Focal mechanisms of intraplate earthquakes in Bolivia,South America

Intraplate seismic activity in Bolivia is mainly located in the central region (16°–19°S, 63°–67°W) which includes the East Andean Cordillera and the Sub-Andean Sierras. At this region there is a bend in the trend of the main geological structures from NW-SE in the north to N-S in the south. Focal mechanisms have been calculated for 10 earthquakes of magnitudes 4.9–5.6, using first motionP-waves from long period instruments. Their solutions correspond to reverse faulting, some with a large component of strike-slip motion. Their solutions can be grouped into two types; one with pure reverse faulting on planes with azimuth NW-SE and the other with a large strike-slip component on planes with azimuths nearly N-S or WNW-ESE. The maximum stress axis (P-axis) is practically horizontal (dipping less than 5°) oriented in a mean N56°E direction. This orientation may be related with the direction of compression resulting from the collision of the Nazca plate against the western margin of the South American continent. Wave-form analysis of long-periodP-waves for one event restricts the focal depth to 8 km in the Sub-Andean region. Seismic moments and source dimensions determined from spectra of Rayleigh waves are in the range of 10¹⁶–10¹⁷Nm and 17–24 km, respectively. The Central Bolivia region can be considered as a zone of intraplate deformation situated between the Bolivian Altiplano and the Brazil shield.     

Anisotropy of P-wave Velocity in the Upper Crust of the Central External Dinarides

The anisotropy of Pg-wave velocity in the area of the central External Dinarides is measured by using arrival time data as reported by local and regional seismological stations. The observed velocity varies between 5.73 km/s (in the ESE-WNWdirection) and 6.20 km/s (in the SSW-NNE direction), indicating azimuthal anisotropy with symmetry axis azimuths of 23°±n 90°. These closely match the orientation of the principal stress axes in the region, as revealed by analyses of available fault-plane solutions. The observed anisotropy may be modelled by assuming a system of vertical/subvertical cracks in the upper crust, aligned under the influence of the regional tectonic stress field.     

Variations in the distribution of magma in the lower crust and at the Moho beneath the East Pacific Rise at 9°-10°N

Measurements of the seafloor deformation under ocean waves (compliance) reveal an asymmetric lower crustal partial melt zone (shear velocity less than 1.8 km/s) beneath the East Pacific Rise axis between 9° and 10°N. At 9°48′N, the zone is less than 8 km wide and is centered beneath the rise axis. The zone shifts west of the rise axis as the rise approaches the westward-stepping 9°N overlapping spreading center discontinuity and is anomalously wide at the northern tip of the discontinuity. The ratio of the compliance determined shear velocity to the compressional velocities (estimated by seismic tomography) suggests that the melt is well-connected in high-aspect ratio cracks rather than in isolated sills. The shear and compressional velocities indicate less than 18% melt in the lower crust on average. The compliance measurements also reveal a separate lower crustal partial melt zone 10 km east of the rise axis at 9°48′N and isolated melt bodies near the Moho beneath four of the 39 measurement sites (three on-axis and one off-axis). The offset of the central melt zone from the rise axis correlates strongly with the offset of the overlying axial melt lens and the inferred center of mantle melting, but its shape appears to be controlled by crustal processes.     

Changes of Ozone Content at Middle Latitudes During Forbush Decreases in Cosmic Rays

The variations of total ozone at Alma-Ata (43°N, 76 °E) and ozone profiles obtained by balloon sounding at Tateno (36°N, 140°E), Wallops Island (38°N, 75°W) and Cagliari (39°N, 9°E) in the periods of Forbush decreases (FD) in galactic cosmic rays have been analysed. A decrease of total ozone was observed in the initial stage of the FD and an increase 10–11 days later. The average total deviations calculated using the superposed epoch method for 9 FD events are equal to 30 D. U. in the positive and to –18 D. U. in the negative phase. The changes of average ozone profiles, associated with 26 FD events, are more significant in the lower stratosphere and upper troposphere. The decrease of the partial ozone pressure at a height of 12–15 km is about 30 mb. These vertical variations of ozone coincide with the average changes of the respective temperature profiles. A cooling, on the average, of 3°C was observed at 12–15 km, and a heating of 4°C below this level.     

