Structure of the upper mantle in the Northeastern Atlantic close to the azores-gibraltar ridge from surface-wave and body-wave observations

Phase velocities of teleseismic Rayleigh waves have been measured in the central North Atlantic on both sides of the Azores-Gibraltar Ridge (AGR) by means of a specially designed long-period station network. The dispersion data obtained were regionalized and then subjected to a “hedgehog” inversion, which gives a set of upper mantle models compatible with the observational data within specified error bounds.Reasonable model solutions were selected by using regional body-wave observations, such as P_n- and S_n-wave velocities determined from earthquakes along the AGR. The S_(itn) velocities measured indicate that the shear-wave velocity in the mantle part of the lithosphere is much higher on the northern side of the AGR. Strongly negative P-wave residuals in this area indicate faster seismic propagation than implied by the Jeffreys-Bullen travel-time tables, while propagation is much slower in the Gulf of Cadiz area. Furthermore the residuals show a clear difference for paths through oceanic and continental domains and suggest that the transition between these two domains extends much further into the ocean on the southern side of the AGR than on the northern side.The proposed model for the structure of the upper mantle in that region shows that there exists a pronounced velocity contrast across the AGR. Thickening of the lithospheric plate with increasing plate age is indicated to the south of the ridge. The greatest thickness is reached close to the continental margin within a zone about 500 km wide, whose velocity close to the Canary Islands and Madeira is significantly lower, probably due to the well-known volcanic activity there. These observations together with the travel time residuals reveal that this zone seems to be of a transitional nature somewhere between a continental and oceanic structure.     

Apparent velocity and azimuths of short period P-Waves recorded at indian WWSSN stations network

Over 200 earthquakes in the distance range 30°–90° and azimuthal range 0°–360°, recorded at Indian WWSSN stations, have been used in the present study. We have treated the four WWSSN recording stations i.e. New Delhi, Poona, Shillong and Kodai-Kanal, as Super Large Aperture Seismic Array (SLASA) Network with New Delhi being its cross-over point. Short period P-wave data as obtained from these stations have been analysed using a least square technique. Slowness and azimuthal anomalies have been computed for all these events. Relative time residuals have also been calculated. A velocity model has been derived on the basis of the slowness and travel-time data. The results do not indicate presence of any triplication in the travel-time curve. Variations in the relative residuals refer to the tectonic features beneath the recording stations. The P-wave velocity increases continuously in the lower mantle region and there is no indication for the presence of any appreciable velocity gradient.     

Teleseismic P-wave residual investigations at Shillong,India

The northeast India region is seismically very active and it has experienced two large earthquakes of magnitude 8.7 during the last eight decades (1897 and 1950). We have analysed teleseismic P-wave residuals at Shillong, the only reliable seismic station operating in the region, to investigate a possible association of travel-time residual anomaly with earthquake occurrence. The period covered is from October 1964 through March 1976. The total number of events is 9479, including 1767 events with depth >/ 100 km. Six-monthly average residuals have been calculated. The standard deviations are less than 0.10 sec for these data sets. During the period of investigations, no major earthquake took place close to Shillong. The earthquake of June 1, 1969 with a magnitude (M_b) of 5.0, at an epicentral distance of 20 km from Shillong is the only significant event. This earthquake is found to be associated with a travel-time increase with a maximum amplitude of 0.4 sec. It appears that, in general, the P-wave velocity has decreased in the neighbourhood of Shillong since 1969. A quadrant-wise analysis of residuals indicates that the residual anomaly is most prominent in the SE quadrant from Shillong.     

