共查询到20条相似文献，搜索用时 15 毫秒
1.
I. Flecha R. Carbonell H. Zeyen D. Martí I. Palomeras F. Simancas A. Prez-Estaún 《Tectonophysics》2006,420(1-2):37
The parameters used for the acquisition of the IBERSEIS deep seismic reflection profile in the SW Iberian Peninsula provide seismic images of the deep crust as well as a high resolution section of the shallow subsurface. A very dense array of sources and receivers allowed high resolution tomographic studies in zones of special interest (granitic plutons). The three dimensional tomographic inversion produced velocity models along a 500 m wide and 1000 m deep strip along the IBERSEIS transect in the areas of La Bazana, La Dehesilla, Feria and Villafranca. In these high resolution velocity models (sampled by 50 × 50 × 50 m cells), high velocity anomalies indicate the geology and extension of the granitic plutons at depth. This directly correlates with the surface outcrops. Moreover, tomographic models provided valuable information for the geometry and characterization of fractured and fresh domains in a rock volume. Furthermore, a piggy back seismic acquisition experiment using additional seismic instrumentation from the University of Paris Sud (40-channel DMT) provided perpendicular, offline recordings of the Vibroseis sources. This additional recording system was deployed perpendicularly to the main IBERSEIS seismic reflection line and provided additional 3D control. 相似文献
2.
First arrival times from P-wave refraction and reflection seismic surveys along Bear Creek Valley on the Oak Ridge Reservation,
Tennessee, were inverted to produce refraction tomographic velocity images showing seismic velocity variations within thinly
mantled karstic bedrock to a depth of approximately 20 m. Inverted velocities are consistent with two distinct bedrock groups:
the Nolichucky Shale (2,730–5,150 m/s) and Maynardville Limestone (3,940–7,575 m/s). Low-velocity zones (2,700–4,000 m/s)
in the tomographic images correspond to previously inferred cross-valley strike-slip faults; in places, these faults create
permeability barriers that offset or block groundwater flowing along Bear Creek Valley. These faults may also force groundwater
contaminants, such as dense non-aqueous phase liquids, to migrate laterally or downward, spreading contamination throughout
the groundwater system. Other, previously unmapped cross-valley faults may also be visible in the tomographic images. Borehole
logs suggest the low-velocity values are caused by low rigidity fractured and vuggy rock, water zones, cavities and collapse
features. Surface streams, including Bear Creek, tend to lie directly above these low-velocity zones, suggesting fault and
fracture control of surface drainage, in addition to the subsurface flow system. In some cases, fault zones are also associated
with bedrock depressions and thicker accumulations of unconsolidated sediment. 相似文献
3.
G. K. Reddy T. Seshunarayana Rajeev Menon P. Senthil Kumar 《Journal of Earth System Science》2010,119(5):745-751
Fracture and fault networks are conduits that facilitate groundwater movement in hard-rock terrains. Soil-gas helium emanometry
has been utilized in Wailapally watershed, near Hyderabad in southern India, for the detection of fracture and fault zones
in a granite basement terrain having a thin regolith. Based on satellite imagery and geologic mapping, three sites were selected
for detailed investigation. High spatial resolution soil-gas samples were collected at every one meter at a depth of >1.5m
along 100m long profiles (3 in number). In addition, deep shear-wave images were also obtained using the multichannel analysis
of surface waves. The study clearly indicates several soil-gas helium anomalies (above 200 ppb) along the profiles, where
the shear-wave velocity images also show many near-surface vertical low velocity zones. We thus interpret that the soil-gas
helium anomalous zones and the vertical low-velocity zones are probable traces of fault/fracture zones that could be efficient
natural recharge zones and potential groundwater conduits. The result obtained from this study demonstrates the efficacy of
an integrated approach of soil-gas helium and the seismic methods for mapping groundwater resource zones in granite/gneiss
provinces. 相似文献
4.
