Effects of subducting lithosphere on p-wave amplitudes and residuals

In studying the structure of the lithosphere and asthenosphere using the kinematic and dynamic parameters of seismic waves, in any area, the tectonics of focal regions should also be considered. Subduction zones represent large-scale inhomogeneities which affect the propagation of seismic waves both at small and teleseismic epicentral distances.A study on the magnitude corrections of seismic stations in Central Europe and Scandinavia revealed that even in the case of close seismic stations the observed differences of the amplitudes of teleseismic P waves depend on the strike and dip of the sinking plates in the Northwestern Pacific. The smaller the angle between a seismic ray and the bottom of the lithospheric plate, the larger the numerical decrease of the magnitude corrections. A similar dependence was found for the P-wave residuals; the rays propagating more along the slab are accelerated more. Besides that, we have observed systematic differences in the P-wave residuals of about 2 s for foci with different positions within the subducting lithosphere of the Kurile arc in investigating the dependence of the residuals on the epicentral distance in the direction of subduction. The differences in the residuals disappear in this region at an epicentral distance of about 52°.     

Stress and deformation patterns in the Aegean region

The Aegean region constitutes the overriding plate of the Africa–Eurasia convergent plate system, in the eastern Mediterranean. To explain the fault kinematics and tectonic forces that controlled rift evolution in the Aegean area, we present fault-slip data from about 900 faults, and summarise the structural analyses of five key structural “provinces”. Five regional tectonic maps are used as the basis for a new stress map for the Aegean region and for discussions on regional geodynamics.Since the Late Miocene, the central Aegean has been affected by WNW- and NE-trending faults which transfer the motion of the Anatolian plate to the southwest, synchronous with arc-normal pull acting on the boundary of the Aegean plate. At the same time, the Hellenic Peninsula has suffered moderate extension by NW-trending grabens formed due to collapse of the Hellenic mountain chain.During intense extension in the southern Aegean in the Plio-Quaternary the arcuate shape of the Hellenic Trench was established. Arc-normal pull in the Aegean plate margin, combined with transform resistive forces along the Hellenic subduction gave rise to widespread strike-slip and oblique-normal faults in the eastern segment and moderate oblique extension in the western segment of the arc. To the north, subduction involves more continental crust and consequently the push of subduction is transmitted to the overriding plate (Hellenic Peninsula), resulting in the formation of NE-trending grabens. WNW-trending grabens in this area are considered to have propagated westward from the Aegean Sea to the Ionian Sea during Plio-Quaternary times, probably acting as pull-apart structures between stable Europe and the rapidly extending southern Aegean area.     

华南东南部上地幔远震P波速度结构及意义

本研究利用114个固定台站记录的121个远震事件,以钦杭结合带为中心,采用天然地震层析成像构建了华南东南部上地幔P波速度结构模型。研究结果表明：（1）钦杭结合带、武夷成矿带以及南岭成矿带的深部结构存在着差异,说明3个成矿带经历了不同的构造演化过程;（2）江绍断裂的上地幔中存在着低速异常,推测该低速异常为从地幔过渡带或者下地幔上涌的热物质,与钦杭结合带和武夷成矿带的成矿作用有着密切的关系;（3）下扬子地区上地幔底部的高速异常可能为拆沉的岩石圈,而华夏板块上地幔顶部的高速异常则有待进一步研究。本研究的结果为认识华南东南部的深部结构提供了新的证据。     

Saudi Arabian seismic-refraction profile: A traveltime interpretation of crustal and upper mantle structure