A dynamic model of the Hellenic Arc deduced from geophysical data

Combined gravity and seismic data from Greece and the adjacent areas have been used to explain the high seismicity and tectonic activity of this area. Computed 2-D gravity models revealed that below the Aegean region a large “plume” of hot upper-mantle material is rising, causing strong attenuation of the crust. The hot “plume” extends to the base of the lithosphere and has very probably been mobilized through compressional processes that forced the lithosphere to sink into the asthenosphere. The above model is supported by: high heat flow in the Aegean region; low velocity of the compressional waves of 7.7 km/sec for the upper mantle; lower density than normal extending to the base of the lithosphere; teleseismic P-wave travel-time residuals of the order of +2 sec for seismic events recorded at the Greek seismic stations; volcanics in the Aegean area with a chemical composition which can be explained by assuming an assimilation of oceanic crust by the upper mantle; deep seismicity (200 km) which has been interpreted by various authors as a Benioff zone.     

Seismicity and structural investigations of the Romanian Vrancea Region: Evidence for azimuthal variations of p-wave velocity and poisson's ratio

A study of the shallow and intermediate depth seismicity of the Romanian Vrancea region in the period 1964–1981 has been performed. The seismic events have been relocated by a standard location procedure using a regional velocity model. From the temporal and spatial distribution of the seismic activity, aspects of the seismicity before the large March 4, 1977 earthquake are treated, in particular the seismic gap in space and time prior to this event, found by Mârze (1979), which is critically discussed and revised. The concept of the precursor time/magnitude relationships of different authors is applied and its validity to the Vrancea region assessed. The hypocentral distribution shows that the intermediate depth seismic activity is confined to a small volume with dimensions of only some tens of kilometers. The results are interpreted in terms of the tectonics of the region. From an analysis of the travel-time residuals at different local stations, evidence for lateral velocity heterogeneities beneath the region is obtained e.g. a high velocity zone southeastwards of the Carpathian chain. Finally mean ratios, (i.e. Poisson's ratios), for various stations are calculated from P- and S-wave travel times. They show azimuthal variations of up to 6% for stations within the area where the intermediate seismic activity occurs in comparison with the station Focsani, situated eastwards in the Carpathian foredeeps. All these results are compatible with the plate tectonic concept for the Vrancea region, that is the subduction of an oceanic lithospheric slab under the Carpathian mountain arc, giving rise to such a highly active seismic zone.     

Relative teleseismic travel-time residuals, North Island, New Zealand, and their relation to upper-mantle structure

Relative travel-time residuals computed from clear P-wave arrivals at fourteen seismograph stations in the North Island, New Zealand, from five deep-focus events in the Banda Sea region, show large spatial variations of up to 3 sec. The variations can be explained by higher than normal velocities in the oceanic lithosphere which is underthrust to depths of 350 km beneath the North Island. After correction for crustal structure, the residuals imply an average P-wave velocity about 11% higher than in the surrounding mantle. The lack of suitable source events at azimuths other than northwest prevents a more detailed investigation by this means.     

http://www.sciencedirect.com/science/article/pii/S1674987112001247

The Sinai Peninsula has been recognized as a subplate of the African Plate located at the triple junction of the Gulf of Suez rift, the Dead Sea Transform fault, and the Red Sea rift. The upper and lower crustal structures of this tectonically active, rapidly developing region are yet poorly understood because of many limitations. For this reason, a set of P- and S-wave travel times recorded at 14 seismic stations belonging to the Egyptian National Seismographic Network (ENSN) from 111 local and regional events are analyzed to investigate the crustal structures and the locations of the seismogenic zones beneath central and southern Sinai. Because the velocity model used for routine earthquake location by ENSN is one-dimensional, the travel-time residuals will show lateral heterogeneity of the velocity structures and unmodeled vertical structures. Seismic activity is strong along the eastern and southern borders of the study area but low to moderate along the northern boundary and the Gulf of Suez to the west. The crustal V_p/V_s ratio is 1.74 from shallow (depth ≤ 10 km) earthquakes and 1.76 from deeper (depth > 10 km) crustal events. The majority of the regional and local travel-time residuals are positive relative to the Preliminary Reference Earth Model (PREM), implying that the seismic stations are located above widely distributed, tectonically-induced low-velocity zones. These low-velocity zones are mostly related to the local crustal faults affecting the sedimentary section and the basement complex as well as the rifting processes prevailing in the northern Red Sea region and the ascending of hot mantle materials along crustal fractures. The delineation of these low-velocity zones and the locations of big crustal earthquakes enable the identification of areas prone to intense seismotectonic activities, which should be excluded from major future development projects and large constructions in central and southern Sinai.     

