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1.
The time evolution of negative buoyancy of a subducting slab is modelled from the beginning of subduction under various kinematic conditions (dip angle and subduction velocity). The calculations take into account the thermal and density effects of the variations of the thermophysical parameters with temperature and pressure, and of phase transitions. The magnitude of the negative buoyancy increases during subduction of oceanic lithosphere, up to values in the (2–4) × 1013 N m−1 range when the tip of the slab reaches a depth of 600–700 km. If continental material arrives at the trench and is subducted, the downward buoyancy decreases by an amount proportional to the volume of the subducted continental crust. Assuming that subduction stops when the buoyancy becomes zero, and that delamination of the continental crust or slab breakoff do not occur, the maximum downdip length of the subductable continental crust is estimated as a function of the dip angle, subduction velocity and geometry of the margin. In most cases, subduction of continental material down to depths of 100–250 km is possible, and continental subduction can continue for times up to 10–15 Ma if the velocity is low. These estimates are not significantly affected by the hypothetical occurrence of a metastable olivine wedge within the slab, and could be lower bounds if the lower continental crust is mafic and transforms to eclogite. 相似文献
2.
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 Lth 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 Lth 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. 相似文献
3.
Anastasia A. Kiratzi 《Pure and Applied Geophysics》1993,140(3):391-402
Recent and historical seismicity as well as reliable fault plane solutions are used to study the active deformation caused by the occurrence of intermediate depth (60–170 km) earthquakes of the Vrancea region, Rumania. In this area, located in the southeastern part of the Carpathian arc, the westward subduction of the Carpathian trench has terminated, leaving continental lithosphere, at present, at the arc. The principalT axis of the intermediate depth events trends N159°E and has a plunge of 74°, which is the same as the dip of the subducted plate. TheP axis has a trend of 314° and a shallow plunge of 15°. The analysis of the moment tensor of six focal mechanisms showed that the dominant mode of deformation of the subducted lithosphere is a down-dip extension at a rate of about 2 cm/yr, based on seismicity data. 相似文献
4.
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. 相似文献
5.
The local subduction geometry at a site south of Puget Sound in western Washington is investigated using teleseismicP-waveforms recorded on a three-component event triggered seismograph. The data are processed using source equalization deconvolution in order to isolate locally convertedP-to-S arrivals and stacked to improve the signal-to-noise ratio. Stable arrivals in the radial component indicate an oceanic Moho within the subducted slab at a depth of about 53 km beneath the station. Observed amplitude variations with azimuth in the radial data, as well as qualitative aspects of the tangential data, are used to establish a slab dip of 16° to the southeast. Our results are compatible with previous results from a site 60 km to the west, and further confirm a substantial warp in the regional geometry of the subducted Juan de Fuca plate. 相似文献
6.
The Arica Elbow region represents that part of Andean South America where the azimuth of the strike of the Peru-Chile trench changes from 150° to 190°. The area under study is roughly bounded by latitudes 17 °S and 23 °S, The shape of the Wadati-Benioff zone was studied in terms of the distribution of ISC hypocentres dated between 1964 and 1993. A system of 22 vertical cross-sections, perpendicular to the trench axis, and a map of epicentres was used to derive the detailed shape of the Wadati-Benioff zone of the presently descending slab. The distribution of earthquake foci indicates a fingerlike shape of the lower part of the Wadati-Benioff zone beneath the aseismic gap. The slab length shows small changes around 350 km in the northern sections, pronounced length oscillations between 350 and 750 km in the neighbouring central sections and a constant value of 650 km in the southern sections. The dip and thickness of the Wadati-Benioff zone are practically constant in all sections. Fault plane solutions, separated spatially into three zones, were used to estimate the state of stress in the slab. 相似文献
7.