The crustal and upper mantle compressional-wave velocity structure across the southwestern Arabian Shield has been investigated by a 1000-km-long seismic refraction profile. The profile begins in Mesozoic cover rocks near Riyadh on the Arabian Platform, trends southwesterly across three major Precambrian tectonic provinces, traverses Cenozoic rocks of the coastal plain near Jizan, and terminates at the outer edge of the Farasan Bank in the southern Red Sea. More than 500 surveyed recording sites were occupied, and six shot points were used, including one in the Red Sea.Two-dimensional ray-tracing techniques, used to analyze amplitude-normalized record sections indicate that the Arabian Shield is composed, to first order, of two layers, each about 20 km thick, with average velocities of about 6.3 km/s and 7.0 km/s, respectively. West of the Shield-Red Sea margin, the crust thins to a total thickness of less than 20 km, beyond which the Red Sea shelf and coastal plain are interpreted to be underlain by oceanic crust.A major crustal inhomogeneity at the northeast end of the profile probably represents the suture zone between two crustal blocks of different composition. Elsewhere along the profile, several high-velocity anomalies in the upper crust correlate with mapped gneiss domes, the most prominent of which is the Khamis Mushayt gneiss. Based on their velocities, these domes may constitute areas where lower crustal rocks have been raised some 20 km. Two intracrustal reflectors in the center of the Shield at 13 km depth probably represent the tops of mafic intrusives.The Mohorovičić discontinuity beneath the Shield varies from a depth of 43 km and mantle velocity of 8.2 km/s in the northeast to a depth of 38 km and mantle velocity of 8.0 km/s depth in the southwest near the Shield-Red Sea transition. Two velocity discontinuities occur in the upper mantle, at 59 and 70 km depth.The crustal and upper mantle velocity structure of the Arabian Shield is interpreted as revealing a complex crust derived from the suturing of island arcs in the Precarnbrian. The Shield is currently flanked by the active spreading boundary in the Red Sea.     

月表构造与深部构造的关系

地球和月球很可能是通过大撞击形成的。在行星地质学中,研究月球的地质-构造现象,对了解月球、地球乃至太阳系的形成与演化历史都有很大帮助。月球的构造分为深部构造与月表构造,寻找它们在分布或成因上的关系,可以为月球甚至地月系的起源和演化历史提供重要参考。利用LROC的宽视角影像数据以及LOLA数据提取解译月表构造,结合深大断裂进行观察分析,并对月球的撞击盆地进行统计,最后以静海地区为例分析构造分布特征,发现月球的质量瘤盆地中具有环状分布的月岭,外侧具有近环状分布的深大断裂,自前酒海纪至酒海纪,具备上述特征的质量瘤盆地占总撞击盆地的比例突然有一个很大的提升,且静海地区西部具有该构造分布特征。推测该特征与撞击、月海沉降等有关,且在酒海纪与雨海纪期间月球有较多的月海玄武岩分布,由此判断静海西部存在质量瘤,发生过撞击与月海沉降。     

2-D velocity structure in Kerala-Konkan basin using traveltime inversion of seismic data

The existence of gas-hydrates in marine sediments increases the seismic velocity, whereas even a small amount of underlying free-gas reduces the velocity considerably. The change in velocities against the background (without gas-hydrates and free-gas) velocity can be used for identification and assessment of gas-hydrates. Traveltime inversion of identifiable reflections from large offset multi channel seismic (MCS) experiment is an effective method to derive the 2-D velocity structure in an area. We apply this method along a seismic line in the Kerala-Konkan (KK) offshore basin for delineating the gas-hydrates and free-gas bearing sediments across a bottom simulating reflector (BSR). The result reveals a four layer 2-D shallow velocity model with the topmost sedimentary layer having velocity of 1,680–1,740 m/s and thickness of 140–190 m. The velocity of the second layer of uniform thickness (110 m) varies from 1,890 to 1,950 m/s. The third layer, exhibiting higher velocity of 2,100–2,180 m/s, is interpreted as the gas-hydrates bearing sediment, the thickness of which is estimated as 100 to 150 m. The underlying sedimentary layer shows a reduction in seismic velocity between 1,620 to 1,720 m/s. This low-velocity layer with 160–200 m thickness may be due to the presence of free-gas below the gas-hydrates layer.     