The seismic travel-time drop in the Charlevoix region from 1979 to 1980: evidence for aligned saturated cracks in the crust

A significant drop in seismic travel times of up to 1.0% occurred in the Charlevoix region between 1979 and 1980, possibly related to the M = 5.0 (Aug., 1979) earthquake in the vicinity. A travel-time drop of this magnitude could have been produced either by the closing of dry or saturated cracks in the upper crustal material or by the saturation of dry or partly saturated cracks. However, the anisotropy of travel-time changes in this area supports the view that this travel-time drop was caused by the closing of water-saturated aligned vertical cracks in the crustal material. Three different crack directions with respect to north were resolved: 0 ° or 90 ° in the Precambrian rocks underneath the St. Lawrence River, −18° or 72° in the shallow rocks (< 5 km) of the Charlevoix crater, region, and +35° or 125° in the Paleozoic cover rocks. Crack closure would require a decrease in the pore volume of the rocks which would be expected to produce subsidence in the Charlevoix area. Since repeated levellings restrict the vertical crustal motion during this time interval to less than 2 cm, we conclude that either the effective aspect-ratio of cracks is less than 0.0001 or the process of crack closure occurred in a number of unconnected regions. More specifically the crack deformation would have to occur in isolated inclusions less than 1 km in diameter and no deeper than about 6 km. The process of crack closure may have been triggered by the passage of seismic waves from the M = 5.0 earthquake.     

Propagation anomalies in Northwestern Italy by inversion of teleseismic residuals

Travel-time residuals of teleseismic P waves were analysed in order to elucidate the crust–upper mantle structure in Northwestern Italy, and the Western Alpine Arc. Using digital data obtained from both fixed seismograph networks operating in NW Italy (notably Liguria–Piedmont) and temporary arrays with the aid of cross-correlation techniques reliable travel-time residuals were calculated which were then inverted to obtain models of propagation anomalies. The reliability of the inversion procedure was tested using synthetic data. The model thus obtained appears to be stable and shows strong lateral heterogeneities at a litho–asthenospheric level; in particular, it confirms the high velocity contrast caused by the ‘Ivrea Body’ in the shallower layers and the presence of Alpine ‘roots’ reaching down to at least 200 km. A statistical analysis performed on the propagation times of rays crossing the resulting four-layered model reveals rms below 0.1 s.     

Statistical Comparison of Geophysical Parameters and Seismicity in Central Italy

Geological and geophysical data of an area of central Italy have been used for statistically comparing with the distribution of the mean seismic deformation. This parameter has been calculated by adding the square root of the energy released by earthquakes inside a cell, the dimensions of which have been fixed after performing an analysis of spatial autocorrelation. Underground temperatures, fault length, and topographic altitude showed a good single linear correlation with seismicity. Factor analysis has been employed to reduce the number of original variables, taking into account their mutual correlation; then, with the new uncorrelated variables, a good multiple linear correlation has been found, considering seismic deformation as a dependent variable.The analysis of residuals for the multiple regression shows the existence of areas with different seismic activity than predicted by the model: a discussion of the residuals pattern follows, considering the geological setting of the area.     

