Possible impact origin for the Middle Miocene (Serravallian) Puffin Structure,Ashmore Platform,northwest Australia

The Puffin Structure is interpreted from high‐quality 3D seismic data as a small multiringed impact structure formed by collision of a meteorite or small asteroid with unconsolidated, water‐saturated shallow‐marine shelf carbonates during the Middle Miocene (mid to late Serravallian). The impact created a dish‐shaped structure about 2.5 km in diameter with annular rings and no central uplift.     

Fohn lamproite and a possible Late Eocene — pre‐Miocene diatreme field,Northern Bonaparte Basin,Timor Sea

Seismic profiles in the Northern Bonaparte Basin, Timor Sea, northern Australia, disclose an east‐northeast‐trending 120 × 25 km swathe of over 40 circular to subcircular features excavated in the pre‐Miocene erosional surface and buried by Lower Miocene sediments. The larger structures, typified by the Fohn Structure, include central structural highs overlying narrow vertical corridors of upward‐bulging seismic horizons (bulge‐forms). Associated with these structural highs are troughs that overlie broader vertical zones of slower seismic velocity (crater‐form features). Smaller circular features (Dc <2.0 km) include both crater‐form and bulge‐form structures. The first type shows zones of seismic velocity crater‐form structures under craters excavated below the base‐Miocene horizon. The second type exhibits vertical seismic bulge‐form features directly beneath palaeotopographic highs developed at the same horizon. Identification in drill cuttings of fragments of olivine‐leucite lamproite at Fohn 1, at depths of 690–880 m, suggests that this structure is a lamproite diatreme consisting of a massive volcanic plug ringed by pyroclastics, analogous to the classic champagne‐glass structure of alkaline volcanic diatremes. The seismic morphometry of several of the larger structures of the field is analogous to Fohn, suggesting that they too may be diatremes. The smaller bulge‐form features may represent deeply eroded diatreme feeder necks. Crater‐like structures underlain by seismic velocity crater‐forms are interpreted as relatively little‐eroded maars with low‐velocity infill, probably consisting of volcanic and country rock breccia as at Fohn 1, as well as unconsolidated clastics of the transgressive Lower Miocene Oliver Sandstone Member. Lamproite fragments at Fohn 1 are dominated by apatite‐rich assemblages consisting of saponite‐altered olivine, analcime and nontronite‐altered zoned leucite, high‐Ti K‐bearing diopside, Ti‐rich richterite, Ti‐rich phlogopite, Mg‐rich ilmenite, Cr‐bearing priderite, Ba and Sr‐rich fluorapatite and rare‐earth element‐rich apatite. Textural relations suggest early crystallisation of olivine, leucite and K‐bearing diopside followed by crystallisation of alkali amphibole, priderite, ilmenite and apatite in alkali‐Tindash;Fe–rich groundmass. Fohn 1 lamproites have very high whole‐rock P levels, high Na,Sr and Y and low K,Ba,Zr and Nb levels compared to Early Miocene West Kimberley lamproites. The mineral paragenesis suggests crystallisation of olivine‐leucite assemblages at temperatures >1100°C and pressures <0.1 GPa, followed by crystallisation of amphibole and phlogopite under water‐saturated conditions at temperatures below ～1020°C.     

Geophysical differences in the lithosphere between phanerozoic and precambrian Australia

Cannikin atomic bomb recordings indicate that there are differences in travel-times from the Aleutian Islands test site to Phanerozoic and Precambrian provinces in Australia of up to 1.1 s. Explosion seismic studies in central and southeastern Australia enable travel-time corrections for crustal and upper mantle structure to be made to recordings of such teleseismic events. Structure in the upper 60 km can account for, at most, about 0.2 s of the residual difference, but attempts to constrain the remaining residual time to the region above the Lehmann discontinuity at about 200 km depth are difficult to reconcile with explosion seismic models. Regional differences in seismic velocity structure between Phanerozoic and Precambrian Australia therefore appear to exist at depths greater than 200 km.Electrical conductivities within the mantle have been investigated using two methods. Long-period electromagnetic depth sounding using magnetometer arrays demonstrates that conductivities increase at about 200 km under Phanerozoic Australia but not until about 500 km depth under Precambrian Australia. Shorter period magnetotelluric measurements can only resolve shallower structures; these too indicate a similar trend but with sub-crustal conductivities increasing at less than 100 km under Phanerozoic Australia. Magma at these depths and shallower may be the source for Cainozoic volcanism in eastern Australia. Under Precambrian central and northern Australia magnetotelluric investigations indicate that pronounced conductivity increases do not occur until depths of 150–200 km are reached.Oceanic magnetic observations indicate that the Australian lithospheric plate as a whole is separating from Antarctica at a rate of about 7 cm/yr. The seismic and conductivity structures under the continental region of this plate indicate that lateral inhomogeneities possibly extend to depths as great as 500 km and are probably caused by the passage of eastern Australia over a hot spot. Hawaiian studies indicate that hot spots are not local features but result from large scale disturbances in the mantle. Conductivity increases commencing in the depth range 100–250 km may give an indication of uppermost zones within which the Palaeozoic lithospherc has been substantially modified resulting in elevated surface heat flow, volcanism and seismic travel-time anomalies.     