利用冈底斯中-东部197个宽频带天然地震台站记录到的数据和远震P波走时层析成像方法,获得了该区域的P波速度扰动图像。层析成像结果显示研究区地壳和上地幔地震波速度结构存在着复杂的空间变化。首先,在藏南拆离系断层(STD)以北的特提斯喜马拉雅地壳中存在着较强的低速异常,但是该低速异常的北端在远离裂谷带的地方并没有明显越过雅鲁藏布江缝合线(YZS),这与前人的观测结果略有不同;在亚东-古露(YGR)和措美-桑日(CSR)裂谷带的下方存在低速异常,但异常强度都没有前者大;在两个裂谷带之间的拉萨地块中-南部,地壳表现为强高速特征。这些结果表明,影响青藏高原地壳构造演化的"地壳通道流(Crustal Channel Flow)"在藏南主要分布在特提斯喜马拉雅地区,在雅鲁藏布江缝合线以北的冈底斯地区,可能主要局限于沿裂谷带分布。其次,被解释为印度岩石圈地幔的上地幔高速异常,在研究区西部,抵达了雅鲁藏布江缝合线以北100km或更远的地方,而在研究区东部,并没有越过雅鲁藏布江缝合线,而是停留在缝合线以南~100km的高喜马拉雅下方,印证了前人给出的印度板块俯冲角度在研究区附近存在东西向变化的层析成像结果。此外,我们的层析成像结果还印证了冈底斯东南侧的上地幔低速异常根植于上地幔底部,我们认为该现象可能与巽他块体的顺时针旋转引起向东俯冲的缅甸弧向西后撤有关。 相似文献
5.
6.
Eastern Anatolia is a region in the early stages of continent–continent collision and so provides a unique opportunity to study the early development of continental plateau. Located within the Alpine–Himalayan fold-thrust fault belt, the Anatolian plateau is geologically very complex, with over half of the surface area covered with late Cenozoic volcanics of diverse composition. The plateau is also seismically active and is dissected by numerous seismogenic faults predominantly of strike-slip motion. In this study, we determine 3-D tomographic images of the crust beneath eastern Anatolia by inverting a large number of arrival time data of P- and S-waves. From the obtained P- and S-wave velocity models, we estimated the Poisson’s ratio structures for a more reliable interpretation of the obtained velocity anomalies. Our tomographic results are generally consistent with the major tectonic features of the region. High P- and S-wave velocity anomalies are recognized near the surface, while at deeper crustal layers, low seismic wave velocities are widely distributed. Poisson’s ratio exhibits significant structural heterogeneities compared to the imaged velocity structure. The seismic activity is intense along highly heterogeneous zones and is closely associated with pre-existing faults in the central and western parts of the study area. Results of the checkerboard resolution test indicate that the imaged anomalies are reliable features down to a depth of about 40 km. The low-velocity/high Poisson’s ratio zones in the middle to lower crust are consistent with many geophysical observations such as strong Sn attenuation, low Pn and Sn velocity, and the absence of mantle lid, implying the presence of partial melt in the uppermost mantle. 相似文献
7.
喜马拉雅东构造结岩石圈板片深俯冲的地球物理证据 总被引：4，自引：0，他引：4
2009～2010年在南迦巴瓦地区进行了宽频带地震和大地电磁探测,分别处理获得东构造结及其邻区的地下300km以上的P波速度图像和两条大地电磁电阻率剖面。通过资料的对比和综合解释,发现电阻率分布与地震波速有较好的对应关系。研究结果表明:南迦巴瓦变质体的上地壳部分呈现明显高速高阻特征,为两侧的雅鲁藏布江缝合带所夹持;中下地壳具有不均匀性,且普遍呈低速低阻特征;印度板块在藏东南向欧亚板块的俯冲前缘越过嘉黎断裂,抵达班公湖-怒江缝合带;在拉萨地体的高速俯冲板片以下100km至200km深度范围内存在大规模的低速异常带,其上盘中下地壳也广泛发育低速高导体,指示青藏高原东南缘可能存在韧性易流动的物质向东、东南逃逸的通道,为印度板块在南迦巴瓦的深俯冲动力学模式提供了地球物理证据。 相似文献
8.
9.