Lithospheric structure of the Tornquist Zone resolved by nonlinear P and S teleseismic tomography along the TOR array

The main aim of the TOR project is to study the lithospheric–asthenospheric boundary structure under the Sorgenfrei–Tornquist Zone, across northern Germany, Denmark and southern Sweden. Relative arrival-time residuals of teleseismic P and S phases from 51 earthquakes, recorded by 150 seismic stations along the TOR array, were used to delineate the transition zone in the studied area. The effects of crustal structures were investigated by correcting the teleseismic residuals for travel-time variations in the crust based on a 3D crustal model derived from other data. The inversion was carried out for S phases. The results were then compared with the corresponding P-wave models. As expected, the derived models show that the relatively old and cold Baltic Shield has higher velocity at depth than the younger lithosphere farther South. The models show two sharp and distinct increases in depth to velocities which are low compared to our reference model, as we move from South to North. The location and sharpness of these boundaries suggests that the features resolved are, at least partially, compositional in origin, presumably related to mantle depletion. A sharp and steep subcrustal boundary is found roughly coincident with the southern edge of Sweden. This is below where the edge of the Baltic Shield is usually placed, based on surface geological evidence (the Sorgenfrei–Tornquist Zone). Another less significant transition is recognised more or less beneath the Elbe-lineament. Relatively high d(V_p / V_s) ratios under the central part of the profile (Denmark) indicate relatively low S-velocity in an area where a gravity high supports the hypothesis of extensive mafic intrusions.     

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°.     

Upper mantle heterogeneities beneath Eastern Europe

P-wave travel-time residuals for seismograph stations in eastern Europe as reported by ISC for the years 1964–1977 were used for constructing a seismic image of upper mantle heterogeneities in the network region. For the depth range 0–100 km, dominant tectonic features like the Pannonian Basin and the Aegean Sea and western Turkey correlate well with pronounced velocity lows which a ppear to extenddown to a 300 km depth. The velocity anomaly patterns in the depth intervals 300–500 km and 500–600 km are broadly similar but quite different from those of shallower depths. The observed seismic heterogeneities are briefly discussed in terms of large-scale tectonic and geophysical (heat-flow) characteristics of eastern Europe.     

塔里木板块早—中二叠世玄武质岩浆作用的沉积响应

塔里木板块内部发育了大量由早—中二叠世板内岩浆作用所形成的、以玄武岩类为主的基性岩浆岩,包括玄武岩、辉绿岩、玄武安山岩等,残余分布面积约20万km2。而二叠纪时期是塔里木板块演化过程中一个非常重要的转折时期,从早二叠世晚期开始塔里木板块结束了海相沉积,进入了陆相沉积阶段。通过对塔里木板块的岩浆作用特征和石炭纪—二叠纪沉积相的分析,笔者讨论了这一大规模的岩浆作用对板块晚石炭世—二叠纪的沉积作用所起的控制作用,提出了塔里木板块早—中二叠世巨量玄武质岩浆作用的沉积响应过程模式。     

基于数字图像分割法的跨孔雷达走时层析成像

跨孔雷达走时层析成像主要利用雷达波的走时进行反演,走时提取的正确与否将直接影响到层析成像的效果。数字图像分割法基于凸集投影（POCS）方法,使用能量比彩色图像分割技术准确提取走时。数字图像分割法提取走时首先应用在折射地震波的数据处理中。笔者首次将数字图像分割法提取走时的方法应用到跨孔雷达走时层析成像中,使用迭代线性反演算法重建了雷达波速度场。反演过程中,使用最小二乘QR分解法（LSQR）求解线性方程组,利用弯曲射线追踪技术构建雅可比矩阵,走时的计算值则由多模板快速推进算法（MSFM）得到。为了验证数字图像分割法在走时层析成像中的效果,使用一组合成数据及一组实测数据分别对比了基于数字图像分割法和基于传统能量比法获得的层析成像反演结果。对比结果表明,使用数字图像分割法得到的层析成像结果更为精确,误差更小,能为判断地下雷达波速度场提供更为有力的帮助。     

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.     