The Sudbury Structure (Ontario,Canada): a tectonically deformed multi-ring impact basin

The occurrence of shock metamorphic features substantiates an impact origin for the 1.85 Ga old Sudbury Structure, but this has not been universally accepted. Recent improvements in knowledge of large-scale impact processes, combined with new petrographic, geochemical, geophysical (LITHOPROBE) and structural data, allow the Sudbury Structure to be interpreted as a multi-ring impact structure. The structure consists of the following lithologies: Sudbury Breccia —dike breccias occurring up to 80 km from the Sudbury Igneous Complex (SIC); Footwall rocks and Footwall Breccia — brecciated, shocked crater floor materials, in part thermally metamorphosed by the overlying SIC; Sublayer and Offset Dikes, Main Mass of the SIC and Basal Member of the Onaping Formation (OF) — geochemically heterogeneous coherent impact melt complex ranging from inclusion-rich basal unit through a dominantly inclusion-free to a capping inclusion-rich impact melt rock; Grey Member of OF — melt-rich impact breccia (suevite); Green Member of OF — thin layer of fall back ejecta; Black Member of OF — reworked and redeposited breccia material; Onwatin and Chelmsford Formations — post-impact sediments. Observational and analytical data support an integrated step-by-step impact model for the genesis of these units. Analysis of the present spatial distribution of various impact-related lithologies and shock metamorphic effects result in an estimated original rim-to-rim diameter of the final crater of 200 or even 280 km for the Sudbury Structure, prior to tectonic thrusting and deformation during the Penokean orogeny.     

Proglacial glaciotectonic deformation at Melabakkar-Ásbakkar, west Iceland

A study of proglacial deformation associated with a Late Weichselian glaciomarine sequence was carried out at Melabakkar-Ásbakkar, west Iceland. At this site, coarse-grained sediments have been deformed into compressive structures with no associated push moraine morphology. Two large structures were examined, Structure A which consists of large-scale reverse (and normal) faulting and overturned bedding; and Structure B, which is more complex, with open folding, high-angle reverse faulting, nappe structures and normal faulting. The structures were interpreted as the result of increasing compressive proglacial deformation, followed by subglacial deformation, which destroyed the surface morphology of the push moraine and incorporated some of the sediments into a subglacial diamicton. The results from this study were compared with other examples of proglacial deformation, and it is suggested that at sites where deformation was restricted to the margin, longitudinal strain was lower than at sites where deformation extended out into the foreland. It is also suggested that if deformation increases downglacier, this is indicative of an overall glacial advance, whilst if the deformation decreases downglacier, this is indicative of a glacial retreat.     