The 2,026 earthquake events registered by the Sichuan regional digital seismic network and mobile seismic array after the April 20 th,2013 Lushan earthquake and 28,188 pieces of data were selected to determine direct P waves arrival times. We applied the tomographic method to inverse the characteristics of the velocity structure for the three-dimensional(3D) P wave in the mid-upper crust of the seismic source region of the Lushan earthquake. The imaging results were combined with the apparent magnetization inversion and magnetotelluric(MT) sounding retest data to comprehensively study the causes of the deep seismogenic environment in the southern section of the Longmenshan fault zone and explore the formation of the Lushan earthquake. Research has shown that there are obvious differences in velocity structure and magnetic distribution between the southern and northern sections of the Longmenshan fault zone. The epicenter of the Lushan earthquake is located near the boundary of the high and low-velocity anomalies and favorable for a high-velocity section. Moreover,at the epicenter of the Lushan earthquake located on the magnetic dome boundary of Ya’an,the development of high velocity and magnetic solid medium favors the accumulation and release of strain energy. Lowvelocity anomalies are distributed underneath the are of seismogenic origin,The inversion results of the MT retest data after the April 20 th Lushan earthquake also indicate that there a high-conductor anomaly occurs under the area of seismogenic origin of the Lushan earthquake,Therefore,we speculated that the presence of a high-conductivity anomaly and low-velocity anomaly underneath the seismogenic body of the Lushan earthquake could be related to the existence of fluids. The role of fluids caused the weakening of the seismogenic layer inside the mid-upper crust and resulted in a seismogenic fault that was prone to rupture and played a triggering role in the Lushan earthquake. 相似文献
10.
G. De Natale C. Troise F. Pingue P. De Gori C. Chiarabba 《Mineralogy and Petrology》2001,73(1-3):5-22
Summary We review recently obtained results about the velocity structure of the Somma-Vesuvius (Southern Italy) volcanic complex
and present an interpretation of structural features, both at local and regional scale, and of the local seismicity. The local
structure of Somma-Vesuvius is reviewed, referring to three depth ranges; i.e. shallow (0–5 km), intermediate (5–15 km) and
deep (from 15 km to the upper mantle). The shallow velocity structure is inferred by the joint inversion of shot and local
earthquake arrival time data. The main feature pointed out by this inversion is a high-velocity anomaly at the crater axis
extending down to a depth of about 5 km. This anomaly can be explained with the presence of residual magma crystallised in
the shallow conduits, which accumulated during the last eruptive cycles. The local seismicity is strongly clustered around
this anomaly, due to the focusing effect of the rigidity contrast. The space-time seismicity pattern at Somma-Vesuvius is
the result of the superposition of background seismicity, mainly due to gravitational instability of the volcanic edifice
and to small external stress perturbations, with intense episodic earthquake swarms possibly due to magmatic or hydrothermal
activity into the shallow system. The velocity structure in the 10–15 km depth range is characterized by the presence of a
low-velocity layer, which has been independently confirmed by multi-channel seismic reflection data and P-Sv conversions from
teleseismic waveforms. The study of the deep structure was performed by regional tomography with teleseisms; it confirmed
the presence of a low-velocity anomaly underneath the volcano, which appears to have roots at greater depths. The regional
structure between the Thyrrenian and the Adriatic sea has been inferred by tomographic inversion of teleseismic arrival times.
The main result from this study which is very important for geodynamic interpretations is the first evidence for a continuous
subducting slab under the Apennines, in an area where previous models hypothesized a slab window.
Received March 3, 2000 revised version accepted July 4, 2001 相似文献
11.
This paper describes seismic velocity tomography applied to the investigation and assessment of karst collapse hazards to facilitate accurate characterization of geological conditions of karst sinkhole formation. In the survey areas of Xiamao, Guangzhou, China, and Huangchi, Foshan, China, seismic velocity tomography was used to explore the structures of rock and soil associated with karst collapse. The results show that sand intercalated with clay or clay intercalated with soft soil dominates the cover of these two areas. The overburden is 20–33 m thick and underlain by Carboniferous limestone. In the limestone, there are well-developed karst caves and cracks as well as highly fluctuating bedrock surfaces. The seismic velocities are less than 2500 m/s in the cover, 2500–4500 m/s in the karst fracture zones and caves of Xiamao, and 1500–2000 m/s in the Huangchi collapse area. The karst fracture zones, relief of bedrock surfaces, and variations of soil thicknesses revealed by seismic velocity tomography are well constrained and in agreement with those in the drilling borehole profiles. This paper demonstrates that seismic velocity tomography can delineate anomalies of rock and soil with the advantages of speed, intuitive images, and high resolution. 相似文献
12.
