Subduction zone seismicity and the thermo-mechanical evolution of downgoing lithosphere

In this paper we discuss characteristic features of subduction zone seismicity at depths between about 100 km and 700 km, with emphasis on the role of temperature and rheology in controlling the deformation of, and the seismic energy release in downgoing lithosphere. This is done in two steps. After a brief review of earlier developments, we first show that the depth distribution of hypocentres at depths between 100 km and 700 km in subducted lithosphere can be explained by a model in which seismic activity is confined to those parts of the slab which have temperatures below a depth-dependent critical valueT _cr.Second, the variation of seismic energy release (frequency of events, magnitude) with depth is addressed by inferring a rheological evolution from the slab's thermal evolution and by combining this with models for the system of forces acting on the subducting lithosphere. It is found that considerable stress concentration occurs in a reheating slab in the depth range of 400 to 650–700 km: the slab weakens, but the stress level strongly increases. On the basis of this stress concentration a model is formulated for earthquake generation within subducting slabs. The model predicts a maximum depth of seismic activity in the depth range of 635 to 760 km and, for deep earthquake zones, a relative maximum in seismic energy release near the maximum depth of earthquakes. From our modelling it follows that, whereas such a maximum is indeed likely to develop in deep earthquake zones, zones with a maximum depth around 300 km (such as the Aleutians) are expected to exhibit a smooth decay in seismic energy release with depth. This is in excellent agreement with observational data. In conclusion, the incoroporation of both depth-dependent forces and depth-dependent rheology provides new insight into the generation of intermediate and deep earthquakes and into the variation of seismic activity with depth.Our results imply that no barrier to slab penetration at a depth of 650–700 km is required to explain the maximum depth of seismic activity and the pattern of seismic energy release in deep earthquake zones.     

深部俯冲板片三维构造重建及其几何学、运动学研究——以汤加—克马德克地区俯冲板片为例

西南太平洋板块与澳大利亚板块之间的汤加—克马德克俯冲带,是研究地球动力学最重要的区域之一.本文研究根据MIT-P08地震数据,结合板块构造边界、地震活动分布、海岸地形数据等,基于GOCAD软件平台建立三维地震层析成像,对西南太平洋板块的汤加—克马德克俯冲板片进行三维解释.地震层析成像显示汤加—斐济地区地幔至少存在三个"高速"异常体.早期汤加—克马德克俯冲板片穿过地幔转换带,并进入下地幔,最大深度达1600 km.三维构造模型揭示了汤加—克马德克板片在深度600~800 km处存在断折形变,该俯冲板片去褶皱恢复后,测量其俯冲的最大位移达2600 km.汤加—克马德克板片开始快速俯冲的时间至少在30 Ma之前,平均移动速率约为68~104 mm /a.俯冲板片三维构造重建和恢复,可以有效揭示俯冲板片几何学、运动学,为研究深源地震成因、地球深部变化过程和动力学机制提供约束.     

Thermal Structure of the Ionian Slab

We propose a thermal model of the subducting Ionian microplate. The slab sinks in an isothermal mantle, and for the boundary conditions we take into account the relation between the maximum depth of seismicity and the thermal parameter L_th of the slab, which is a product of the age of the subducted lithosphere and the vertical component of the convergence rate. The surface heat-flux dataset of the Ionian Sea is reviewed, and a convective geotherm is calculated in its undeformed part for a surface heat flux of 42 mW m^–2, an adiabatic gradient of 0.6 mK m^–1, a mantle kinematic viscosity of 10¹⁷ m² s^–1 and an asthenosphere potential temperature of 1300°C. The calculated temperature-depth distribution compared to the mantle melting temperature indicates the decoupling limit between lithosphere and asthenosphere occurs at a depth of 105 km and a temperature of 1260°C. A 70–km thick mechanical boundary layer is found. By considering that the maximum depth of the seismic events within the slab is 600 km, a L_th of 4725 km is inferred. For a subduction rate equal to the spreading rate, the corresponding assimilation and cooling times of the microplate are about 7 and 90 Myr, respectively. The thermal model assumes that the mantle flow above the slab is parallel and equal to the subducting plate velocity of 6 cm yr^–1, and ignores the heat conduction down the slab dip. The critical temperature, above which the subduced lithosphere cannot sustain the stress necessary to produce seismicity, is determined from the thermal conditions governing the rheology of the plate. The minimum potential temperature at the depth of the deepest earthquake in the slab is 730°C.     