The influence of different rheological parameters on the surface deformation and stress field of the Aegean–Anatolian region

The Aegean–Anatolian region is characterised by an inhomogeneous deformation pattern with high strain rates and a high seismicity both at the boundaries and in the plate interior. This pattern is controlled by the geometry and rheology of the structural units involved and their tectonic setting. A numerical analysis with a finite-element model of the region is used to quantify the influence of different rheological parameters. Viscoelastic material behaviour is implemented for the mantle lithosphere, whereas the crust is modelled with an elastic–plastic rheology. The variation of the inelastic material properties (viscosity and plastic strength) quantifies the influence of these material parameters on the deformation, stress, and strain patterns. Comparison of the modelled results with geodetic and geophysical observations reveals that the viscosity of the mantle lithosphere is the key to explaining the inhomogeneous deformation pattern. The best-fit model yields a viscosity of 10²⁰ Pa s beneath Anatolia, whereas adjacent regions have viscosities between 10²¹ and 10²³ Pa s. The model also explains the intra-plate seismicity and the stress field as well as its partitioning into regions with strike-slip and normal faulting. The final model is in good agreement with seismological, geodetic, and geological observations. Local deviations can be tracked down to small-scale structures, which are not included in the model.     

Crustal structure in the Barents-Kara region from detailed surveys by the method of deep seismic sounding. Paper 2

Seismogeologic sections for the Barents-Kara region along geotraverses 1-AR, 2-AR, and 3-AR with a total length of about 4000 km were obtained using the GODOGRAF software package developed at the Department of Seismometry and Geoacoustics of the Moscow State University. The data were travel times of refracted waves excited by approximately 100 sources along each traverse. This paper reports sections for the 3-AR traverse covering areas of the White Sea, Pechora Sea, and Kara Sea, and a geological interpretation of these. The sections cover depths down to 40–50 km and show basic crustal discontinuities, fold-thrust, rift, and paleospreading structural features, and paleosubduction zones. We characterize the possible character of the junction between the South Kara and North Kara basins. A geodynamic interpretation of the structures is provided for the Barents-Kara region.     

Long-term prediction of intermediate depth earthquakes in the southern Aegean region based on a time-predictable model

Repeat times of strong intermediate depth (60 km h 180 km) earthquakes have been determined by the use of instrumental and historical data for six seismogenic sources in the Benioff zone of the southern Aegean area. For four of these sources, at least two interevent times (three mainshocks) are available for each source. By using the repeat times for these four sources, the following relation has been determined: logT _t = 0.20M _min + 0.19M _p +a, whereT _t is the repeat time (in years),M _min the surface wave magnitude of the smallest earthquake considered,M _p the magnitude of the preceding mainshock and a parameter which varies from source to source. A multilinear correlation coefficient equal to 0.91 was determined for this relation, which indicates that the time predictable model holds to a satisfactory degree for the strong mainshocks of intermediate focal depth in the southern Aegean.By assuming that the ratioT/T _t, whereT is the observed andT _t the calculated repeat time, follows a lognormal distribution, the conditional probabilities for the occurrence of strong (M _s 6.5) and very strong (M _s 7.5) earthquakes during the period 1991–2001 in these four seismogenic sources have been calculated. These probabilities are very high (P > 0.9) for the strong and high (P > 0.5) for the very strong intermediate depth earthquakes which occur in the three sources of the shallower (h < 100 km) part of the Benioff zone where coupling occurs between the front parts of the Mediterranean lithosphere (downgoing) and the Aegean lithosphere.     

Probable regional geothermal field reconnaissance in the Aegean region from modern multi-temporal night LST imagery