— Alexandria City has suffered great damage due to earthquakes from near and distant sources, both in historical and recent times. Sometimes the source of such damages is not well known. Seismogenic zones such as the Red Sea, Gulf of Aqaba-Dead Sea Hellenic Arc, Suez-Cairo-Alexandria, Eastern-Mediterranean-Cairo-Faiyoum and the Egyptian costal area are located in the vicinity of this city. The Egyptian coastal zone has the lowest seismicity, and therefore, its tectonic setting is not well known. The 1998 Egyptian costal zone earthquake is a moderate complex source. It is composed of two subevents separated by 4 sec. The first subevent initiated at a depth of 28 km and caused a rupture of strike (347°), dip (29°) and slip (125°). The second subevent occurred at a shallower depth (24 km) and has a relatively different focal parameter (strike 334°, dip 60° and slip 60°). The available focal mechanisms strongly support the manifestation of a complex stress regime from the Hellenic Arc into the Alexandria offshore area. In the present study a numerical modeling technique is applied to estimate quantitative seismic hazard in Alexandria. In terms of seismic hazard, both local and remote earthquakes have a tremendous affect on this city. A local earthquake with magnitude Ms = 6.7 at the offshore area gives peak ground acceleration up to 300 cm/sec2. The total duration of shaking expected from such an earthquake is about three seconds. The Fourier amplitude spectra of the ground acceleration reveals that the maximum energy is carried by the low frequency (1–3 Hz), part of the seismic waves. The largest response spectra at Alexandria city is within this frequency band. The computed ground accelerations due to strong earthquakes in the Hellenic Arc, Red Sea and Gulf of Aqaba are very small (less than 10 cm/sec2) although with long duration (up to 3 minutes). 相似文献
8.
G.D. Garland 《Physics of the Earth and Planetary Interiors》1975,10(3):220-230
Anomalies of electrical conductivity are considered in relation to other geophysical parameters, such as seismic wave velocity, attenuation, seismicity and density, and to tectonic features. In the case of active subduction zones there appears to be a good correlation between low conductivity and the seismic quality factor Q. Beneath western North America, a conductive zone in the uppermost mantle apparently is controlled by the thickness and severity of the low-velocity layer. Anomalies in conductivity beneath rift valleys can be related to regions of intermediate seismic P-wave velocity, typically about 7.0 km/sec, which is suggestive of partial melting of mantle material. Within the continental crust, anomalies in conductivity are not, in general, thermally controlled, but they can show correlations with seismicity, and may indicate intra-plate boundaries. 相似文献
9.
—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. 相似文献
10.
Sean C. Solomon 《Physics of the Earth and Planetary Interiors》1975,11(2):97-108
The top of the olivine-spinel phase change in subducted oceanic lithosphere can be located by the travel times of seismic waves which have propagated through the slab. P-wave travel-time residuals from deep earthquakes in the Tonga island are observed at Australian seismic stations are grouped according to the depth of the earthquake. The change in mean residual with a change in earthquake depth is related to the velocity contrast between slab and normal mantle at that depth. The curve mean residual versus earthquake depth displays a region of markedly increased slope between earthquake depths of about 250 and 350 km. The most probable explanation of this observation is an elevation by 100 km of the olivine-spinel phase change within the relatively cooler slab. No evidence was found for vertical displacements within the slab of any deeper phase changes.A temperature contrast between slab and normal mantle of about 1,000°C at 250 km depth is implied. This finding confirms current thermal models for subducted lithosphere but is inconsistent with the global intraplate stress field unless only a few percent of the negative buoyancy force at subduction zones is transmitted to the surface plates. 相似文献
11.
汤加—克马德克俯冲带是太平洋板块向澳大利亚板块俯冲碰撞的动力作用区,是全球俯冲带动力学研究的热点区域.本研究基于EHB地震目录,对汤加—克马德克俯冲带(18.5°S—28.5°S)区域进行平面拟合,得到该范围内俯冲带走向约为196°,倾角约为48°;利用该俯冲带研究区域内Global CMT目录,对不同位置、不同深度进行区域应力张量反演,得到汤加—克马德克俯冲带研究区内精细的应力图像.结果显示:(1)俯冲带浅部(60~300km)应力结构非均匀特征明显,主应力轴倾伏角变化多样,并且最大主压应力轴方位在24°S左右发生明显偏转,我们推测这可能与洋底构造路易斯维尔海链俯冲有关;(2)中部(300~500km)最大主压、主张应力轴由北向南逐渐发生偏转,这可能与由北向南流动的地幔流对俯冲板片产生推挤作用有关,并且这种推挤作用向南逐渐减弱;(3)深部(500~700km)最大主压应力轴沿俯冲方向分布;(4)本文的结果还发现了主俯冲带深部西侧"偏移"板片与主俯冲带应力结构不同,表明"偏移"板片与主俯冲带是分离的. 相似文献
12.
