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1.
—?We present results from a large-scale study of surface-wave group velocity dispersion across the Middle East, North Africa, southern Eurasia and the Mediterranean. Our database for the region is populated with seismic data from regional events recorded at permanent and portable broadband, three-component digital stations. We have measured the group velocity using a multiple narrow-band filter on deconvolved displacement data. Overall, we have examined more than 13,500 seismograms and made good quality dispersion measurements for 6817 Rayleigh- and 3806 Love-wave paths. We use a conjugate gradient method to perform a group-velocity tomography. Our current results include both Love- and Rayleigh-wave inversions across the region for periods from 10 to 60 seconds. Our findings indicate that short-period structure is sensitive to slow velocities associated with large sedimentary features such as the Mediterranean Sea and Persian Gulf. We find our long-period Rayleigh-wave inversion is sensitive to crustal thickness, such as fast velocities under the oceans and slow along the relatively thick Zagros Mts. and Turkish-Iranian Plateau. We also find slow upper mantle velocities along known rift systems. Accurate group velocity maps can be used to construct phase-matched filters along any given path. The filters can improve weak surface wave signals by compressing the dispersed signal. The signals can then be used to calculate regionally determined M S measurements, which we hope can be used to extend the threshold of m b :M S discriminants down to lower magnitude levels. Other applications include using the group velocities in the creation of a suitable background model for forming station calibration maps, and using the group velocities to model the velocity structure of the crust and upper mantle. 相似文献
2.
Crustal and uppermost mantle structure of SE Tibetan plateau from Rayleigh-wave group-velocity measurements 
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下载免费PDF全文 A shear-wave velocity model of the crust and uppermost mantle beneath the SE Tibetan plateau was derived by inverting Rayleigh-wave group-velocity measurements of periods between 10 and 70 s. Rayleigh-wave group-velocity dispersions along more than 3,000 interstation paths were measured based on analysis of teleseismic waveform data recorded by temporary seismic stations. These observations were then utilized to construct 2D group-velocity maps in the period range of 10–70 s. The new group-velocity maps have an enhanced resolution compared with previous global and regional group-velocity models in this region because of the denser and more uniform data coverage. The lateral resolution across the region is about 0.5° for the periods used in this study. Local dispersion curves were then inverted for a 3D shear-wave velocity model of the region by applying a linear inversion scheme. Our 3D shear-wave model confirms the presence of low-velocity zones (LVZs) in the crust beneath the northern part of this region. Our imaging shows that the upper-middle crustal LVZ beneath the Tengchong region is isolated from these LVZs beneath the eastern and northern part of this region. The upper–middle crustal LVZ may be regarded as evidence of a magma chamber in the crust beneath the Tengchong Volcanoes. Our model also reveals a slow lithospheric structure beneath Tengchong and a fast shield-like mantle beneath the stable Yangtze block. 相似文献
3.
In the present study, fundamental Rayleigh waves with varying period from 10 to 80 s are used to obtain group velocity maps in the northwest Deccan Volcanic Province of India. About 350 paths are obtained using 53 earthquakes (4.8 ≤ M ≥ 7.9) recorded by the SeisNetG (Seismic Network of Gujarat). Individual dispersion curves of group velocity of Rayleigh wave for each source-station path are estimated using multiple filter technique. These curves are used to determine lateral distribution of Rayleigh wave group velocity by tomographic inversion method. Our estimated Rayleigh group velocity at varying depths showed conspicuous corroboration with three tectonic blocks [Kachchh Rift Basin (KRB), Saurashtra Horst (SH), and Mainland Gujarat (MG)] in the region. The seismically active KRB with a thicker crust is characterized as a low velocity zone at a period varying from 10 to 30 s as indicative of mantle downwarping or sagging of the mantle beneath the KRB, while the SH and MG are found to be associated with higher group velocities, indicating the existence of the reduced crustal thickness. The trend of higher group velocity was found prevailed adjacent to the Narmada and Cambay rift basins that also correspond to the reduced crust, suggesting the processes of mantle upwarping or uplifting due to mantle upwelling. The low velocities at periods longer than 40 s beneath the KRB indicate thicker lithosphere. The known Moho depth correlates well with the observed velocities at a period of about 30 s in the Gujarat region. Our estimates of relatively lower group velocities at periods varying from 70 to 80 s may correspond to the asthenospheric flow beneath the region. It is interesting to image higher group velocity for the thinner crust beneath the Arabian Sea adjacent to the west coast of Gujarat at the period of 40 s that may correspond to the upwarped or upwelled mantle beneath the Arabian Sea. Our results have better resolution estimated by a radius of equivalent circular averaging area for each period. 相似文献
4.