The effect of the descending lithosphere beneath the Tonga island arc on P-wave travel-time residuals at the Warramunga Seismic Array

P-wave travel-time residuals at the Warramunga Seismic Array (WRA) in the Northern Territory, Australia, have been studied from 49 earthquakes with epicenters south of 19°S in the Fiji-Tonga region. Focal depths are between 42 and 679 km as determined from pP-P. Using the Jeffreys-Bullen and the Herrin travel-time tables the epicentral parameters have been redetermined by considering only “normal” seismic stations in the location procedure. These are those stations where P-wave travel times are probably not affected by lateral heterogeneities caused by the lithosphere descending beneath the Tonga trench. Epicenters of deep earthquakes below 300 km have been relocated by using stations at Δ > 25° only. Epicenters from shallower-depth earthquakes have been recalculated without using stations between 35 < Δ < 75° epicentral distance. In both cases focal depths were determined from pP-P times. The resulting pattern of P-residuals at WRA does not show any significant change with depth below 350 km. The residuals become more negative for shallower earthquakes above about 250 km. P-waves to WRA are advanced by approximately 2 s compared with those from deep earthquakes. The results do not essentially differ for the two different travel-time tables used. The observations can be interpreted by P-wave velocities that are higher in the sinking slab down to 350–400 km by 5±2% than in both the Jeffreys-Bullen and Herrin models. Without considering possible elevations of phase boundaries this estimate yields a temperature contrast of 1000±450°C between slab and normal mantle material in this depth range.     

基于P波三重震相研究青藏高原上地幔速度结构及其动力学意义

青藏高原因其复杂的结构和演化历史,一直都是研究大陆碰撞、构造运动及其动力学的热点区域。本文采用三重震相波形拟合技术,基于中国地震观测台网和大型流动台阵记录到的某地震P波垂向记录,获得了包括拉萨、南羌塘和松潘甘孜地块在内的青藏高原上地幔P波速度结构。结果表明:①拉萨和南羌塘地块下方地幔过渡带存在高速异常,推测是俯冲的印度板片滞留体,过渡带底部的板片残余温度较低,使得660-km相变滞后约3～8km。而松潘甘孜地块下方过渡带同样存在高速异常,可能是欧亚岩石圈发生拆沉进入地幔过渡带所致。这说明印度板块俯冲作用的影响已经到达地幔过渡带,其俯冲前缘位于班公怒江缝合带附近。②从拉萨、南羌塘到松潘甘孜地块,200km之上的地幔岩石圈高速盖层速度由南向北逐渐减小,松潘甘孜地块则出现盖层缺失。推测受小规模地幔对流或者热不稳定性的影响,在南羌塘和松潘甘孜地块,增厚的欧亚岩石圈发生拆沉作用,岩石圈被减薄和弱化,造成羌塘地块上地幔低速和松潘甘孜地块上地幔高速盖层缺失。拆沉的冷的欧亚岩石圈可能部分停留在410-km上方,使得410-km抬升约10km,部分沉入地幔过渡带,表现为松潘甘孜地块地幔过渡带中存在高速异常。低温造成660-km下沉约8km,导致地幔过渡带增厚。      

Deep dynamical processes in the central-southern Qinghai-Tibet Plateau—Receiver functions and travel-time residuals analysis of north Hi-Climb

