct a density model that satisfies Bouguer gravity data. Finally, after correcting for temperature and pressure dependencies, the velocity and density information is used to generate a compositional model of the orogen. This model indicates a varied structure with at least four distinct blocks between the Yilgarn and Pilbara cratons, bounded by major shear zones. We suggest that this variation is linked to multiple accretion events during the amalgamation of the West Australian Craton.

Outlining tectonic inheritance and construction of the Min Shan region, eastern Tibet, using crustal geometry

The ongoing collision between India and Eurasia has created the Tibetan Plateau, which features high elevations and large crustal thicknesses. The easternmost portion of the plateau has long been a key region for studying the uplift mechanism of the Tibetan Plateau, especially after the 2008 Ms. 7.9 Wenchuan earthquake. However, previous studies have assumed that easternmost Tibet is tectonically homogeneous, and the tectonic significance of the Min Shan has been overshadowed by that of its more conspicuous neighbour, the Longmen Shan region. Here, we describe the crustal geometry of the Min Shan region using two newly obtained deep seismic reflection profiles. In this study, we identify an upper-lower crust mechanical decoupling within the Min Shan region; the Min Shan region is tectonically delineated by an inherited boundary fault zone, the Huya fault zone, which was responsible for triggering the 2017 Jiuzhaigou M 7.0 earthquake. Together with the gravity dataset and previous studies in this area, the outlined crustal geometry indicated that crustal-scale shortening at the eastern plateau margin is a primary mechanism driving uplift, although extensive uplift might have occurred due to the decoupled shortening between the upper and lower crust.

Nonuniform subduction of the Indian crust beneath the Himalayas

Himalayan tectonic activity is triggered by downward penetration of the Indian plate beneath the Asian plate. The subsurface geometry of this interaction has not been fully investigated. This study presents novel constraints on this geometry provided by two newly obtained, deep seismic reflection profiles. The profiles cover 100- and 60-km transects across the Yarlung-Zangbo suture of the Himalaya-Tibet orogen at c. 88°E. Both profiles show a crustal-scale outline of the subducting Indian crust. This outline clearly shows Indian understhrusting southern Tibet, but only to a limited degree. When combined with a third seismic reflection profile of the western Himalayas, the new profiles reveal progressive, eastward steepening and shortening in the horizontal advance of the subducting Indian crust.

Crustal structure, gravity anomalies, and subsidence history of the Parnaíba cratonic basin, Northeast Brazil

AbstractCratonic basins cover more than 10% of Earth's continental surface area, yet their origin remains enigmatic. Here we use a 1400 km long, deep (20 s two‐way travel time) seismic reflection profile, five wide‐angle split‐spread receiver gathers, gravity anomaly, and well data to constrain the origin of the Parnaíba Basin, a cratonic basin in Northeast Brazil. In the center of the basin, the depth to pre‐Paleozoic basement is ~3.3 km, a zone of midcrustal reflectivity (MCR) can be traced laterally for ~250 km at depths between 17 and 25 km and Moho depth is ~42 ± 2 km. Gravity and P wave modeling suggests that the MCR represents the upper surface of a high density (2985 kg m−3) and Vp (6.75–7.0 km s−1) lower crustal body, likely of magmatic origin. Backstripping of well data shows a concave up decreasing tectonic subsidence, similar in form to that observed in rift‐type basins. We show, however, that our seismic and gravity data are inconsistent with an extensional origin. Other basin forming mechanisms are therefore required. We show that an intrusive body in the lower crust that has loaded and flexed the surface of the crust, combined with sediment loading, provides a satisfactory fit to the observed gravity anomaly. A buried load model is consistent with seismic data, which suggest that the Moho is as deep or deeper beneath the basin center than its flanks and accounts for at least part of the tectonic subsidence through a viscoelastic stress relaxation that occurs in the lithosphere following load emplacement.

Growth of lithosphere-scale fault system in NE Tibet: Numerical modeling constrained by high-resolution seismic reflection data

The east Kunlun fault is an important strike-slip shear zone to understand continental deformation in NE Tibet. The recent high-resolution deep seismic reflection profiling across the Kunlun fault reveals a thrust fault system and two decollements in the crust, along with another thrust fault system which reaches to the depth of upper mantle and cuts off the Moho. The mechanism of the growth of these shear zones at lithosphere-scale, which reveals the type of deep deformation, is investigated using the finite-element method with an elastic-plastic constitutive relationship. The results show that the thrust fault system in the crust may be explained by the conjugated plastic deformation belts under compression from the Indian plate. The pre-existing fault in the depth may develop to cut the Moho toward two new directions rather than along the original direction. The vertical and lateral heterogeneity of material, frictional property and geometry of the models all affect the feature of the fault growth. The growth of the thrust fault system on both sides of the Kunlun fault is only located in the crust, it means that the Kunlun fault does not reach to the Moho depth.