Rayleigh and Love wave group velocities in the Arctic islands and adjacent mainland of northern Canada exhibit large regional variations which can be attributed to variations in crustal structure. The shield and platform regions, the eastern Sverdrup basin, and the western Sverdrup basin are characterized by progressively slower velocities. Inversions of the regionalized group velocity data lead to three distinct shear-velocity models. The shield and platform model is similar to earlier models obtained for the Canadian shield. The Sverdrup basin models have low near-surface velocities which may correspond to thick accumulations of sediment. A sediment thickness of about 12 km obtained for the eastern basin corresponds to the maximum thickness inferred from earlier geological studies. An even thicker zone of low velocities occurs in the upper crust of the western basin. This zone may represent the true thickness of a very deep basin or slow velocities in basement rock may underlie the shallower sediments. A third alternative is that low apparent velocities for the western Sverdrup basin result from systematic errors in group velocity determinations produced by epicentral mislocations or origin time errors of the earthquakes used, but such mislocation or errors would have to be quite large to explain the observations. It is not possible to explain the group velocities across the basin areas by simply adding sediments to the shield and platform model. The resulting discrepancies with the data suggest an altogether different crustal structure beneath the basin than that beneath the shield and platform areas. 相似文献
5.
The lithosphere of the South American continent has been studied little, especially in northern Brazil (the Amazonian region). A 3D lithospheric S-velocity model of South America was obtained by first carrying out Rayleigh and Love wave group-velocity tomography, and then inverting the regionalized dispersion curves. Fundamental mode group velocities were measured using a Multiple Filtering Technique. More than 12,000 paths were examined and about 6000 Rayleigh- and 3500 Love-wave dispersion curves with good quality were retrieved. Checkerboard tests showed that our dataset permits the resolution of features 400-800 km across laterally in the central part of the continent from crustal to upper mantle depths. Our results confirm previous tomographic results and correlate well with the major geological provinces of South America. The 3D S-velocity model confirms both regional features of SE Brazil from P-wave travel-time tomography and continental-scale features of central and western South America from waveform inversion, e.g., lowest velocities in the Andean upper mantle; three parts of the Nazca plate with flat subduction; strong low-velocity anomalies in the upper-mantle depth beneath the Chaco basin. Furthermore, our 3D model revealed new features in the South American continent: (1) high velocities in the lower crust were consistently found in regions with high Bouguer or free-air anomalies; (2) the NE-SW trending TransBrasiliano shear zone was delineated by a NE-SW low-velocity belt at lithospheric depths; (3) the eastern Amazonian craton appears to have thicker lithosphere than the western craton; (4) in areas of Archean nuclei located in the northeastern Guaporé shield and southeastern São Francisco craton, high velocity anomalies were found down to 150 km. 相似文献
6.
V. Midzi 《Journal of Seismology》2001,5(4):559-574
Group velocities estimated from fundamental mode Love and Rayleigh waves are used in a tomography process in central-southern Africa. The waves were generated by eighteen earthquakes, which occurred along the East African Rift and recorded at BOSA, LBTB and SLR seismic stations in southern Africa. The group velocities from Love and Rayleigh waves were isolated using the Multiple Filter Technique (MFT) at the period range of 10 to 50 seconds. The tomography method developed by Ditmar and Yanovskaya (1987) and Yanovskaya and Ditmar (1990), was applied to calculate the lateral distribution of surface wave group velocities in central-southern Africa. The results of the tomographic inversion were plotted as distribution maps. In addition to the maps, I also produced two velocity cross-sections across the area of study. The velocity distribution maps show the regional tectonic units, though with poor resolution. The azimuthal bias of the surface wave paths is reflected in the distribution of the group velocities. The Moho depth appears to correlate with velocities at a period of about 30 s. A low velocity feature observed beneath the Zimbabwe craton implies a thickening upper asthenosphere and lithospheric thinning beneath the Zimbabwe craton. Also estimated was a shear wave velocity model beneath the Zimbabwe craton. 相似文献
7.
Variations in crustal thickness in the Zagros determined by joint inversion of P wave receiver functions (RFs) and Rayleigh wave group and phase velocity dispersion. The time domain iterative deconvolution procedure was employed to compute RFs from teleseismic recordings at seven broadband stations of INSN network. Rayleigh wave phase velocity dispersion curves were estimated employing two-station method. Fundamental mode Rayleigh wave group velocities for each station is taken from a regional scale surface wave tomographic imaging. The main variations in crustal thickness that we observe are between stations located in the Zagros fold and thrust belt with those located in the Sanandaj–Sirjan zone (SSZ) and Urumieh–Dokhtar magmatic assemblage (UDMA). Our results indicate that the average crustal thickness beneath the Zagros Mountain Range varies from ～46 km in Western and Central Zagros beneath SHGR and GHIR up to ～50 km beneath BNDS located in easternmost of the Zagros. Toward NE, we observe an increase in Moho depth where it reaches ～58 km beneath SNGE located in the SSZ. Average crustal thickness also varies beneath the UDMA from ～50 km in western parts below ASAO to ～58 in central parts below NASN. The observed variation along the SSZ and UDMA may be associated to ongoing slab steepening or break off in the NW Zagros, comparing under thrusting of the Arabian plate beneath Central Zagros. The results show that in Central Iran, the crustal thickness decrease again to ～47 km below KRBR. There is not a significant crustal thickness difference along the Zagros fold and thrust belt. We found the same crystalline crust of ～34 km thick beneath the different parts of the Zagros fold and thrust belt. The similarity of crustal structure suggests that the crust of the Zagros fold and thrust belt was uniform before subsidence and deposition of the sediments. Our results confirm that the shortening of the western and eastern parts of the Zagros basement is small and has only started recently. 相似文献
8.