Seismic interferometry and ambient noise tomography in the British Isles

Traditional methods of imaging the Earth's subsurface using seismic waves require an identifiable, impulsive source of seismic energy, for example an earthquake or explosive source. Naturally occurring, ambient seismic waves form an ever-present source of energy that is conventionally regarded as unusable since it is not impulsive. As such it is generally removed from seismic data and subsequent analysis. A new method known as seismic interferometry can be used to extract useful information about the Earth's subsurface from the ambient noise wavefield. Consequently, seismic interferometry is an important new tool for exploring areas which are otherwise seismically quiescent, such as the British Isles in which there are relatively few strong earthquakes. One of the possible applications of seismic interferometry is ambient noise tomography (ANT). ANT is a way of using interferometry to image subsurface seismic velocity variations using seismic (surface) waves extracted from the background ambient vibrations of the Earth. To date, ANT has been used successfully to image the Earth's crust and upper-mantle on regional and continental scales in many locations and has the power to resolve major geological features such as sedimentary basins and igneous and metamorphic cores. Here we provide a review of seismic interferometry and ANT, and show that the seismic interferometry method works well within the British Isles. We illustrate the usefulness of the method in seismically quiescent areas by presenting the first surface wave group velocity maps of the Scottish Highlands using only ambient seismic noise. These maps show low velocity anomalies in sedimentary basins such as the Moray Firth, and high velocity anomalies in igneous and metamorphic centres such as the Lewisian complex. They also suggest that the Moho shallows from south to north across Scotland which agrees with previous geophysical studies in the region.     

Contribution de la gravimétrie et de la sismique réflexion pour une nouvelle interprétation géodynamique des fossés d'effondrement en Tunisie : exemple du fossé de Grombalia

Troughs in Tunisia are interpreted as Plio-Quaternary structures associated to normal faults (grabens) or to flexure faults. Gravity data and seismic sections are used in this study to clarify the structure and the geodynamic evolution of an example of trough: the Grombalia trough (northeastern Tunisia), since the Upper Miocene to the Quaternary. A high residual negative gravity anomaly, which reaches ?15 mGal, is interpreted as being related to the thickening of Mio-Plio-Quaternary deposits (and probably older), as illustrated by seismic data. This subsidence has been the result of a negative flower structure related to strike-slip faults that have been reactivated with normal component during the Upper Miocene and with reverse component during the Pliocene. Seismic and gravity data demonstrate that the fault system is rooted, and more than four kilometres deep. The Grombalia example outlines the association between troughs and strike-slip faults; such a system is recognized in Tunisia, in the Ionian Sea and in the Pelagian Sea. To cite this article: M. Hadj Sassi et al., C. R. Geoscience 338 (2006).     

基于三维地质模型的地下洞室群地震动力响应分析

工程区域地质条件是影响地下洞室群稳定的重要因素,而通用数值分析软件在建立复杂地质模型时存在建模时间长、准确度低、不能真实反映复杂地质特征等困难。针对此问题,采用以非均匀有理B样条（NURBS)为主的混合数据结构,对复杂区域地质构造进行了三维建模,建立了西南某水电站大型地下洞室群的三维地质模型。并以此模型为基础,建立了包括空间曲面断层、地层等复杂地质因素的数值模型,结合ABAQUS的隐式和显式分析,对洞室开挖和地震作用下的动力时程响应进行了仿真分析。分析结果表明,该方法不仅较大程度地反映了实际地质情况,而且有利于数值仿真分析的网格剖分与计算,从而为分析地下洞室群地震反应提供了三维数值模型,为地震作用下洞室的安全性评价提供了重要的技术支持。     

复杂表层引起的生物礁解释陷阱分析

生物礁储层是良好的油气储集场所,具有高孔隙度、丰度大、产能高等特点,在油气勘探开发中占有十分重要的地位。常规的生物礁解释主要依据地震时间剖面上的外形隆起,顶底反射,上覆盖层的披覆等特征,但是在实际的生物礁解释中,还存在着很多解释陷阱。这里从常规地震资料处理误差分析的角度出发,首先提出了一种新的适用于起伏地形的波动方程地震叠后正演方法,然后通过对复杂表层条件下的生物礁模型进行地震数值模拟,重点分析了复杂表层条件可能引起的,生物礁的各种解释陷阱,如假隆起、生物礁体分布范围的变化等假象,为生物礁的识剐提供了重要的理论参考。     

三峡库区泥灰质岩石斜坡带构造对岩溶地质灾害的控制机理

三峡库区三叠系巴东组(T26)泥灰质岩石岩溶是移民迁建中发现的重大工程地质问题。泥灰质岩石中的构造非常复杂，包括老构造、新构造和表生构造，它们共同控制了岩溶作用。老构造中，褶皱和断裂带等局部构造控制着岩溶的重要部位和重要层位，节理和层理等小构造使岩溶普遍存在。新构造时期地表隆升和河流切割使岩体卸荷松动，岩溶通道扩宽。表生岩溶构造加密了岩溶通道，使岩溶作用增强。三峡库区泥灰质岩石斜坡带地质灾害形成的机理遵循着构造控制下岩溶发育的规律性，致使岩溶地质灾害具有范围广、规模大和结构复杂的特点。岩溶地质灾害的形式包括地面不均匀沉降、地裂缝、滑坡、崩塌、泥石流和地面塌陷。     