Wavefield features of teleseismic receiver function in laterally inhomogeneous media

IntroductionSince1980(s,theteleseismicreceiverfunctionhasbeenbroadlyappliedtoexploringthecrustalanduppermantlestructure.Thatthismethoddrawssomanyattentionsisnotonlybecauseitscheapercostandhighverticalresolution,butalsobecauseitcanbeusedforexploringthelateralvariationofthecrustalstructure.IntheUSnationalresearchprogramofthecontinentaldynamicsmuchemphasiswasgiventothereceiverfunctionmethod(Phinney,1989).Uptonow,the1-Dreceiverfunctioninversiontechniquehasbecomesophisticatedandmanypracticalresul…     

Quasi-Rayleigh Waves in Transversely Isotropic Half-Space with Inclined Axis of Symmetry

A method for determination of characteristics of quasi-Rayleigh (qR) wave in a transversely isotropic homogeneous half-space with inclined axis of symmetry is outlined. The solution is obtained as a superposition of qP, qSV and qSH waves, and surface wave velocity is determined from the boundary conditions at the free surface and at infinity, as in case of Rayleigh wave in an isotropic half-space. Though the theory is simple enough, a numerical procedure for calculation of surface wave velocity presents some difficulties. The difficulty is caused by necessity to calculate complex roots of a non-linear equation, which in turn contains functions determined as roots of non-linear equations with complex coefficients. Numerical analysis shows that roots of the equation corresponding to the boundary conditions do not exist in the whole domain of azimuths and inclinations of the symmetry axis. The domain of existence of qR wave depends on the ratio of the elastic parameters: for some strongly anisotropic models the wave cannot exist at all. For some angles of inclination qR-wave velocities deviate from those calculated on the basis of the perturbation method valid for weak anisotropy, though they have the same tendency of variation with azimuth. The phase of qR wave varies with depth unlike Rayleigh wave in an isotropic half-space. Unlike Rayleigh wave in an isotropic half-space, qR wave has three components - vertical, radial and transverse. Particle motion in horizontal plane is elliptic. Direction of the major axis of the ellipsis coincides with the direction of propagation only in azimuths 0° (180°) and 90° (270°).     

横向非均匀介质远震体波接收函数的波场特征

系统研究了台站下方存在倾斜界面时,远震体波接收函数的波场特征及多方位接收函数对台站下方介质横向非均匀特征的反映能力．我们利用合成三维横向非均匀介质接收函数的Maslov方法,具体模拟了台站下方存在倾斜界面时的接收函数响应．结果表明,当台站下方存在倾向一致的倾斜界面时,远震体波接收函数的径向分量和切向分量分别随震源方位角的变化呈现对称和反对称的规律性变化．利用不同震源方位角接收函数径向分量和切向分量的变化规律,可以估计台站下方界面的倾向和倾角．当台站下方各界面的倾向不同时,随方位角的变化,接收函数只能直观给出界面的整体倾向．实际观测数据的分析结果表明,对于利用单个台站接收函数研究台站下方介质的横向非均匀特征来说,简单倾斜界面可以看作较好的一级近似．      

Slant-stack velocity analysis for one-dimensional upper mantle structure using short-period data from MAJO

In this paper, regionalP-wave upper mantle structure is investigated using slant-stack velocity analysis of short-period earthquake data recorded at station MAJO (Matsushiro, Japan). Shallow earthquakes from 1980–1986 within 35° of MAJO are used to construct a common receiver gather. Processing of the wavefield data includes focal depth and static time corrections, as well as deterministic deconvolution, in order to equalize pulse shapes and align wavelets on the first arrivals. The processed wavefield data are slant stacked and interatively downward continued to obtain a regional upper mantle velocity model. The model includes a low velocity zone between 107 and 220 km. Beneath the LVZ, the velocity increases smoothly down to the discontinuity at 401 km. In the transition zone, the velocity model again increases linearly, although there is some suggestion of further complexity in the downward continued wavefield data. At the base of the transition zone, a second velocity discontinuity occurs at 660 km, with a linear velocity gradient below. In addition to slant-stack analysis, travel times and synthetic seismograms are computed and compared with the processed and unprocessed wavefield data.     