Teleseismic receiver functions and travel-time residuals along the north Hi-Climb broadband seismic array in the central-southern Qinghai-Tibet Plateau show that the lithosphere structures in the central and western Qinghai-Tibet Plateau are different. In the central Qinghai-Tibet Plateau, the Indian Plate is northward subducted beneath the Qiangtang block and arrives at the greatest depth beneath the central-southern Qiangtang block. The delaminated Indian lithospheric slab remains beneath the central Lhasa block to a depth possibly greater than that of the upper interface of the mantle transform zone. In the western Qinghai-Tibet Plateau, the Indian lithospheric plate is gently northward subducted and may have arrived to the south of Tarim plate. Due to the resistance from the gently northward subduction of the Indian mantle lithosphere in the western Qinghai-Tibet Plateau, the upwelling mantle material be-neath the Qiangtang block moves mostly toward the east to bring about the lateral eastward flow of the deep mantle hot material in the central Qinghai-Tibet Plateau.     

Joint inversion of P- and S-receiver functions and dispersion curves of Rayleigh waves: The results for the Central Anatolian Plateau

The P- and S-wave receiver functions and dispersion curves of the fundamental Rayleigh wave are used to study the lithosphere within the Central Anatolian Plateau. The results for eight broadband seismic stations are presented. It is established that within the plateau, the crust with a thickness of about 35 km is underlain by the mantle lid with its bottom at a depth of about 60 km. The velocities of longitudinal (Vp) and shear (Vs) waves in this layer are at most 7.6 and 4.5 km/s, respectively, and the Vp/Vs ratio is close to 1.7 (i.e., by 6% lower than in the standard IASP91 and PREM models). Such a low velocity ratio is characteristic of rocks having high orthopyroxene content. Beneath the high-velocity mantle lid, the S-wave velocity decreases to 4.0–4.2 km/s and the Vp/Vs ratio is close to its standard value (1.8). At most stations, the P-wave receiver functions do not contain seismic phase P410s, which is formed at the global seismic boundary at a depth of 410 km. The seismic boundary at a depth of 410 km is related to the olivine-spinel phase transformation, and its absence can indicate the anomalously low olivine content and high basalt content. This anomaly is probably associated with the subduction of a large amount of oceanic crust during the closure of the Tethys. The results of the study overall indicate the high informativity of the used method.     

Space Variations of Stress Along the Tyrrhenian Wadati-Benioff Zone

—Gephart and Forsyth’s (1984) algorithm for stress inversion of earthquake fault-plane solutions has been applied to a set of ninety intermediate and deep events occurring in the southern Tyrrhenian region between 1976 and 1995. P- and S-wave data from local seismic networks in southern Italy, the Italian National Network and international bulletins, have been used for hypocenter and focal mechanism computations. Stress inversion runs performed after accurate selection and weighting of fault-plane solutions have allowed us to identify stress space variations at a higher level of detail than available from all previous investigations carried out in the study area. The maximum compressive stress has been shown to follow the depth-decreasing dip of the Wadati-Benioff zone, along the entire zone from a depth of 90 km, to the depth of the deepest events (about 500 km). Variations to such a stress pattern have been found, possibly related to mantle dynamics and the complex composition of the subducting structure. The diffused state of down-dip compression suggests that the Tyrrhenian subduction has already evolved to the point where the lower end of the slab has reached high-strength mantle materials, the load of the excess mass is entirely supported from below and most of the subducted slab is under compression. In agreement with the lack of large, shallow thrusting events in the immersion zone, the findings of the present study appear to agree well with geodynamic models assuming a passive subduction process with eastward roll-back of the Ionian lithosphere in the study area. In this context, the depth-decrease of the slab dip may also find a reasonable explanation.     