Structure and Evolution of the East Antarctic Lithosphere: Tectonic Implications for the Development and Dispersal of Gondwana

Lithospheric evolution of the Antarctic shield is one of the keystones for understanding continental growth during the Earth's evolution. Architecture of the East Antarctic craton is characterized by comparison with deep structures of the other Precambrian terrains. In this paper, we review the subsurface structure of the Lower Paleozoic metamorphic complex around the Lützow-Holm area (LHC), East Antarctica, where high-grade metamorphism occurred during the Pan-African orogenic event. LHC is considered to be one of the collision zones in the last stage of the formation of Gondwana. A geoscience program named ‘Structure and Evolution of the East Antarctic Lithosphere (SEAL)’ was carried out since 1996-1997 austral summer season as part of the Japanese Antarctic Research Expedition (JARE). Several geological and geophysical surveys were conducted including a deep seismic refraction/wide-angle reflection survey in the LHC. The main target of the SEAL seismic transect was to obtain lithospheric structure over several geological terrains from the western adjacent Achaean Napier Complex to the eastern Lower Paleozoic Yamato-Belgica Complex. The SEAL program is part of a larger deep seismic profile, LEGENDS (Lithospheric Evolution of Gondwana East iNterdisciplinary Deep Surveys) that will extend across the Pan-African belt in neighboring fragments of Gondwana.     

Gnargoo: a possible 75 km-diameter post-Early Permian – pre-Cretaceous buried impact structure,Carnarvon Basin,Western Australia

The Gnargoo structure is located on the Gascoyne Platform, Southern Carnarvon Basin, Western Australia, and is buried beneath about 500 m of Cretaceous and younger strata. The structure is interpreted as being of possible impact origin from major geophysical and morphometric signatures, characteristic of impact deformation, and its remarkable similarities with the proven Woodleigh impact structure, about 275 km to the south on the Gascoyne Platform. These similarities include: a circular Bouguer anomaly (slightly less well-defined at Gnargoo than at Woodleigh); a central structurally uplifted area comprising a buried dome with a central uplifted plug; and the lack of a significant magnetic anomaly. Gnargoo shows a weakly defined inner 10 km-diameter circular Bouguer anomaly surrounded by a broadly circular zone, ～75 km in diameter. The north?–?south Bouguer anomaly lineament of the Giralia Range (a regional topographic and structural feature) terminates abruptly against the outer circular zone which is, in turn, intersected on the eastern flank by the Wandagee Fault. A <?28 km-diameter layered sedimentary dome of Ordovician to Lower Permian strata, surrounding a cone-shaped, central uplift plug of 7?–?10 km diameter, are inferred from the seismic data. Seismic-reflection data indicate a minimum central structural uplift of 1.5 km, as compared to a model uplift of 7.3 km calculated from the outer structural diameter. An interpretation of Gnargoo in terms of a plutonic or volcanic caldera/ring origin is unlikely as these features display less regular geometry, are typically smaller and no volcanic rocks are known in the onshore Gascoyne Platform. An interpretation of Gnargoo as a salt dome is likewise unlikely because salt structures tend to have irregular geometry, and no extensive evaporite units are known in the Southern Carnarvon Basin. Morphometric estimates of the rim-to-rim diameter based on seismic data for the central dome correspond to the observed diameter deduced from gravity data, and fall within the range of morphometric parameters of known impact structures. The age of Gnargoo is constrained between the deformed Lower Permian target rocks and unconformably overlying undeformed Lower Cretaceous strata. Because of its large dimensions, if Gnargoo is an impact structure, it may have influenced an environmental catastrophe during this period.     