Source model of Noto-Hanto-Oki earthquake tsunami of 7 February 1993

A source model was discussed for a small tsunami accompanied by the Noto-Hanto-Oki earthquake (M _s 6.6), striking Japan on 7 February, 1994. Assuming a fault model under the sea bottom, we estimated the focal parameters jointly, using synthesized tsunami source spectra as well as the tsunami numerical simulation. The fault proposed by this study consists of a plane sized 15×15 km, dipping N47°W with the dip angle of 42°, which is almost pure reverse fault (slip angle 87°) with a dislocation of 1 meter. The numerical simulation shows that the shallow sea in the source region caused a comparatively long recurring tsunami (the periods are 12–18 minutes) in spite of its small size. The model fault is corresponding to an aftershock area of this earthquake.     

Seismic Anisotropy Observations in the Mexicali Valley,Baja California,México

A study of seismic anisotropy was performed using data from earthquakes of the Mexicali Valley. The investigated region encompasses the Cerro Prieto Geothermal Field (CPGF), one of the most important fields in the world. The results showed that at most of our stations the average polarization directions of the fast S waves range from N14°W to N17°E. A N-S polarization direction was obtained for the whole area by averaging the polarization directions from all stations used. In terms of the EDA hypothesis, this average trend agrees with the postulated state of stress for southern California, and with fault plane solutions for events of the Mexicali Valley. Notorious deviations from the N-S global trend were found southeast of the CPGF, with polarization trends between N25°E and N67°E, and in the geothermal field, with polarization directions between N7°W and N14°W. The polarization results for these zones indicated stress conditions that are different from the more regional stress pattern. The delay times that were measured between the fast and slow shear waves reached values of up to 0.6 sec, with a mean value of 0.35 sec. Consistent with our polarization results, the larger delay times (0.2–0.6 sec) were found in the CPGF. Smaller or null values were observed at the periphery of the study area. No temporal trends in the delay times were apparent, as shown by data from the two stations that recorded the larger number of events. Overall, we conclude that the splitting effects of this study result from a shallow anisotropy volume. The splitting results are thus interpreted as caused by the preferred orientation of vertical fluid-filled microcracks aligned in a direction that is parallel to the regional stress field. The stronger splitting effects that were observed in the area of the CPGF were found consistent with the geothermal reservoir that is embedded in the sedimentary cover of the zone, at depths of 1 to 4–5 km from the surface. We thus believe that such marked splitting effects have a direct relation with the reservoir of the CPGF.We are grateful to Miguel Navarro, Tito Valdez, and Manuel Luna for their contribution in the operation of RANM and for processing and cataloguing the strong-motion data. Ignacio Méndez and Francisco Farfán helped us with data from the RESNOM system. The study benefited from funding provided by CICESE and from grants awarded by CONACYT to Luis Munguía (Grants F195T and PCCNCNA-031339).     

Climatological monthly characteristics of middle atmosphere gravity waves (10 min-10 h) during 1979–1993 at Saskatoon

Saskatoon (52° N, 107°W) medium frequency (MF) radar data from 1979 to 1993 have been analyzed to investigate the climatology of irregular wind components in the height region 60–100 km. This component is usually treated in terms of internal gravity waves (IGW). Three different band-pass filters have been used to separate the intensities of IGWs having periods 0.2-2.5; 1.5-6 and 2–10 h, respectively. Height, seasonal and inter-annual variations of IGW intensities, anisotropy and predominant directions of propagation are investigated. Mean over 14 years’ seasonal variation of the intensity of long-period IGWs shows a dominant annual component with winter maximum and summer minimum. Seasonal variations of the intensity of short-period waves have a strong semi-annual component as well, which forms a secondary maximum in summer. Predominant azimuths of long-period IGWs are generally zonal, though they vary with season. For short-period IGWs, the predominant azimuth is closer to the meridional direction. Anisotropy of IGW intensity is larger in summer, winter and at lower altitudes. The IGW intensity shows apparent correlation with both solar and geomagnetic activity. In most cases, this correlation appears to be negative. The variations versus solar activity is larger for longer-period IGW. Possible reasons and consequences of the observed climatological variations of IGW intensity are discussed.     