Seismic scattering at the top of the mantle Transition Zone

We document strong seismic scattering from around the top of the mantle Transition Zone in all available high resolution explosion seismic profiles from Siberia and North America. This seismic reflectivity from around the 410 km discontinuity indicates the presence of pronounced heterogeneity in the depth interval between 320 and 450 km in the Earth’s mantle. We model the seismic observations by heterogeneity in the form of random seismic scatterers with typical scale lengths of kilometre size (10-40 km by 2-10 km) in a 100-140 km thick depth interval. The observed heterogeneity may be explained by changes in the depths to the α-β-γ spinel transformations caused by an unexpectedly high iron content at the top of the mantle Transition Zone. The phase transformation of pyroxenes into the garnet mineral majorite probably also contributes to the reflectivity, mainly below a depth of 400 km, whereas we find it unlikely that the presence of water or partial melt is the main cause of the observed strong seismic reflectivity. Subducted oceanic slabs that equilibrated at the top of the Transition Zone may also contribute to the observed reflectivity. If this is the main cause of the reflectivity, a substantial amount of young oceanic lithosphere has been subducted under Siberia and North America during their geologic evolution. Subducted slabs may have initiated metamorphic reactions in the original mantle rocks.     

Siberian traps: Hypotheses and seismology data

Siberian traps are the result of huge basalt eruptions which took place about 250 Ma ago over a vast territory of Siberia. The genesis of Siberian traps is attributed to a mantle plume with a center in the region of Iceland or beneath the central Urals in terms of their present coordinates. The eruption mechanism is associated with delamination—replacement of the mantle lithosphere by the deep magma material. The receiver function analysis of the records from the Norilsk seismic station (NRIL) allows comparing these hypotheses with the factual data on the depth structure of the region of Siberian traps. The S-wave velocity section place the seismic lithosphere/asthenosphere boundary (LAB) at a depth of 155–190 km, commensurate with the data for the other cratons. The mantle lithosphere has a high S-wave velocity characteristic of cratons (4.6–4.8 km/s instead of the typical value 4.5 km/s). The seismic boundary, which is located at a depth around 410 km beneath the continents is depressed by ~10 km in the region of the NRIL station. The phase diagram of olivine/wadsleyite transformation accounts for this depression by a 50–100°С increase in temperature. At the depths of 350–400 km, the S-wave velocity drops due to partial melting. A new reduction in the S-wave velocities is observed at a depth of 460 km. The similar anomalies (deepening of the 410-km seismic boundary and low shear wave velocity at depths of 350–400 and 460–500 km, respectively) were previously revealed in the other regions of the Meso-Cenozoic volcanism. In the case of a differently directed drift of the Siberian lithosphere and underlying mantle at depths down to 500 km, these anomalies are barely accountable. In particular, if the mantle at a depth ranging from 200 to 500 km is fixed, the anomalies should be observed at the original locations where they emerged 250 Ma ago, i.e. thousands of km from the Siberian traps. Our seismic data suggest that despite the low viscosity of the asthenosphere, the mantle drift at depths ranging from 200 to 500 km is correlated with the drift of the Siberian lithospheric plate. Furthermore, the position of the mantle plume beneath the Urals is easier to reconcile with the seismic data than its position beneath Iceland because of the Siberian traps being less remote from the Urals.     

帕米尔—兴都库什深俯冲残留体对410km间断面起伏形态的影响

受俯冲残留体影响的410km间断面起伏形态的研究对于确定地球内部物质构成及地球动力学过程具有重要作用.帕米尔—兴都库什俯冲区域拥有全球少有的中、深源地震,为研究410km间断面起伏提供了良好的资源.利用日本Hi-net地震台网和美国TA台阵记录的帕米尔—兴都库什俯冲区域的6个震源深度为154.0~220.9km、震级为Mb5.6~6.4的中、深源地震的短周期/宽频带波形资料,经过4次根倾斜叠加处理,获得了36组Hi-net子台网和TA记录资料的倾斜叠加灰度图,从中提取了与410km间断面相关的次生转换震相SdP,发现受俯冲残留体影响下的410km间断面的深度位于372~398km.较之持续俯冲的西太平洋地区海洋岩石圈,研究区域俯冲滞留体对于410km间断面的相变线的影响要小得多.     