Estimation of the 2D crustal velocity structure in the Hidaka and Iburi Subprefectures, Hokkaido, Japan

Applying the iterative shooting/bisection technique for rapid forward modeling to the seismic explosion data, we could refine the crustal velocity structure model of the western part of the Hidaka collision zone, Hokkaido, Japan. We used only the precise P-wave first arrival data obtained by the Research Group for Explosion Seismology, which set up a 113.4-km-long profile in August 2000 along with 327 observation points and four shot points with TNT charges from 100 to 300 kg. We could estimate a two-dimensional inhomogeneous crustal velocity structure model with a velocity decrease in the eastern direction at a depth of 15.7 km, several portions of velocity reversals with depth and a low velocity anomaly proposed in previous studies. The root-mean-square of travel-time residuals was improved from 0.398 s for the previous structure model to 0.176 s for the present model with a reduction of 55.8%.     

Crustal structure in Tengchong Volcano-Geothermal Area, western Yunnan, China

Based upon the deep seismic sounding profiles carried out in the Tengchong Volcano-Geothermal Area (TVGA), western Yunnan Province of China, a 2-D crustal P velocity structure is obtained by use of finite-difference inversion and forward travel-time fitting method. The crustal model shows that a low-velocity anomaly zone exists in the upper crust, which is related to geothermal activity. Two faults, the Longling–Ruili Fault and Tengchong Fault, on the profile extend from surface to the lower crust and the Tengchong Fault likely penetrates the Moho. Moreover, based on teleseismic receiver functions on a temporary seismic network, S-wave velocity structures beneath the geothermal field show low S-wave velocity in the upper crust. From results of geophysical survey, the crust of TVGA is characterized by low P-wave and S-wave velocities, low resistivity, high heat-flow value and low Q. The upper mantle P-wave velocity is also low. This suggests presence of magma in the crust derived from the upper mantle. The low-velocity anomaly in upper crust may be related to the magma differentiation. The Tengchong volcanic area is located on the northeast edge of the Indian–Eurasian plate collision zone, away from the eastern boundary of the Indian plate by about 450 km. Based on the results of this paper and related studies, the Tengchong volcanoes can be classified as plate boundary volcanoes.     

Morphology of the subducted Indian plate in the Indo-Burmese convergence zone

Earthquake hypocenters and travel time residuals have been analysed to constrain the geometry and physical state of the subducted Indian plate in the Indo-Burmese convergence zone. A critical analysis of earthquake hypocenters reveals the existence of a non-uniform Benioff zone, progressively shortening from north to south. The deepest level of seismicity is observed beneath the Naga hills (160 km) followed by that under the Chin hills (120 km) and Arakan-Yoma ranges (80 km). The region seems to be devoid of moderate sized shallow (< 40 km) earthquakes. Differential travel time residuals from pairs of shallow and intermediate depth earthquakes recorded at teleseismic distances show significantly faster travel time (up to l.2s) in the north-northeast and south-southwest azimuths, whilst slower arrivals (1.2 to 1.5 s) are recorded in the transverse direction. This observation points to the presence of a high velocity slab possibly linked to the subduction of the Indian oceanic lithosphere.     

庐枞铁多金属矿集区龙桥铁矿反射地震初至波层析成像与隐伏矿床预测

为落实安徽庐枞地区北部的龙桥铁矿含矿层位在第二找矿深度的空间分布,本文应用小道距、长排列接收的地震反射初至波走时数据,运用层析成像方法,得到该地区近地表1000m以上的速度结构。研究结果显示,反射地震初至波携带了丰富的上地壳速度结构特征和界面变化的构造信息,精细的速度结构揭露出龙桥铁矿隐伏岩体顶界面深度与起伏形态,发现岩体顶面凸起及陡变化部位与矿体分布存在着明显的对应关系。长排列的深地震反射初至波走时层析成像方法能够为金属矿集区深部隐伏矿床的勘探与研究提供岩体的空间分布信息。