Some results of short wave insolation measurements during the IGY along the 10° E meridian and a comparison with theoretical values

Summary The observational material of 138 stations, which had during the July 1957 to December 1958 IGY period made obervations of global radiation along the longitude 10°E (±10°) between the latitudes 47°S and 70°N, had been evaluated to obtain mean monthly and annual totals of global radiation for 12 different latitudes along thea/n-meridian. The sums are compared with theoretical results obtained byBudyko, Black, andBernhardt andPhilipps. These theoretically computed values are drawn graphically or numerically from charts or tables of thea/n-authors for the coordinates corresponding to the means of the relevant IGY stations. In hardly a case the measured annual totals deviate by more than 10% from theoretical values. In hardly a case the measured annual totals deviate by more than 10% from theoretical values. TheBudyko data are the nearest approximation to the measured ones;Black's computation shows the greatest deviation. In the 0 to 47° S zone theoretical values are actually exceeded by to 20%.The annual variation is marked by the following deviations as compared to the theoretical results: In the region north to 50° latitude in October, November and January only 80% of the theoretically computed amount of radiation was observed, while in the remaining months the monthly totals are within a ±10% boundary of the theoretical values; in March about 20% above normal global radiation was observed. Between 30° and 50°N the measured data in autumn, winter and spring 10 to 20% below the theoretical values, in the remaining time of the year the deviation is less than ±10%. A comparison with cloud conditions (relative sunshine duration) suggests that deviations cannot be interpreted merely by deviating cloud conditions of this zone. Between 0° and 30°N only January and February are 10% below theoretical values, while in August to October the totals were 10 to 15% above. South of the equator from October to May (southern summer) the radiation totals were too high by up to 25%, while in the remaining time of the year the data were about 10% below theoretical values; it should be noted that deviation increases with increasing southern latitude. For comparison theAshbel IGY global radiation charts were used which indicate similar deviations from theoretical values; there are also some differences to microcard values.There is reason to suggest that there is a complex relation between the observed deviations and variation of the large-scale circulation during IGY. The results of the small number of available turbidity measurements (Central European area only) can be interpreted in the same way. To what extent the observed deviations from theoretical values are due to weather conditions deviating during the IGY period from normal or originate from inconsistencies in the theoretical consideration cannot definition be said until inspection of the complete IGY cloud and turbidity data and the availability of the climatological material which served for the theoretical work. The latter was only the case in theBernhardt andPhilipps paper.

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Zusammenfassung Aus 138 Stationen auf dem Meridian 10°E (±10°) zwischen –47° und +70° Breite, die während der IGJ-Periode die Globalstrahlung beobachtet haben, wurden mittlere Monats-und Jahressummen der Globalstrahlung für 12 verschiedene Breiten auf diesem Meridian berechnet. Diese Summen wurden mit den theoretischen Werten vonBudyko, Black, undBernhardt undPhilipps verglichen. Die gemessenen Summen weichen meist um höchstens 10% von den theoretischen Werten ab. Die Werte vonBudyko kommen den gemessenen am nächsten, die vonBlack zeigen die grössten Abweichungen. Für die Zone zwischen 0° und 47° südlicher Breite liegen die gemessenen Werte jedoch durchwegs um 8 bis 20% über den theoretischen. Im jahreszeitlichen Verlauf traten folgende bemerkenswerte Abweichungen gegen die theoretischen Werte auf: Nördlich 50° Breite erhielten die Monate Oktober, November und Januar nur etwa 80% der theoretischen Strahlungssummen, in den anderen Monaten stimmten die Summen innerhalb 10% mit den theoretischen Werten überein, der März erhielt etwa 20% übernormale Globalstrahlung. Zwischen 30° und 50° nördlicher Breite lagen die gemessenen Werte besonders im Herbst, Winter und Frühling 10 bis 20% unter den theoretischen. Die Abweichungen können nicht allein der Bewölkung in dieser Zone zugeschrieben werden. Südlich des Äquators wiesen die Monate Oktober bis Mai (Südsommer!) bis zu 25% zu hohe Strahlungssummen auf. Die zum Vergleich herangezogenen Karten vonAshbel ergeben ähnliche Abweichungen gegen die theoretischen Werte, wenn auch einige Unterschiede gegen die hier veröffentlichten Summen auftreten.Ein komplexer Zusammenhang zwischen diesen Abweichungen und der grossräumigen Zirkulation im IGJ scheint zu bestehen. Auch die wenigen Trübungsmessungen (nur mitteleuropäische Stationen) können im gleichen Sinne gedeutet werden. Inwieweit die beobachteten Abweichungen der Strahlung von den theoretischen Werten auf den im Mittel abweichenden Witterungsverhältnissen der IGJ-Periode beruhen oder auf Unstimmigkeiten der theoretischen Werte, lässt sich aus dem vorliegenden Material noch nicht definitiv entscheiden.