Olivine-spinel Phase Boundary Rheology, Strength and Deep Seismicity

In this paper,we discuss (1) whether a significant change in dominant creep mechanism will occur at 400 km discontinuity in some subducting slabs as a result of olivine-spinel phase transition;(2) could the result influence phase boundary strength and deep seismicity? Through this study,we noticed that a transition of creep mechanism from dislocation to diffusion (or superplasticity) could occur at the olivine-spinel phase boundary where temperature effect on phase transition dominates over pressure,which will result in a weakening phase boundary.Triggered by this phase transition weakening,a deep strong earthquake might be generated in the relatively strengthening region above the phase boundary so that the phase boundary is naturally the ceasing boundary of deep seismicity.Contrasting to this,the transition of the creep mechanism from dislocation glide to dislocation climb may be common at the phase boundary where pressure effect on phase transition dominates over temperature.In this case,olivine-spin     

Passive Seismology and Deep Structure in Central Italy

—In the last decade temporary teleseismic transects have become a powerful tool for investigating the crustal and upper mantle structure. In order to gain a clearer picture of the lithosphere-asthenosphere structure in peninsular Italy, between 1994 and 1996, we have deployed three teleseismic transects in northern, central, and southern Apennines, in the framework of the project GeoModAp (European Community contract EV5V-CT94–0464). Some hundreds of teleseisms were recorded at each deployment which lasted between 3 and 4 months. Although many analyses are still in progress, the availability of this high quality data allowed us to refine tomographic images of the lithosphere-asthenosphere structure with an improved resolution in the northern and central Apennines, and to study the deformation of the upper mantle looking at seismic anisotropy through shear-wave splitting analysis. Also, a study of the depth and geometry of the Moho through the receiver function technique is in progress. Tomographic results from the northernmost 1994 and the central 1995 teleseismic experiments confirm that a high-velocity anomaly (HVA) does exist in the upper 200–250 km and is confined to the northern Apenninic arc. This HVA, already interpreted as a fragment of subducted lithosphere is better defined by the new temporary data, compared to previous works, based only on data from permanent stations. No clear high-velocity anomalies are detected in the upper 250 km below the central Apennines, suggesting either a slab window due to a detachment below southern peninsular Italy, or a thinner, perhaps continental slab of Adriatic lithosphere not detectable by standard tomography. We found clear evidence of seismic anisotropy in the uppermost mantle, related to the main tectonic processes which affected the studied regions, either NE–SW compressional deformation of the lithosphere beneath the mountain belt, or arc-parallel asthenospheric flow (both giving NW–SE fast polarization direction), and successive extensional deformation (～E–W trending) in the back-arc basin of northern Tyrrhenian and Tuscany. Preliminary results of receiver function studies in the northern Apennines show that the Moho depth is well defined in the Tyrrhenian and Adriatic regions while its geometry underneath the mountain belt is not yet well constrained, due to the observed high complexity.     

Variation of seismic source parameters and stress drops within a descending slab and its implications in plate mechanics