利用环渤海地区的天然地震P波到时资料,采用纬度和经度方向分别为05°×06°的网格划分,反演了该地区地壳上地幔的三维P波速度结构.初步结果表明,环渤海地区地壳上地幔的速度结构具有明显的横向不均匀性：京津唐地区地壳中上部的速度异常反映了浅表层的地质构造特征,造山带和隆起区对应于高速异常,坳陷区和沉积盆地对应于低速异常;地壳下部出现大规模的低速异常与华北地区广泛存在的高导层相对应,估计与壳内的滑脱层和局部熔融、岩浆活动有关;莫霍面附近的速度异常反映了地壳厚度的变化及壳幔边界附近热状态的差异;上地幔顶部大范围的低速异常可能是上地幔软流层热物质大规模上涌所致. 相似文献
9.
The fundamental mode Love and Rayleigh waves generated by ten earthquakes and recorded across the Tibet Plateau, at QUE, LAH, NDI, NIL, KBL, SHL, CHG, SNG and HKG are analysed. Love- and Rayleigh-wave attenuation coefficients are obtained at time periods of 5–120 s using the spectral amplitudes of these waves for 23 different paths. Love wave attenuation coefficient varies from 0.0021 km?1, at a period of 10 s, to 0.0002 km?1 at a period of 90 s, attaining two maxima at time periods of 10 and 115 s, and two minima at time periods of 25 and 90 s. The Rayleigh-wave attenuation coefficient also shows a similar trend. The very low value for the dissipation factor, Qβ, obtained in this study suggests high dissipation across the Tibetan paths. Backus-Gilbert inversion theory is applied to these surface wave attenuation data to obtain average Qβ?1 models for the crust and uppermost mantle beneath the Tibetan Plateau. Independent inversion of Love- and Rayleigh-wave attenuation data shows very high attenuation at a depth of ～50–120 km (). The simultaneous inversion of the Love and Rayleigh wave data yields a model which includes alternating regions of high and low Qβ?1 values. This model also shows a zone of high attenuating material at a depth of ～40–120 km. The very high inferred attenuation at a depth of ～40–120 km supports the hypothesis that the Tibetan Plateau was formed by horizontal compression, and that thickening occurred after the collision of the Indian and Eurasian plates. 相似文献
10.
—Rayleigh waves are used in a tomographic inversion to obtain group velocity maps of East Asia (40° E–160° E and 20° N–70° N). The period range studied is 30 to 70 seconds. Seismograms used for this study were recorded at CDSN stations, at a temporary broadband seismic array in Tibet, at several SRO stations, and Kirnos-equipped stations established in Asia by the former Soviet Union, in Siberia, in the Sakhalin and in Mongolia. Altogether more than 1200 paths were available in the tomographic inversion. The study area includes the Angara craton, the geologically ancient core of Asia, and the subsequently accreted units, the Altaids (a Paleozoic collision complex), the Sino-Korean platform (a chain of Archaen terranes separated by belts of active structures), the south China platform (a collage of Precambrian, Paleozoic and Mesozoic metamorphic and igneous terranes), as well as the Tibetan plateau (an active tectonic feature created in late Cenozoic through collision of the Indian subcontinent and the Asian continent). Many of these main units are recognizable in the tomographic images as distinctive units; Tibet appears as a prominent low velocity (about ?15% from the average) structure, with western and central Tibet often appearing as the areas with the lowest velocities, the Central Asian fold-belt, and the Angara craton are consistently high group velocity areas. Some lesser tectonic features are also recognizable. For example, Lake Baikal is seen as a high velocity feature at periods greater than 40 seconds. However, the high group velocity feature does not stop near the southern end of Lake Baikal; it extends south-southwestward across Mongolia. The North China Plain, a part of the platform where extensional tectonics dominate, is an area of high velocities as a result of relatively thin crust. The south China block, the least tectonically active region of China, is generally an area of high velocity. For periods longer than 40 seconds, a NNE trending high group velocity gradient clearly exists in eastern China; the velocities are noticeably higher in the east. From the group velocity maps, average dispersion curves at twelve locations were determined and inverted to obtain velocity structures. Main results of group velocity inversion include: (1) a Tibetan crust of around 60?km thick, with low crustal and upper mantle shear velocities, at 3.3?km/s and 4.2?km/s, respectively; (2) with the Moho constrained at 40–43?km, the Angara craton and the Central Asian foldbelt have a V S in excess of 4.6?km/s; (3) relatively low shear velocities are obtained for tectonically active areas. In many parts of the study area, where Precambrian basement is exposed, the process in the crust and upper mantle due to recent tectonic activities have modified the crust and upper mantle velocity structures under the Precambrian terranes, they are no longer underlain by high velocity crust and mantle. 相似文献
11.