We applied a seismic tomography technique to arrival time data generated by local crustal earthquakes in central Anatolia in order to study the three-dimensional velocity and Vp/Vs structures and their relation with the complex tectonic processes and seismic activity occurring in the study region. The relatively equal and large number of both P- and S-wave arrival times comprising a total of 51,650 arrivals and the relatively uniform distribution of the recording stations imply that the obtained velocity anomalies are reliable features down to a depth of 40 km. This is also evident from the results of the checkerboard resolution test, hit count, and the ray-path coverage. The inversion results indicate the existence of strong lateral heterogeneities in the crust and uppermost mantle beneath central Anatolia. Prominent low-velocity anomalies are clearly imaged at all layers especially beneath existing volcanoes and the active fault segments. Higher-than-average Vp/Vs ratios are widely distributed, indicating the possible existence of over-pressurized fluids that may be responsible for the triggering of the large crustal earthquakes along the north and east Anatolian fault zones. We noticed that the seismic activity occurs mainly at the low-velocity areas and to a lesser extent in some high-velocity zones, perhaps because of the complex tectonics and geological structures. These observations imply that all the zones with velocity anomalies—either low or high—are potential sites for strain energy accumulation and subsequent release. The obtained velocity and Vp/Vs models are consistent with previous geophysical measurements conducted beneath central Anatolia and give much deeper understanding of the current seismotectonic processes occurring in the region. 相似文献
13.
A detailed three-dimensional (3-D) P-wave velocity model of the crust and uppermost mantle under the Chinese capital (Beijing) region is determined with a spatial resolution of 25 km in the horizontal direction and 4–17 km in depth. We used 48,750 precise P-wave arrival times from 2973 events of local crustal earthquakes, controlled seismic explosions and quarry blasts. These events were recorded by a new digital seismic network consisting of 101 seismic stations equipped with high-sensitivity seismometers. The data are analyzed by using a 3-D seismic tomography method. Our tomographic model provides new insights into the geological structure and tectonics of the region, such as the lithological variations and large fault zones across the major geological terranes like the North China Basin, the Taihangshan and the Yanshan mountainous areas. The velocity images of the upper crust reflect well the surface geological and topographic features. In the North China Basin, the depression and uplift areas are imaged as slow and fast velocities, respectively. The Taihangshan and Yanshan mountainous regions are generally imaged as broad high-velocity zones, while the Quaternary intermountain basins show up as small low-velocity anomalies. Velocity changes are visible across some of the large fault zones. Large crustal earthquakes, such as the 1976 Tangshan earthquake (M=7.8) and the 1679 Sanhe earthquake (M=8.0), generally occurred in high-velocity areas in the upper to middle crust. In the lower crust to the uppermost mantle under the source zones of the large earthquakes, however, low-velocity and high-conductivity anomalies exist, which are considered to be associated with fluids. The fluids in the lower crust may cause the weakening of the seismogenic layer in the upper and middle crust and thus contribute to the initiation of the large crustal earthquakes. 相似文献
14.
We applied a tomographic method to image an aseismic strike–slip fault in North Morocco and found that the occurrence of earthquakes is not only controlled by the state of tectonic stress but also by material heterogeneity in the crust. We have constructed an integrated model of seismic, electric, magnetic and heat flow properties across northeastern Morocco primarily based on a tomography inversion of local earthquake arrival times. The seismic images obtained show a pronounced low-velocity zone at 5 km depth parallels to the Nekor fault, coinciding with an anomalously high conductive and low gravity structure, which is interpreted as a fault gouge zone and/or a fluid-filled subsurface rock matrix. Below 10 km depth, a weak positive velocity zone indicates that the fault gouge is stable. The seismicity and the seismic velocity results for the Al-Hoceimas region show that the concentrations of earthquakes are confined in the high velocity area. This anomaly is interpreted to be a brittle and competent layer of the upper crust that sustains seismogenic stress. On the eastern coast line of Morocco, we infer that a high density, high velocity body exists in the shallowest layers of the upper crust, probably formed by Miocene volcanic rocks. 相似文献
15.