The Hellenic Peninsula and the adjacent Aegean and Ionian seas are segmented into regions on the basis of Terra satellite-MODIS instrument derived land surface temperatures (LST) to test if they can be used in the field reconnaissance for potential geothermal targets, each region representing a different thermal signature. The method has been used successfully to identify hot spot and geothermal activity in the Afar Triangle and in the Red Sea. Night monthly average LST values per pixel, since 2001, are used in this work for geothermal field identification. Average LST seasonal variability is expressed by a common centroid curve of pixel cluster. Clusters were subsequently ranked in increasing LST according to their centroids. Cluster-2 represents by far the Aegean volcanic arc (AVA) which comprises the high enthalpy (320 and 350°C, respectively) geothermal fields of Milos and Nisyros. The interpretation of LST terrain segmentation into sub-clusters of Cluster-2 is consistent with thermal anomalies related to the volcano-islands of the AVA, the occurrence of thermal anomalies related to granodiorite plutons in the Cyclades and to the volcano-related anomalies of the Dodecanese Province. We conclude that the identification of such thermal anomalies obtained by a combination of remote sensing (LST), regional geology and field data (borehole, thermal spring and subsurface hydrothermal reservoir temperatures) can be a particularly useful exploration tool for localizing geothermal anomalies.     

Cenozoic geodynamic evolution of the Aegean

The Aegean region is a concentrate of the main geodynamic processes that shaped the Mediterranean region: oceanic and continental subduction, mountain building, high-pressure and low-temperature metamorphism, backarc extension, post-orogenic collapse, metamorphic core complexes, gneiss domes are the ingredients of a complex evolution that started at the end of the Cretaceous with the closure of the Tethyan ocean along the Vardar suture zone. Using available plate kinematic, geophysical, petrological and structural data, we present a synthetic tectonic map of the whole region encompassing the Balkans, Western Turkey, the Aegean Sea, the Hellenic Arc, the Mediterranean Ridge and continental Greece and we build a lithospheric-scale N-S cross-section from Crete to the Rhodope massif. We then describe the tectonic evolution of this cross-section with a series of reconstructions from ~70 Ma to the Present. We follow on the hypothesis that a single subduction has been active throughout most of the Mesozoic and the entire Cenozoic, and we show that the geological record is compatible with this hypothesis. The reconstructions show that continental subduction (Apulian and Pelagonian continental blocks) did not induce slab break-off in this case. Using this evolution, we discuss the mechanisms leading to the exhumation of metamorphic rocks and the subsequent formation of extensional metamorphic domes in the backarc region during slab retreat. The tectonic histories of the two regions showing large-scale extension, the Rhodope and the Cyclades are then compared. The respective contributions to slab retreat, post-orogenic extension and lower crust partial melting of changes in kinematic boundary conditions and in nature of subducting material, from continental to oceanic, are discussed.     

Structure and seismicity of the Aegean subduction zone

Tomographic results show the presence of a high-velocity anomaly dipping north beneath the Aegean Sea (Hellenic arc), down to a depth of at least 600 km. This anomaly is interpreted as the image of the subducting lithosphere of the African plate. No deep seismicity, however, is associated with this downgoing slab, although this would be expected on the basis of the age of the downbending lithosphere (approximately 100 Myr) and the inferred duration of the present ongoing episode of subduction. Using a thermo-mechanical model for the subduction zone we find that the non-stationary input of the subduction zone-both in convergence rate and in thermal structure of the downgoing lithosphere - adequately accounts for both the presence of a velocity anomaly associated with a slab and the absence of deep seismicity. The non-stationarity follows from the large-scale tectonic setting of the Eastern Mediterranean basin.     

Seismotectonics of the northern Aegean area

Data concerning the focal mechanism and the spatial distribution of earthquakes have been used to investigate the active tectonics of the northern Aegean and the surrounding area.A thrust region, which includes the northernmost part of the Aegean and at least part of the Marmara Sea, has been defined. An amphitheatrical Benioff zone dipping towards the thrust region from south, east and probably from west, at a mean angle of about 30°, has been detected.The thrust region is surrounded by a region of normal faulting. An eastward progression of the seismic activity in this normal faulting region between 1954 and 1971 has been observed.A correspondence between the earthquake occurrence in the thrust and normal faulting regions has also been observed. Each large shock produced by tensional mechanism in the region of normal faulting is preceded or followed by one or more shocks of compressional mechanism in the thrust region.The focal mechanism, the distribution of the earthquake foci with intermediate focal depth, as well as some magnetic and gravimetric observations can be interpreted by assuming that dense oceanic crust sinks in the northern part of this area and that the adjacent lithosphere moves by segmentation to fill the void with the consequence of producing tensile stresses associated with normal faulting. Such a mechanism of lithospheric interaction suggests that accretion probably takes place in this area.     