Failure modes of the lineaments on Jupiter's moon, Europa: Implications for the evolution of its icy crust

Lineaments referred to as ridges, troughs, bands, and faults on the icy surface of Jupiter's moon, Europa, have long been interpreted as extensional structures due to brittle fracturing of ice and intrusion of mobile materials from the interior of the satellite. Based on detailed mapping and possibly analogous structures present on Earth, we propose that the kinematics and failure mechanisms of these structures are variable and more complex than previously thought. A dense network of structures of multiple generations, forming the background on the surface of the planet, is here interpreted as localized zones of volumetric strain, likely compaction and/or dilation bands. The next class of linear failure structures is shear bands with significant offset of pre-existing markers. A few additional phases of less pervasive but more prominent volumetric deformation bands overprint the shear zones and background network. The mode of younger features can be characterized as sharp, dilational, brittle fracturing and subsequent shearing, thereby producing comminution and fragmentation in various sizes, leading to a series of younger faults with detectable lateral, as well as vertical, offset. This rich variability in the nature of the distribution, localization, kinematics, and formation mechanisms, if true, suggests that the conditions prevailing within the crust of Europa must have changed dramatically over time. The implication of this conclusion is that structures interpreted to be compaction/dilation bands and shear bands on Europa are composed of deformed materials similar to the surrounding ice, whereas only the younger faults, developed by brittle fracturing and fragmentation, may be conduits for mobile substrate to reach the surface and thus offer the highest potential for recovering evidence for life in the satellite.     

Study of subsurface structures using seismic reflection data for Kalar–Khanaqin area/Kurdistan region,Iraq

The study is carried out to detect the subsurface structures that have geological and economic importance by interpreting the available seismic reflection data of an area estimated to be about 1,752 km². The study comprises of the Kalar–Khanaqin and surrounding area, which is located at Zagros folded zone. Twenty-five seismic sections had been interpreted. The total length of all the seismic lines is about 650.4 km. Interpretation of the seismic data is focused on two reflectors, lower Fars and Jeribe formation. The lower Fars reflector picked at the two-way time ranging from 0.1 to 2.6 second, while the Jeribe reflector picked at the two-way time ranging from 1.0 to 2.7 second. The constructed maps denote to the existence of many closed and nose structures, in addition, to numerous fault types. All these features were detected in the area having the NW–SE trend. The depth of the lower Fars formation is ranging from 100.0 to 4,800.0 m, while the depth of the Jeribe formation is ranging from 1,700.0 to 5,000.0 m. The depth maps for the two formations also refer to the similarity of the major geological structures. These structures appear in both formations with existence of slight variation in dimensions. The closed structure no. (1) is located at the north of the study area. The nose structure no. (2) is located at the south of the area. At the west of the area, the elongated structure no. (3). The longitudinal reveres fault intersects the SW limb of the structure. The SW limb of elongated structure no. (4), intersect by longitudinal reveres fault, is located at the east of the area. There is also the semi-closed structure no. (5), which appears at the west of the area around the Qr-1 well. Most of detected faults are of reverse and thrust types having a variable amount of throws and horizontal displacements. Some seismic sections explained the existence of the decollement surface within lower Fars formation, which caused the thrusting and faulting of the overlaying beds.     

Seismic evidence of Caledonian deformed crust and uppermost mantle structures in the northern part of the Trans-European Suture Zone, SW Baltic Sea

Collisional structures from the closure of the Tornquist Ocean and subsequent amalgamation of Avalonia and Baltica during the Caledonian Orogeny in the northern part of the Trans-European Suture Zone (TESZ) in the SW Baltic Sea are investigated. A grid of marine reflection seismic lines was gathered in 1996 during the DEKORP-BASIN '96 campaign, shooting with an airgun array of 52 l total volume and recording with a digital streamer of up to 2.1 km length. The detailed reflection seismic analysis is mainly based on post-stack migrated sections of this survey, but one profile has also been processed by a pre-stack depth migration algorithm. The data provides well-constrained images of upper crustal reflectivity and lower crustal/uppermost mantle reflections. In the area of the Caledonian suture, a reflection pattern is observed with opposing dips in the upper crust and the uppermost mantle. Detailed analysis of dipping reflections in the upper crust provides evidence for two different sets of reflections, which are separated by the O-horizon, the main decollement of the Caledonian deformation complex. S-dipping reflections beneath the sub-Permian discontinuity and above the O-horizon are interpreted as Caledonian thrust structures. Beneath the O-horizon, SW-dipping reflections in the upper crust are interpreted as ductile shear zones and crustal deformation features that evolved during the Sveconorwegian Orogeny. The Caledonian deformation complex is subdivided into (1) S-dipping foreland thrusts in the north, (2) the S-dipping suture itself that shows increased reflectivity, and (3) apparently NE-dipping downfaulted sedimentary horizons south of the Avalonia–Baltica suture, which may have been reactivated during Mesozoic normal faulting. The reflection Moho at 28–35 km depth appears to truncate a N-dipping mantle structure, which may represent remnant structures from Tornquist Ocean closure or late-collisional compressional shear planes in the upper mantle. A contour map of these mantle reflections indicates a consistent northward dip, which is steepest where there is strong bending of the Caledonian deformation front. The thin-skinned character of the Caledonian deformation complex and the fact that N-dipping mantle reflections do not truncate the Moho indicate that the Baltica crust was not mechanically involved in the Caledonian collision and, therefore, escaped deformation in this area.     