2006年12月26日12点26分27秒(GMT)台湾南部滨海发生Ms7.2(Harvard CMT)级地震.震中位于台湾南部滨海之南海次板块与菲律宾海板块碰撞引发造山作用生成的海洋增生楔内, 这次地震是该区域百年来震级最大的地震.我们利用中国数字地震台网(CSDN)和美国地震学联合研究所(IRIS) 提供的上地幔及远场范围宽频带P波垂向记录资料,基于点源和有限断层模型进行波形拟合反演,获得这次地震的震源机制解并给出了震源破裂过程.反演结果表明,本次地震为东倾正断层兼小幅度走滑破裂事件, 断层面走向为341.5°,倾角为77.9°,震源深度6 km.所得正断层震源机制解表明,地震可能与板块的拆离（break-off）作用引发的在台湾造山带局部存在伸张作用力有关. 相似文献
13.
J.Arthur Snoke I.Selwyn Sacks Hiromu Okada 《Physics of the Earth and Planetary Interiors》1974,9(3):199-206
Various workers have constructed models to explain a class of anomalous arrivals at Peruvian and Chilean stations from deep-focus South American earthquakes. These arrivals are shear waves with a later arrival time, a higher frequency content, a longer duration and a lower apparent velocity than direct S. Our models assume that there is a sufficiently sharp discontinuity at the upper interface of the descending lithospheric slab between depths of 80 and 250 km to provide efficient reflection (≈0.1) for S-waves incident from below. The observed travel times require a single S-to-S reflection at this interface if the J-B velocity-depth model is modified to allow for 7% higher velocities down to a depth of 300 km (excluding the crust). The locus of required reflection points correlates well with the upper boundary of the observed seismicity (strike and dip angles within 5°) and Q for the proposed path is consistent with the frequency content of the anomalous arrivals. Thus the existence of these arrivals requires a dipping interface down to about 250 km, but, contrary to the wave-guide model of Isacks and Barazangi, cannot be used to infer a continuous lithospheric slab down to the deep-focus earthquakes (h #62; 500 km). 相似文献
14.
Past studies have shown that high coastal uplift rates are restricted to active areas, especially in a subduction context. The origin of coastal uplift in subduction zones, however, has not yet been globally investigated. Quaternary shorelines correlated to the last interglacial maximum (MIS 5e) were defined as a global tectonic benchmark (Pedoja et al., 2011). In order to investigate the relationships between the vertical motion and the subduction dynamic parameters, we cross-linked this coastal uplift database with the “geodynamical” databases from Heuret (2005), Conrad and Husson (2009) and Müller et al. (2008). Our statistical study shows that: (1) the most intuitive parameters one can think responsible for coastal uplift (e.g., subduction obliquity, trench motion, oceanic crust age, interplate friction and force, convergence variation, dynamic topography, overriding and subducted plate velocity) are not related with the uplift (and its magnitude); (2) the only intuitive parameter is the distance to the trench which shows in specific areas a decrease from the trench up to a distance of ∼300 km; (3) the slab dip (especially the deep slab dip), the position along the trench and the overriding plate tectonic regime are correlated with the coastal uplift, probably reflecting transient changes in subduction parameters. Finally we conclude that the first order parameter explaining coastal uplift is small-scale heterogeneities of the subducting plate, as for instance subducting aseismic ridges. The influence of large-scale geodynamic setting of subduction zones is secondary. 相似文献
15.