Peculiarities in the deep structure of the Sakhalin-Hokkaido-Primorye zone

The results of detailed seismic crustal investigations in the marginal part of the transition zone from the Asian continent to the Pacific Ocean are discussed. The observations were carried out during 1963 and 1964 by expeditions of the Institute of Physics of the Earth of the U.S.S.R. Academy of Sciences and the Sakhalin Complex Research Institute, Siberian Division of the U.S.S.R. Academy of Sciences. The results of these measurements have been described in detail in a monograph, by Zverev and Tulina, entitled Deep Seismic Sounding of the Earth's Crust in the Sakhalin-Hokkaido-Primorye Zone (1971).

The distribution of seismic crustal characteristics is studied and schematic maps of the surface of presumably Cretaceous and Meso-Paleozoic (or pre-Paleozoic) sediments near Sakhalin Island as well as of the mantle surface in the Sakhalin-Hokkaido-Primorye zone are presented.     

月球雨海盆地多环结构的厘定及其深部构造研究

雨海盆地是月球正面最大、月球上研究程度最高的多环结构撞击盆地,已有很多学者对其多环结构的边界进行恢复研究,但在多环结构最初始形状、多环位置/数量、盆地大小等方面,至今未能达成共识。本文利用GRAIL自由空气重力异常数据、LOLA激光测高数据进行了多源数据的融合,结果表明,雨海盆地是具有偏心圆的三环结构特点,其直径从外到内分别为1 500 km、1 100 km、665 km。基于欧拉反演结果研究表明,在雨海撞击盆地中部存在两种不同深度、构造运动性质及方向的断裂构造,即：(1)深度大于40 km,向下逐渐向内倾斜、延伸的深部断裂构造;(2)深度在40 km以内,由月表向下逐渐向外倾斜、延伸的浅部断裂构造。结合物质成分及地球物理特征的研究,雨海地区的地质构造演化过程可分为两个阶段：(1)在月球早期阶段(45~38.5亿年),主要以内动力地质作用即岩浆洋冷凝过程为主,形成了雨海盆地深度在40 km以下逐渐向内倾斜、延伸的构造断裂,其为本区在月球早期深部岩浆洋产生、分异及运移提供了通道,该构造断裂代表了雨海盆地撞击前的月球早期深部岩浆洋的构造地质演化阶段;(2)在月球晚期阶段(≤38.5亿年),主要以内、外动力地质作用并重,形成了雨海盆地深度在40 km以内逐渐向外倾斜、延伸的构造断裂,其应为本区不同期次的玄武质岩浆喷出或溢流到月表提供了运移通道,该构造断裂代表了雨海盆地撞击后的月球晚期不同期次玄武质岩浆喷发、充填溢流的月海岩浆活动作用的构造地质演化阶段。     

Revising the urban structure of the Ruhr region

The historical processes of industrialization and urbanization have caused a variety of urban patterns in the largest conurbation of Central Europe, the Ruhr region, that now has to pass a period of economic recession and stagnation.After an outline of the historical development the existing economic regionalization is enlarged by a system of subregional areas which is based on the various urban patterns of the region. This system shows striking coincidence with the present central place structure. Small scale population trends can be explained by and future planning objectives may be derived from it.     

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

Understanding 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.     

Crustal structure in the region of the British Isles

Progress in the application of seismic refraction methods to the determination of crustal structure for the British Isles and surrounding sea areas, is reviewed for the period which follows the publications of 1965. The work has been strongly oriented towards the application of “Time Term” interpretation to land-based observations of explosions in the English Channel, to the south of Ireland, in the Irish Sea, in the North Sea, and off the west coast of Scotland.

All of the surveys have included determinations of velocities for P_n and P_g, with some indication of an increase in P_n-velocity with range. In part of the area, indications of lower-crust velocities ranging from 6.9 to 7.3 km/sec have been found.