Major crustal boundaries of Australia,and their significance in mineral systems targeting

For over 35 years, deep seismic reflection profiles have been acquired routinely across Australia to better understand the crustal architecture and geodynamic evolution of key geological provinces and basins. Major crustal-scale breaks have been interpreted in some of the profiles, and are often inferred to be relict sutures between different crustal blocks, as well as sometimes being important conduits for mineralising fluids to reach the upper crust. The widespread coverage of the seismic profiles now allows the construction of a new map of major crustal boundaries across Australia, which will better define the architecture of the crustal blocks in three dimensions. It also enables a better understanding of how the Australian continent was constructed from the Mesoarchean through to the Phanerozoic, and how this evolution and these boundaries have controlled metallogenesis. Starting with the locations in 3D of the crustal breaks identified in the seismic profiles, geological (e.g. outcrop mapping, drill hole, geochronology, isotope) and geophysical (e.g. gravity, aeromagnetic, magnetotelluric) data are used to map the crustal boundaries, in plan view, away from the seismic profiles. Some of the boundaries mapped are subsurface boundaries, and, in many cases, occur several kilometres below the surface; hence they will not match directly with structures mapped at the surface. For some of these boundaries, a high level of confidence can be placed on the location, whereas the location of other boundaries can only be considered to have medium or low confidence. In other areas, especially in regions covered by thick sedimentary successions, the locations of some crustal boundaries are essentially unconstrained, unless they have been imaged by a seismic profile. From the Mesoarchean to the Phanerozoic, the continent formed by the amalgamation of many smaller crustal blocks over a period of nearly 3 billion years. The identification of crustal boundaries in Australia, and the construction of an Australia-wide GIS dataset and map, will help to constrain tectonic models and plate reconstructions for the geological evolution of Australia, and will provide constraints on the three dimensional architecture of Australia. Deep crustal-penetrating structures, particularly major crustal boundaries, are important conduits to transport mineralising fluids from the mantle and lower crust into the upper crust. There are several greenfields regions across Australia where deep crustal-penetrating structures have been imaged in seismic sections, and have potential as possible areas for future mineral systems exploration.     

Artificial intelligence in seismology:Advent,performance and future trends

Realistically predicting earthquake is critical for seismic risk assessment,prevention and safe design of major structures.Due to the complex nature of seismic events,it is challengeable to efficiently identify the earthquake response and extract indicative features from the continuously detected seismic data.These challenges severely impact the performance of traditional seismic prediction models and obstacle the development of seismology in general.Taking their advantages in data analysis,artificial intelligence(AI) techniques have been utilized as powerful statistical tools to tackle these issues.This typically involves processing massive detected data with severe noise to enhance the seismic performance of structures.From extracting meaningful sensing data to unveiling seismic events that are below the detection level,AI assists in identifying unknown features to more accurately predicting the earthquake activities.In this focus paper,we provide an overview of the recent AI studies in seismology and evaluate the performance of the major AI techniques including machine learning and deep learning in seismic data analysis.Furthermore,we envision the future direction of the AI methods in earthquake engineering which will involve deep learning-enhanced seismology in an internet-of-things(IoT) platform.     