     

乌鲁木齐地区现今构造应力场综合分析

乌鲁木齐地处天山中段.震源机制解研究表明,北天山中段区域主压应力方向为N10°E左右,且具有自西向东逐渐东偏的特点.在乌鲁木齐地区,由中强地震震源机制解反演的主压应力方向为N15°-20°E;由断层滑动资料反演的乌鲁木齐周边构造应力场的主压应力方向为N17°W-N2°E.上述两种资料反演的乌鲁木齐构造应力场主压应力方向...     

Preliminary stress measurements in central California using the hydraulic fracturing technique

Use of the hydraulic fracturing technique for determiningin situ stress is reviewed, and stress measurements in wells near the towns of Livermore, San Ardo, and Menlo Park, California are described in detail. In the Livermore well, four measurements at depths between 110 and 155 m indicate that the least principal compressive stress is horizontal and increases from 1.62 to 2.66 MPa. The apparent direction of maximum compression is N 70° E (±40°). At the San Ardo site the least principal stress is that due to the overburden weight. At depths of 240.2 and 270.7 m the minimum and maximum horizontal stresses are estimated to be 11.4 and 22.5 MPa, and 12.0 (±1.1) and 15.8 (±3.3) MPa, respectively. From an impression of the fracture at 240.2 m, the direction of maximum compression appears to be about N 15° E. The rock in the Menlo Park well is too highly fractured to yield a reliable measurement of the horizontal stresses. The data indicate, however, that the least principal stress is vertical (due to the overburden weight) to a depth of 250 m.     

三维倾斜界面PS转换波CMP道集时距及参数估计

在PS转换波资料处理过程中,往往需要联合P波资料提供相应的模型.在实际应用中存在P波和PS转换波层位对比困难.本文仅利用PS转换波数据,通过三维倾斜界面PS转换波CMP道集精确时距关系推导了近似时距解析表达式;分析了PS波的精确与近似时距关系随测线方位、界面倾角与倾向的变化规律及其拟合误差;并讨论了近似时距关系的三个时距参数随方位的变化特征;理论上给出描述时距的三维倾斜界面倾角、倾向、深度、纵波速度和横波速度这5个独立参数的估计方法,并通过理论模拟数据证明了该方法的可行性.     

Receiver function study in northern Sumatra and the Malaysian peninsula

In this receiver function study, we investigate the structure of the crust beneath six seismic broadband stations close to the Sunda Arc formed by subduction of the Indo-Australian under the Sunda plate. We apply three different methods to analyse receiver functions at single stations. A recently developed algorithm determines absolute shear-wave velocities from observed frequency-dependent apparent incidence angles of P waves. Using waveform inversion of receiver functions and a modified Zhu and Kanamori algorithm, properties of discontinuities such as depth, velocity contrast, and sharpness are determined. The combination of the methods leads to robust results. The approach is validated by synthetic tests. Stations located on Malaysia show high-shear-wave velocities (V _S) near the surface in the range of 3.4–3.6 km s^− 1 attributed to crystalline rocks and 3.6–4.0 km s^− 1 in the lower crust. Upper and lower crust are clearly separated, the Moho is found at normal depths of 30–34 km where it forms a sharp discontinuity at station KUM or a gradient at stations IPM and KOM. For stations close to the subduction zone (BSI, GSI and PSI) complexity within the crust is high. Near the surface low V _S of 2.6–2.9 km s^− 1 indicate sediment layers. High V _S of 4.2 km s^− 1 are found at depth greater than 6 and 2 km at BSI and PSI, respectively. There, the Moho is located at 37 and 40 km depth. At station GSI, situated closest to the trench, the subducting slab is imaged as a north-east dipping structure separated from the sediment layer by a 10 km wide gradient in V _S between 10 and 20 km depth. Within the subducting slab V _S ≈ 4.7 km s^− 1. At station BSI, the subducting slab is found at depth between 90 and 110 km dipping 20° ± 8° in approximately N 60° E. A velocity increase in similar depth is indicated at station PSI, however no evidence for a dipping layer is found.     