Structural characteristics of the Yilan–Yitong and Dunhua–Mishan faults as northern extensions of the Tancheng–Lujiang Fault Zone: New deep seismic reflection results

The Tancheng–Lujiang Fault Zone (TLFZ) can be subdivided into three segments that exhibit sharp contrasts in their deep structures. A deep seismic reflection profile (length ~600km) across the north part of the TLFZ, which provides new constraints on the structural styles of the northern TLFZ, was recently completed by the Chinese Sinoprobe Project. Here, the TLFZ branches into the Yilan–Yitong Fault (YYF) to the west and the Dunhua–Mishan Fault (DMF) to the east. The YYF developed as an internal fault in the Songnen–Zhangguangcai massif, while the DMF serves as the tectonic boundary between the Nadanhada terrane and the Khanka massif. Both faults developed large-scale flower structures, with that of the YYF being negative and that of the DMF being positive with reverse faults. The Moho in the profile is at a depth of 25–39km and is offset by the faults. The north part of the TLFZ extends into the upper mantle as thin shear zones with the reflectors truncated in the middle/lower crust. This feature differs from most crustal-scale strike-slip faults that distribute over a discrete shear zone in the lower crust, such as the San Andreas Fault.

Crustal Structure, Seismic Anisotropy and Deformations of the Ediacaran/Cambrian of the Ma\u0142opolska Block in SE Poland Based on Data from Two Seismic Wide-Angle Experiments

The area of SE Poland represents a complex contact of tectonic units of different consolidation age—from the Precambrian East European Craton, through Palaeozoic West European Platform (including Małopolska Block) to Cenozoic Carpathians and Carpathian Foredeep. In order to investigate the anisotropic properties of the upper crust of the Małopolska Block and their relation to tectonic evolution of the area, two seismic datasets were used: seismic wide-angle off-line recordings from POLCRUST-01 deep seismic reflection profile and recordings from active deep seismic experiment CELEBRATION 2000. During acquisition of deep reflection seismic profile POLCRUST-01 in 2010, a 35-km-long line of 14 recorders (PA-14), oriented perpendicularly to the profile, was deployed to record the refractions from the upper crust (Pg) at wide range of azimuths. These data were used for an analysis of the azimuthal anisotropy of the MB with the modified delay-time inversion method. The results of modelling of the off-line refractions from the MB suggest ~6% HTI anisotropy of the Cambrian/Ediacaran basement, with ~130º azimuth of the fast velocity axis and mean Vp of 4.9 km/s. To compare this result with previous, independent information about anisotropy at larger depth, a subset of previously modelled data from CELEBRATION 2000 experiment, recorded in the MB area, was also analysed by inversion. The recordings of Pg phase at up to 120 km offsets were analysed using anisotropic delay-time inversion, providing information down to ~12 km depth. The CELEBRATION 2000 model shows ~9% HTI anisotropy with ~126º orientation of the fast axis. Thus, local-scale anisotropy of this part of MB confirms the large-scale anisotropy suggested by previous studies based on data from a broader area and larger depth interval. The azimuthal anisotropy (i.e. HTI symmetry of the medium) is interpreted as a result of strong compressional deformation during the accretion of terranes to the EEC margin, leading to tight (sub-vertical) folding and fracturing of intrinsically anisotropic metasediments forming the MB basement. Obtained anisotropy models are compared with data about stratal dips of the MB sequences and implications of assuming more realistic TTI model are discussed. Wide-angle recordings from off-line measurements along a reflection profile provided new information about seismic velocity and anisotropy, not available from standard near-vertical profiling, and contributed to more complete image of the upper crustal structure of Małopolska Block.

An integrated shear-wave velocity model for the Groningen gas field, The Netherlands

A regional shear-wave velocity (VS) model has been developed for the Groningen gas field in the Netherlands as the basis for seismic microzonation of an area of more than 1000 km2. The VS model, extending to a depth of almost 1 km, is an essential input to the modelling of hazard and risk due to induced earthquakes in the region. The detailed VS profiles are constructed from a novel combination of three data sets covering different, partially overlapping depth ranges. The uppermost 50 m of the VS profiles are obtained from a high-resolution geological model with representative VS values assigned to the sediments. Field measurements of VS were used to derive representative VS values for the different types of sediments. The profiles from 50 to 120 m are obtained from inversion of surface waves recorded (as noise) during deep seismic reflection profiling of the gas reservoir. The deepest part of the profiles is obtained from sonic logging and VP–VS relationships based on measurements in deep boreholes. Criteria were established for the splicing of the three portions to generate continuous models over the entire depth range for use in site response calculations, for which an elastic half-space is assumed to exist below a clear stratigraphic boundary and impedance contrast encountered at about 800 m depth. In order to facilitate fully probabilistic site response analyses, a scheme for the randomisation of the VS profiles is implemented.