A least-squares searching technique has been developed to estimate the source dimensions of intermediate and deep focus earthquakes using azimuthal variations of body wave pulse-widths. With this method and also amplitude data, modes of rupture propagation, seismic moments, and stress drops of 17 intermediate and deep focus earthquakes in the Tonga-Kermadec region have been determined in order to investigate variations in source properties and the state of stress within the descending slab there. Three different modes of rupture; unilateral, bilateral, and circular faults, are compared and tested against observations. Results indicate that the unilateral fault is the best model for most of the earthquakes studied. Stress drops of the 17 events vary within a very large range, from 20 bar to about 4.6 kbar, and change significantly with depth. The magnitude of stress drops for depths between 220 and 430 km is very much higher than at shallower depths. This change in stress drop magnitude at about 220 km-depth seems to reflect a change in material properties both in the mantle and within the slab. Two regions of high stress drop are observed at depths of about 360 and 640 km. A relative minimum of stress drop is found at about 450–560 km where the earthquake frequency is particularly high. Earthquakes at the northern end to the Tonga arc, where the Benioff zone is laterally bent, show systematically higher stress drops than other events at comparable depths, but away from the bend. Also, events in regions of low seismicity appear to have higher stress drops than those in regions of high seismicity. The upper bound of seismic efficiency is found to decrease with depth, implying an increase of frictional force with depth at the earthquake source.     

The influence of trench migration on slab penetration into the lower mantle

A two-dimensional numerical convection model in cartesian geometry is used to study the influence of trench migration on the ability of subducted slabs to penetrate an endothermic phase boundary at 660 km depth. The transient subduction history of an oceanic plate is modelled by imposing plate and trench motion at the surface. The viscosity depends on temperature and depth. A variety of styles of slab behaviour is found, depending predominantly on the trench velocity. When trench retreat is faster than 2–4 cm/a, the descending slab flattens above the phase boundary. At slower rates it penetrates straight into the lower mantle, although flattening in the transition zone may occur later, leading to a complex slab morphology. The slab can buckle, independent of whether it penetrates or not, especially when there is a localised increase in viscosity at the phase boundary. Flattened slabs are only temporarily arrested in the transition zone and sink ultimately into the lower mantle. The results offer a framework for understanding the variety in slab geometry revealed by seismic tomography.     

Formation time of the big mantle wedge beneath eastern China and a new lithospheric thinning mechanism of the North China craton—Geodynamic effects of deep recycled carbon

High-resolution P wave tomography shows that the subducting Pacific slab is stagnant in the mantle transition zone and forms a big mantle wedge beneath eastern China. The Mg isotopic investigation of large numbers of mantle-derived volcanic rocks from eastern China has revealed that carbonates carried by the subducted slab have been recycled into the upper mantle and formed carbonated peridotite overlying the mantle transition zone, which becomes the sources of various basalts. These basalts display light Mg isotopic compositions(δ26 Mg = –0.60‰ to –0.30‰) and relatively low87 Sr/86 Sr ratios(0.70314–0.70564) with ages ranging from 106 Ma to Quaternary, suggesting that their mantle source had been hybridized by recycled magnesite with minor dolomite and their initial melting occurred at 300-360 km in depth. Therefore, the carbonate metasomatism of their mantle source should have occurred at the depth larger than 360 km, which means that the subducted slab should be stagnant in the mantle transition zone forming the big mantle wedge before 106 Ma. This timing supports the rollback model of subducting slab to form the big mantle wedge. Based on high P-T experiment results, when carbonated silicate melts produced by partial melting of carbonated peridotite was raising and reached the bottom(180–120 km in depth) of cratonic lithosphere in North China, the carbonated silicate melts should have 25–18 wt% CO2 contents, with lower Si O2 and Al2 O3 contents, and higher Ca O/Al2 O3 values, similar to those of nephelinites and basanites, and have higher εNdvalues(2 to 6). The carbonatited silicate melts migrated upward and metasomatized the overlying lithospheric mantle, resulting in carbonated peridotite in the bottom of continental lithosphere beneath eastern China. As the craton lithospheric geotherm intersects the solidus of carbonated peridotite at 130 km in depth, the carbonated peridotite in the bottom of cratonic lithosphere should be partially melted, thus its physical characters are similar to the asthenosphere and it could be easily replaced by convective mantle. The newly formed carbonated silicate melts will migrate upward and metasomatize the overlying lithospheric mantle. Similarly, such metasomatism and partial melting processes repeat, and as a result the cratonic lithosphere in North China would be thinning and the carbonated silicate partial melts will be transformed to high-Si O2 alkali basalts with lower εNdvalues(to-2). As the lithospheric thinning goes on,initial melting depth of carbonated peridotite must decrease from 130 km to close 70 km, because the craton geotherm changed to approach oceanic lithosphere geotherm along with lithospheric thinning of the North China craton. Consequently, the interaction between carbonated silicate melt and cratonic lithosphere is a possible mechanism for lithosphere thinning of the North China craton during the late Cretaceous and Cenozoic. Based on the age statistics of low δ26 Mg basalts in eastern China, the lithospheric thinning processes caused by carbonated metasomatism and partial melting in eastern China are limited in a timespan from 106 to25 Ma, but increased quickly after 25 Ma. Therefore, there are two peak times for the lithospheric thinning of the North China craton: the first peak in 135-115 Ma simultaneously with the cratonic destruction, and the second peak caused by interaction between carbonated silicate melt and lithosphere mainly after 25 Ma. The later decreased the lithospheric thickness to about70 km in the eastern part of North China craton.     