The present relative motion between Africa and Antarctica

Movement between the Africa and Antarctica plates is at present accomplished by sea-floor spreading on the Southwest Indian Ocean Ridge. This movement may be described in terms of an angular rotation vector. Bathymetric and magnetic observations from marine geophysical surveys near the Bouvet triple junction, at 52°S, 15°E and in the environs of the Prince Edward Islands are combined with spreading azimuths derived from earthquake fault plane solutions to define this vector. The rotation pole which describes the motion is located at 10.7°N, 40.9°W and the angular velocity is 1.44 × 10^?7 deg/yr. This pole is significantly different from some other poles obtained by global closure or vector addition. The possibility that the differences may be due to Africa being split into two plates is investigated but there would have to be convergence across the African Rift system for this possibility to be true. Closure of the vector velocity triangle around the Central Indian triple junction is checked by using the pole derived in this study and published poles and rates for the Africa/India and Antarctica/India motions to determine this triangle. The triangle is found to close when errors in the Africa/India and Antarctica/India motions are taken into account. This suggests that it is errors in the data that cause the differences between the observed and predicted poles.     

Full-correlation analysis of turbulent scattering layers in the mesosphere observed by the MU radar

We have applied a full-correlation analysis technique to the echo power fluctuations observed by the MU radar (35°N, 136°E), and analyzed the horizontal structure of the scattering pattern in the mesosphere as well as their horizontal motions. The velocity of the scattering pattern did not agree with the background wind velocity, but was associated with the horizontal propagating direction of a saturated inertia gravity wave identified in the wind field. The length of the long axis of the characteristic ellipse of the scattering pattern was approximately 50 km, and the direction was almost perpendicular to the propagating direction of the wave. The correlation time of the scattering pattern was approximately 700 s, which is much longer than the lifetime of the isolated turbulence itself. This implies that the observed scattering pattern is associated with a region where the saturated inertia gravity wave generates turbulence.     

Intermediate and deep earthquakes in Spain

Recent improvements in the seismological networks on the Ibero-Maghrebian region have permitted estimation of hypocentral location and focal mechanisms for earthquakes which occurred at South Spain, Alboran Sea and northern Morocco of deep and intermediate depth, with magnitudes between 3.5 and 4.5. Intermediate depth shocks, range from 60 to 100 km, with greater concentration located between Granada and Málaga. Fault-plane solutions of 5 intermediate shocks have been determined; they present a vertical plane in NE-SW or E-W direction. Seismic moments of about 10¹⁵ Nm and dimensions of about 1 km have been determined from digital records of Spanish stations.P-wave forms are complex. This may be explained by the crustal structure near the station, discontinuities in the upper mantle and inhomogeneities near the source. Deep activity at about 650 km has only 3 shocks since 1954 (1954, 1973, 1990). Shocks are located at a very small region. Fault-plane solutions show a consistent direction of the pressure axis dipping 45° in E direction. For the 1990 shock seismic moment is 10¹⁶ Nm and dimensions 2.6 km. TheP-waves are of simpler form with a single pulse. The intermediate and deep activities are not connected and no activity has been detected between 100 and 650 km. The intermediate shocks may be explained in terms of a recent subduction from Africa under Iberia in SE direction. The very deep activity must be related to a sunk detached block of lithospheric material still sufficiently cold and rigid to generate earthquakes.     

3-D velocity structure beneath the crust and upper mantle of Aegean Sea region

The region of the Aegean Sea and the surrounding areas in the Eastern Mediterranean lies on the boundary zone between the Eurasian and the African plates. It is a zone of widespread extensive deformation and, therefore, reveals a high level of seismicity.Three-dimensional velocity structure, beneath the crust and upper mantle of the region between 33.0°N–43.0°N and 18.0°E–30.6°E, is determined.The data used are arrival times ofP-waves from 166 earthquakes, recorded at 62 seismological stations. In total, 3973 residual data are inverted.The resultant structure reveals a remarkable contrast of velocity. In the top crustal layer, low velocities are dominant in Western Turkey and on the Greek mainland, while a high velocity zone is dominant in the Ionian Sea and in the southern Aegean Sea.In the upper mantle, high velocity zones dominate along the Hellenic arc, corresponding to the subducting African plate and in the northern part of the region, corresponding to the subducting African plate and in the northern part of the region, corresponding to the margin of Eurasian plate.A low velocity zone is dominant in the Aegean Sea region, where large-scale extension and volcanic activity are predominant, associated with the subduction of the African plate.