Detailed regional body wave tomographic inversion of the Western Pacific region has been performed using P and S travel times from common sources and receivers, with a joint inversion in terms of bulk-sound and shear wave-speed variations in the mantle. This technique allows the separation of the influence of bulk and shear moduli, and hence a more direct comparison with mineral physics information. The study region is parameterized with cells of side 0.5° to 2° and 19 layers to a depth of 1500 km, while the rest of the mantle was parameterized with 5×5° cells with 16 layers between the surface and the core–mantle boundary. A simultaneous inversion is made for regional and global structures to minimize the influence of surrounding structures on the regional image. A nested iterative inversion scheme is employed with local linearization and three-dimensional ray tracing through the successive model updates. The results of the regional tomographic inversion reveal the penetration of a subducted slab below the 660 km discontinuity at the Kurile–Kamchatka trench, while flattening of slabs above this depth is observed in the Japan and Izu–Bonin subduction zones on both the bulk-sound and shear wave-speed images. The penetration of a subducted slab down to a depth of at least 1200 km is seen below the southern part of the Bonin trench, Mariana, Philippine, and Java subduction zones. Fast shear wave-speed perturbations associated with the subducted slabs, down to the 410 km transition zone, are larger than the comparable bulk-sound perturbations for all these subduction zones except the Philippines. The bulk-sound signature for the subducted slab is more pronounced than for shear in the Philippines, Talaud, New Guinea, Solomon, and Tonga subduction zones, where penetration of the slab into the middle mantle is observed. Variation in the amplitude ratio between bulk-sound and shear wave-speed anomalies correlates well with the subduction parameters of the descending slab. Slabs younger than 90 Ma at the trench show bulk-sound dominance in the upper mantle, while older slabs have a stronger shear wave-speed signature. Spreading of the fast shear wave-speed zone between 800 and 1000 km is observed in the areas of deep subducted slab penetration, but has no comparable expression in the bulk-sound images. This high-velocity feature may reflect physical or chemical disequilibria introduced to the lower mantle by subducted slabs. 相似文献
16.
M. Cocco C. Chiarabba M. Di Bona G. Selvaggi L. Margheriti A. Frepoli F. P. Lucente A. Basili D. Jongmans M. Campillo 《Journal of Seismology》1999,3(1):105-117
The analysis of the Irpinia earthquake of 3 April 1996 (ML = 4.9), based on strong motion and short period local data, shows that it was a normal faulting event located within the epicentral area of the MS 6.9, 1980, earthquake. It was located at 40.67° N and 15.42° E at a depth of 8 km. The local magnitude (4.9) has been computed from the VBB stations of the MedNet network. The moment magnitude is Mw = 5.1 and the seismic moment estimated from the ground acceleration spectra is 5.0 1023 dyne cm. Spectral analysis of the strong motion recordings yields a Brune stress drop of 111 bars and a corner frequency of 1 Hz. The source radius associated to these values of seismic moment and stress drop is 1.3 km. The focal mechanism has two nodal planes having strike 297°, dip 74°, rake 290° and strike 64°, dip 25° and rake 220°, respectively. A fault plane solution with strike 295° ± 5°, dip 70° ± 5°, and rake 280° ± 10° is consistent with the S-wave polarization computed from the strong motion data recorded at Rionero in Vulture. We discuss the geometry and the dimensions of the fault which ruptured during the 1996 mainshock, its location and the aftershock distribution with respect to the rupture history of the 1980 Irpinia earthquake. The distribution of seismicity and the fault geometry of the 1996 earthquake suggest that the region between the two faults that ruptured during the first subevents of the 1980 event cannot be considered as a strong barrier (high strength zone), as it might be thought looking at the source model and at the sequence of historical earthquakes revealed by paleoseismological investigations. 相似文献
17.
利用NECESSArray宽频带地震台阵记录的P波三重震相波形资料,采用遗传算法,对千岛俯冲板块内部及附近410-km间断面的结构进行了非线性反演.其中,选取了发生在俯冲带的发震时刻为2009年10月10日21时24分(GMT时间)震级为M w5.9的地震;其三重震相的射线回折点处射线路径的方向与俯冲板块的走向大致一致,克服了间断面在俯冲板块内部沿俯冲方向起伏剧烈、不易识别的困难,设计以震中为顶点、方位角范围分别为275°~280°、269°~274°、264°~266°的北、中、南三个扇形区域,用于研究410-km间断面逐渐靠近俯冲板块直至处在其中的起伏情况;“先对齐、后反演”的具体计算方案极大地减小了浅部结构不确定性对反演结果的影响;同时,整体归一化策略充分利用了台阵的振幅信息,有效地加强了对深部结构的约束.反演结果显示,“410-km间断面”在北区抬升了10~20 km,在中区抬升了20~30 km,在南区抬升了60~70 km,与橄榄石-瓦兹利石平衡态相变界面的矿物物理学预测结果一致;其波速跃变在北区为10%,在中区为10%,在南区为7%.扣除了前人在层析成像显示的地震源区及目标区速度异常的影响后,约4%的波速跃变可能由橄榄石-瓦兹利石的相变所产生,与IASP91模型的速度跃变值相当.目前的研究结果表明俯冲带内部似乎不存在大量的亚稳态橄榄石.基于更多资料并对波形细节进行拟合,可望刻画俯冲板块内部410-km间断面的精细结构,给出关于这一问题的确定性回答. 相似文献
18.