Effects of relative plate motion on the deep structure and penetration depth of slabs below the Izu-Bonin and Mariana island arcs

An increasing number of seismological studies indicate that slabs of subducted lithosphere penetrate the Earth's lower mantle below some island arcs but are deflected, or, rather, laid down, in the transition zone below others. Recent numerical simulations of mantle flow also advocate a hybrid form of mantle convection, with intermittent layering. We present a multi-disciplinary analysis of slab morphology and mantle dynamics in which we account explicitly for the history of subduction below specific island arcs in an attempt to understand what controls lateral variations in slab morphology and penetration depth. Central in our discussion are the Izu-Bonin and Mariana subduction zones. We argue that the differences in the tectonic evolution of these subduction zones—in particular the amount and rate of trench migration—can explain why the slab of subducted oceanic lithosphere seems to be (at least temporarily) stagnant in the Earth's transition zone below the Izu-Bonin arc but penetrates into the lower mantle below the Mariana arc. We briefly speculate on the applicability of our model of the temporal and spatial evolution of slab morphology to other subduction zones. Although further investigation is necessary, our tentative model shows the potential for interpreting seismic images of slab structure by accounting for the plate-tectonic history of the subduction zones involved. We therefore hope that the ideas outlined here will stimulate and direct new research initiatives.     

Seismicity pattern: an indicator of source region of volcanism at convergent plate margins

The results of detailed investigation into the geometry of distribution of earthquakes around and below the volcanoes Korovin, Cleveland, Makushin, Yake-Dake, Oshima, Lewotobi, Fuego, Sangay, Nisyros and Montagne Pelée at convergent plate margins are presented. The ISC hypocentral determinations for the period 1964-1999, based on data of global seismic network and relocated by Engdahl, van der Hilst and Buland, have been used.The aim of this study has been to contribute to the solution of the problem of location of source regions of primary magma for calc-alkaline volcanoes spatially and genetically related to the process of subduction. Several specific features of seismicity pattern were revealed in this context. (i) A clear occurrence of the intermediate-depth aseismic gap (IDAG) in the Wadati-Benioff zone (WBZ) below all investigated active volcanoes. We interpret this part of the subducted slab, which does not contain any teleseismically recorded earthquake with magnitude greater than 4.0, as a partially melted domain of oceanic lithosphere and as a possible source of primary magma for calc-alkaline volcanoes. (ii) A set of earthquakes in the shape of a seismically active column (SAC) seems to exists in the continental wedge below volcanoes Korovin, Makushin and Sangay. The seismically active columns probably reach from the Earth surface down to the aseismic gap in the Wadati-Benioff zone. This points to the possibility that the upper mantle overlying the subducted slab does not contain large melted domains, displays an intense fracturing and is not likely to represent the site of magma generation. (iii) In the continental wedge below the volcanoes Cleveland, Fuego, Nisyros, Yake-Dake, Oshima and Lewotobi, shallow seismicity occurs down to the depth of 50 km. The domain without any earthquakes between the shallow seismically active column and the aseismic gap in the Wadati-Benioff zone in the depth range of 50-100 km does not exclude the melting of the mantle also above the slab. (iv) Any earthquake does not exist in the lithospheric wedge below the volcano Montagne Pelée. The source of primary magma could be located in the subducted slab as well as in the overlying mantle wedge. (v) Frequent aftershock sequences accompanying stronger earthquakes in the seismically active columns indicate high fracturing of the wedge below active volcanoes. (vi) The elongated shape of clusters of epicentres of earthquakes of seismically active columns, as well as stable parameters of the available fault plane solutions, seem to reflect the existence of dominant deeply rooted fracture zones below volcanoes. These facts also favour the location of primary magma in the subducting slab rather than in the overlying wedge.We suppose that melts advancing from the slab toward the Earth surface may trigger the observed earthquakes in the continental wedge that is critically pre-stressed by the process of subduction. However, for definitive conclusions it will be necessary to explain the occurrence of earthquake clusters below some volcanoes and the lack of seismicity below others, taking into account the uncertainty of focal depth determination from global seismological data in some regions.     