Seismic reflectivity of detachment faults of the Iberian and Tethyan distal continental margins based on geological and petrophysical data

Low-angle detachment faults are key to our understanding of the tectonic evolution of magma-poor rifted continental margins. In seismic images of present-day rifted margins the identification and interpretation of such features is, however, notoriously difficult and ambiguous. We address this problem by studying the structure and seismic response of such faults through a synoptic interpretation of petrophysical data and geological evidence from the distal segments of the present-day West Iberian and the ancient Tethyan margins. On the basis of the geologically well-constrained remnants of the Tethyan margins, which are spectacularly preserved and exposed in the Alps of Eastern Switzerland, vertical profiles at four key geological settings of a typical magma-poor rifted margin are constructed and their synthetic seismic responses are compared to the observed seismic data from corresponding locations in the present-day Iberian margin. The seismic structure of these profiles is considered as the sum of deterministic large-scale and the stochastic small-scale components. Both components are analyzed for all pertinent lithologies. The large-scale structures are derived from laboratory measurements on samples from both, the West Iberian and Tethyan margins, whereas the small-scale fluctuations are constrained predominantly on the basis of well-log data from the Iberian margin. Different realizations of the simulated stochastic small-scale velocity fluctuations illustrate the potential variability of impedance contrasts and its impact on the seismic response from lithological interfaces and fault structures. Our results indicate that the nature of the seismic response from low-angle detachment faults is largely determined through the fracture-healing behavior of the surrounding rocks. Geological evidence from the exposed fragments of the Tethyan margins indicate that fracture-healing is generally well developed in crustal lithologies, but largely absent in mantle lithologies. It is for this reason that low-angle, intra-crustal detachment faults tend to be seismically undetectable. Conversely, crust–mantle detachments have a complex and variable seismic response, depending on the nature of the damaged zone and on the frequency content of the seismic data. These model-based inferences are consistent with the available evidence from the present-day Iberian passive margin and thus open new perspectives for the interpretation of the corresponding seismic images.     

密集线性台阵地震背景噪声速度成像及其在湖南沃溪金钨锑矿勘探中的应用

多道面波分析技术在近地表勘探领域有着广泛应用,准确的提取频散曲线成为面波勘探成像的关键。文章介绍了一种新的地震背景噪声互相关面波频散成像方法——拓距相移法。该方法在传统相移法的基础上,利用阵内相移对小孔径范围的面波中高频信号进行提取,并利用阵外相移对大孔径范围的面波中低频信号进行提取,然后将两部分频散曲线融合从而得到更宽频带的面波频散曲线用于地下速度结构的反演。该方法在保证对近地表结构进行较高分辨率成像的同时,大大增加对深部结构的有效约束。2019年9月到10月期间,作者在湖南沃溪布设了8条密集测线,进行了1个月的地震背景噪声数据采集,并利用上述拓距相移法提取了0.1~2 s的瑞利面波宽频带相速度频散曲线。通过初步反演其中3条测线的背景噪声数据,获得了该矿区深度2.5 km以浅的地震横波速度结构。经与已知地质资料比对,160测线的地震横波速度反演结果与断层、岩性分界面及矿脉有着较好的对应关系,表明获得的沃溪矿区地震横波速度结构较好地反映了控矿构造、岩性分界以及矿体的分布位置等信息,为该区中—深部找矿提供了重要依据。该研究利用实际数据检验了拓距相移法的有效性,为今后深部找矿提供了一个有效的高精度成像方法。     

Application of sequential Gaussian simulation to quantify the surface wave magnitude uncertainty within the faulted regions

Geostatistical simulations have been recently widely used in the geological and mining investigations. Variogram, the fundamental tools of geostatistics, can identify the spatial distribution of the regionalized variable within the area. One of the important issues of geostatistical simulation in seismotectonics is producing uncertainty maps, which could be applicable to predict earthquake parameters through the site locations especially for civil structures like bridges. It can help engineers to design the structure of interest better. Earthquake parameters as for example seismic fault and surface wave magnitude (Ms) have significant impact on the feasibility study of the civil structures. In this research, a method is presented to produce uncertainty maps for seismic fault and surface wave magnitude, Ms. For this aim, information related to surface wave magnitude and fault trace in Zagros region (SW of Iran) has been collected. Then, the relationships between them through the site location have been investigated and analyzed by conditional geostatistical simulation. In order to quantify the uncertainty of each parameter, the uncertainty formula after generating the E-type maps has been provided and discussed. Finally, in “Talgah Bridge” site, these uncertainty maps were produced to interpret the impact of the surface wave magnitude and fault trace in this specific civil structure.