19.
Recent seismic and magnetotelluric experiments, aimed at better characterizing the shape and state of the subducting slab and continental crust beneath Central Mexico, exposed significant differences with conclusions of previous studies. A new slab geometry is revealed in which the subducting Cocos slab is perfectly flat between 120 to 290?km from the trench, after which it plunges into the asthenosphere at a dip angle of ~65°, in sharp contrast with the previously proposed ~20° dip angle. Seismic tomography studies show negative P-wave velocity anomalies (?2 to ?4%) in the mantle wedge beneath the Mexican Volcanic Belt, and positive anomalies (+2 to +3%) for the subducted Cocos slab. Magnetotelluric experiments exposed a very low-resistivity area (1?C10? ??m) located within the continental crust just below the Mexican Volcanic Arc. Finally, several spots of non-volcanic tremors (NVTs) have been recorded inside the continental crust above the flat-slab segment. While all these experiments provide a better picture of the subduction system beneath Central Mexico, several key processes need further investigation. In this study, we take advantage of these new observations to better constrain the thermal structure beneath Central Mexico. Two different thermal models are computed for a mantle potential temperature (T p) of 1,350 and 1,450°C, respectively. The new thermal structures are then converted into P-wave velocity anomalies and compared with the observed V p anomalies. We found that a T p of 1,450°C produced larger V p anomalies that do not fit the observations. However, using a T p of only 1,350°C, our predicted V p anomalies are positive (+2 to +3%) for the cold slab and negative (?2 to ?4%) in the mantle wedge. These V p estimates are consistent with the observed seismic tomography from P-wave arrivals, and therefore we conclude that a T p of 1,350°C is a better estimate for the mantle potential temperature beneath Central Mexico. The new thermal model, in conjunction with phase diagrams for sediments, hydrated basalt and lithospheric mantle, have been used to estimate the amount and location of fluids released from the subducting Cocos slab. Several dehydration pulses have been identified along the slab interface where most of the fluids stored in sediments and oceanic crust are released into the overlying continental crust above the flat-slab. We found a good correlation between the pattern of these dehydration pulses and the location of NVTs, suggesting that slab dehydration is responsible for triggering the tremors. We suggest that NVT bursts localized above the flat slab segment represent the manifestation of ongoing continental crust hydration and weakening, a process that has been going on since 15?Ma ago when the Cocos slab entered into a flat-slab regime. Such continuous weakening would have reduced the suction forces that kept the slab in a flat regime in the last 15?Ma, allowing the slab to easily roll back. The continuous low-resistivity region recorded beneath the volcanic front in Central Mexico might represent the evidence of slab dehydration and crust weakening over time. 相似文献
20.
Benjun Wu Clinton P. Conrad Arnauld Heuret Carolina Lithgow-Bertelloni Serge Lallemand 《Earth and Planetary Science Letters》2008,272(1-2):412-421
Although subducting slabs undergo a bending deformation that resists tectonic plate motions, the magnitude of this resistance is not known because of poor constraints on slab strength. However, because slab bending slows the relatively rapid motions of oceanic plates, observed plate motions constrain the importance of bending. We estimated the slab pull force and the bending resistance globally for 207 subduction zone transects using new measurements of the bending curvature determined from slab seismicity. Predicting plate motions using a global mantle flow model, we constrain the viscosity of the bending slab to be at most ~ 300 times more viscous than the upper mantle; stronger slabs are intolerably slowed by the bending deformation. Weaker slabs, however, cannot transmit a pull force sufficient to explain rapid trenchward plate motions unless slabs stretch faster than seismically observed rates of ~ 10− 15 s− 1. The constrained bending viscosity (~ 2 × 1023 Pa s) is larger than previous estimates that yielded similar or larger bending resistance (here ~ 25% of forces). This apparent discrepancy occurs because slabs bend more gently than previously thought, with an average radius of curvature of 390 km that permits subduction of strong slabs. This gentle bending may ultimately permit plate tectonics on Earth. 相似文献