Composition,temperature, and thickness of the lithosphere of the Archean Kaapvaal craton

A new model is proposed for the structure of the Kaapvaal craton lithosphere. Based on chemical thermodynamics methods, profiles of the chemical composition, temperature, density, and S wave velocities are constructed for depths of 100–300 km. A solid-state zone of lower velocities is discovered on the S velocity profile in the depth interval 150–260 km. The temperature profiles are obtained from absolute values of P and S velocities, taking into account phase transformations, anharmonicity, and anelastic effects. The examination of the sensitivity of seismic models to the chemical composition showed that relatively small variations in the composition of South African xenoliths result in lateral temperature variations of ～200°C. Inversion of some seismic profiles (including IASP91) with a fixed bulk composition of garnet peridotites (the primitive mantle material) leads to a temperature inversion at depths of 200–250 km, which is physically meaningless. It is supposed that the temperature inversion can be removed by gradual fertilization of the mantle with depth. In this case, the craton lithosphere should be stratified in chemical composition. The depleted lithosphere composed by garnet peridotites exists to depths of 175–200 km. The lithospheric material at depths of 200–250 km is enriched in basaltoid components (FeO, Al₂O₃, and CaO) as compared with the material of garnet peridotites but is depleted in the same components as compared with the fertile substance of the underlying primitive mantle. The material composing the craton root at a depth of ～275 km does not differ in its physical and chemical characteristics from the composition of the normal mantle, and this allows one to estimate the thickness of the lithosphere at 275 km. The results of this work are compared with data of seismology, thermal investigations, and thermobarometry.     

马尼拉俯冲带的地震层析成像研究

基于国际地震中心的P波走时数据和层析成像反演方法,获得了具有较高分辨率的马尼拉俯冲带的深部速度模型.结果表明,（1）高速的南海俯冲板片沿马尼拉俯冲带的俯冲形态随纬度发生变化,在14°N和16°N之间,板片俯冲角度较大,俯冲深度可达400~500 km,在17°N附近,俯冲板片角度和深度较南部变小,而在18°N附近,俯冲板片以近垂直角度俯冲到地幔转换带;（2）17°N和18°N之间俯冲角度的变化意味着南海板片发生了撕裂;（3）在14°N附近,南海板片由300 km以上的近垂直俯冲转为200~300 km深度的近水平展布,与震源分布存在较大的差异,表明南海板片发生了撕裂,并且导致410 km间断面抬升.根据成像结果计算的不同位置南海板片的俯冲长度和时间表明,南海板片俯冲之前的面积为现今面积的两倍,14°N最先开始发生俯冲,并由南向北扩展.