We present a new three-dimensional SV-wave velocity model for the upper mantle beneath South America and the surrounding oceans, built from the waveform inversion of 5850 Rayleigh wave seismograms. The dense path coverage and the use of higher modes to supplement the fundamental mode of surface waves allow us to constrain seismic heterogeneities with horizontal wavelengths of a few hundred kilometres in the uppermost 400 km of the mantle.The large scale features of our tomographic model confirm previous results from global and regional tomographic studies (e.g. the depth extent of the high velocity cratonic roots down to about 200–250 km).Several new features are highlighted in our model. Down to 100 km depth, the high velocity lid beneath the Amazonian craton is separated in two parts associated with the Guyana and Guapore shields, suggesting that the rifting episode responsible for the formation of the Amazon basin has involved a significant part of the lithosphere. Along the Andean subduction belt, the structure of the high velocity anomaly associated with the sudbduction of the Nazca plate beneath the South American plate reflects the along-strike variation in dip of the subducting plate. Slow velocities are observed down to about 100 km and 150 km at the intersection of the Carnegie and Chile ridges with the continent and are likely to represent the thermal anomalies associated with the subducted ridges. These lowered velocities might correspond to zones of weakness in the subducted plate and may have led to the formation of “slab windows” developed through unzipping of the subducted ridges; these windows might accommodate a transfer of asthenospheric mantle from the Pacific to the Atlantic ocean. From 150 to 250 km depth, the subducting Nazca plate is associated with high seismic velocities between 5°S and 37°S. We find high seismic velocities beneath the Paraná basin down to about 200 km depth, underlain by a low velocity anomaly in the depth range 200–400 km located beneath the Ponta Grossa arc at the southern tip of the basin. This high velocity anomaly is located southward of a narrow S-wave low velocity structure observed between 200 and 500–600 km depth in body wave studies, but irresolvable with our long period datasets. Both anomalies point to a model in which several, possibly diachronous, plumes have risen to the surface to generate the Paraná large igneous province (LIP). 相似文献
16.
《Tectonophysics》1987,140(1):115-120
The question was examined as to whether the same gravity anomaly is produced for a range of crustal velocity structures all of which produce similar seismic record sections. The results indicate that the maximum errors in the computed gravity anomalies for the various velocity structures is around 20–30 mGal for some models and is around 50 mGal for many others. A major source of error is underestimating the effect of a seismic low-velocity zone which is at one end of the correct velocity structure and which was not detected by several of the interpretations presented at a recent workshop. If no low-velocity layer is included, the error in the gravity anomaly is 30 mGal. The choice of a particular velocity-density relationship does not seem to be a serious problem in the analysis, except for the case of the low-velocity zone, as long as we use a continuous velocity-density curve. 相似文献
17.
A football stadium with a capacity of a hundred thousand spectators is under construction over a karst terrain, 10 km west of the old town of Istanbul, Turkey. A large cavity of approximately 30 m3 was detected beneath the sports field through a number of boreholes so that a geophysical survey was required to further investigate a portion of the sports field. We utilized seismic refraction tomography and dc-electrical method with rotated Wenner array to delineate zones with solution voids and cavities. Total core recovery (TCR) was 5–15% from boreholes where zones with low velocities were identified through tomographic inversion, whereas TCR values were above 60% in zones with higher velocities. Both low velocity zones in the tomographic images and increasing resistivity anisotropy with depth appear to indicate that the cavity extends toward the west and south at a depth of approximately 8 m, although the southward and westward extension changes in character. 相似文献
18.
A dense nationwide seismic network recently constructed in Japan has resulted in the production of a large amount of high-quality data that have enabled the high-resolution imaging of deep seismic structures in the Japanese subduction zone. Seismic tomography, precise locations of earthquakes, and focal mechanism research have allowed the identification of the complex structure of subducting slabs beneath Japan, revealing that the subducting Philippine Sea slab underneath southwestern Japan has an undulatory configuration down to a depth of 60–200 km, and is continuous from Kanto to Kyushu without disruption or splitting, even within areas north of the Izu Peninsula. Analysis of the geometry of the Pacific and Philippine Sea slabs identified a broad contact zone beneath the Kanto Plain that causes anomalously deep interplate and intraslab earthquake activity. Seismic tomographic inversions using both teleseismic and local events provide a clear image of the deep aseismic portion of the Philippine Sea slab beneath the Japan Sea north of Chugoku and Kyushu, and beneath the East China Sea west of Kyushu down to a depth of ∼450 km. Seismic tomography also allowed the identification of an inclined sheet-like seismic low-velocity zone in the mantle wedge beneath Tohoku. A recent seismic tomography work further revealed clear images of similar inclined low-velocity zones in the mantle wedge for almost all other areas of Japan. The presence of the inclined low-velocity zones in the mantle wedge across the entirety of Japan suggests that it is a common feature to all subduction zones. These low-velocity zones may correspond to the upwelling flow portion of subduction-induced convection systems. These upwelling flows reach the Moho directly beneath active volcanic areas, suggesting a link between volcanism and upwelling. 相似文献
19.
A.V. Jakovlev N.A. Bushenkova I.Yu. Koulakov N.L. Dobretsov 《Russian Geology and Geophysics》2012,53(10):963-971
We present a new three-dimensional model of P-velocity anomalies in the upper mantle beneath the Circum-Arctic region based on tomographic inversion of global data from the catalogues of the International Seismological Center (ISC, 2007). We used travel times of seismic waves from events located in the study area which were recorded by the worldwide network, as well as data from remote events registered by stations in the study region. The obtained mantle seismic anomalies clearly correlate with the main lithosphere structures in the Circum-Arctic region. High-velocity anomalies down to 250–300 km depth correspond to Precambrian thick lithosphere plates, such as the East European Platform with the adjacent shelf areas, Siberian Plate, Canadian Shield, and Greenland. It should be noted that lithosphere beneath the central part of Greenland appears to be strongly thinned, which can be explained by the effect of the Iceland plume which passed under Greenland 50–60 million years ago. Beneath Chukotka, Yakutia, and Alaska we observe low-velocity anomalies which represent weak and relatively thin actively deformed lithosphere. Some of these low-velocity areas coincide with manifestations of Cenozoic volcanism. A high-velocity anomaly at 500–700 km depth beneath Chukotka may be a relic of the subduction zone which occurred here about 100 million years ago. In the oceanic areas, the tomography results are strongly inhomogeneous. Beneath the North Atlantic, we observe very strong low-velocity anomalies which indicate an important role of the Iceland plume and active rifting in the opening of the oceanic basin. On the contrary, beneath the central part of the Arctic Ocean, no significant anomalies are observed, which implies a passive character of rifting. 相似文献
20.
CHANG Xu LIU YikeInstitute of Geology Geophysics Chinese Academy of Sciences All Datun Ro Chaoyang District Beijing E-mail: changxu@mail.c-geos.ac.cn 《《地质学报》英文版》2000,74(1):102-112
In this paper, 3-D velocity images of the crust and upper mantle beneath the Nanbei tectonic zone of China are constructed using P-wave travel time residuals of earthquakes, with the data supplied by China's seismic networks and the International Seismic Centre.During the model parameterization in the tomographic inversion the interpolation function of grid node velocities is used as the space function of velocity, and the velocity function is allowed to be discontinuous. The difficulties caused by large memory demand and high computing cost in solving the system of equations are avoided by utilizing the least squares QR decomposition algorithm. Thus, the stability of the algorithm is ensured. Though resolution images are not derived in the inversion process based on the resolution theory of Backus and Gilbert, a covariance resolution method is put forward by the authors. The resolution analyses have proved the reliability of the image results. 3-D ray tracing is conducted to obtain the ray paths in a s 相似文献