除了使用前人提取的面波频散曲线外, 还从秦岭及周边地区布设的59个宽频流动地震台的数据和中国地震局及各省局台网的数据中筛选出的地震事件波形和台站间噪声互相关格林函数中提取瑞雷波群速度频散曲线. 利用二维面波层析成像反演获得了瑞雷波周期为10—50 s的各向同性群速度及方位各向异性分布. 结果显示: 周期为10 s的各向同性群速度和方位各向异性分布与各构造单元存在明显的对应关系; 周期为10—50 s的面波在四川盆地和鄂尔多斯盆地内的快波方向多为近NS向. 与前人研究结果不同的是, 本文得到的秦岭、 大巴山构造带周期为10—50 s的面波快波方向均与山脉走向近似平行, 且与SKS波分析得到的快波方向一致. 这表明秦岭和大巴山之下整个岩石圈的快波方向都与山脉走向平行, 预示着秦岭和大巴山整个地壳, 甚至岩石圈发生了类似的变形. 由于四川盆地和鄂尔多斯盆地面波快波方向与SKS波结果差别较大, 推测青藏高原隆升扩展对这两个盆地的地壳基本无影响, 但对其岩石圈上地幔却产生了重大影响. 相似文献
12.
A detailed dispersion analysis of Rayleigh waves generated by local earthquakes and occasionally by blasts that occurred in
southern Spain, was undertaken to obtain the shear-wave velocity structure of the region at shallow depth. Our database includes
seismograms generated by 35 seismic events that were recorded by 15 single-component short-period stations from 1990 to 1995.
All these events have focal depths less than 10 km and body-wave magnitudes between 3.0 and 4.0, and they were all recorded
at distances between 40 and 300 km from the epicentre. We analysed a total of 90 source-station Rayleigh-wave paths. The collected
data were processed by standard digital filtering techniques to obtain Rayleigh-wave group-velocity dispersion measurements.
The path-averaged group velocities vary from 1.12 to 2.25 km/s within the 1.0-6.0 s period interval. Then, using a stochastic
inversion approach we obtained 1-D shear-wave velocity–depth models across the study area, which were resolved to a depth
of circa 5 km. The inverted shear-wave velocities range approximately between 1.0 and 3.8 km/s with a standard deviation range
of 0.05–0.16 km/s, and show significant variations from region to region. These results were combined to produce 3-D images
via volumetric modelling and data visualization. We present images that show different shear velocity patterns for the Betic
Cordillera. Looking at the velocity distribution at various depths and at vertical sections, we discuss of the study area
in terms of subsurface structure and S-wave velocity distribution (low velocity channels, basement depth, etc.) at very shallow
depths (0–5 km). Our results characterize the region sufficiently and lead to a correlation of shear-wave velocity with the
different geological units features. 相似文献
13.
Özcan Çakır 《Acta Geophysica》2018,66(6):1303-1340
The Turkish plate is covered by hundreds of accelerometer and broadband seismic stations with less than 50 km inter-station distance providing high-quality earthquake recordings within the last decade. We utilize part of these stations to extract the fundamental mode Rayleigh and Love surface wave phase and group velocity data in the period range 5–20 s to determine the crust structure beneath the Aegean region in southwest Turkey. The observed surface wave signals are interpreted using both single-station and two-station techniques. A tomographic inversion technique is employed to obtain the two-dimensional group velocity maps from the single-station group velocities. One-dimensional velocity–depth profiles under each two-dimensional mesh point, which are jointly interpreted to acquire the three-dimensional image of the shear-wave velocities underneath the study area, are attained by utilizing the least-squares inversion technique, which is repeated for both Rayleigh and Love surface waves. The isotropic crust structure cannot jointly invert the observed Rayleigh and Love surface waves where the radial anisotropic crust better describes the observed surface wave data. The intrusive magmatic activity related to the northward subducting African plate under the Turkish plate results the crust structure deformations, which we think, causing the observed radial anisotropy throughout complex pattern of dykes and sills. The magma flow resulting in the mineral alignment within dykes and sills contributes to the observed anisotropy. Due to the existence of dykes, the radial anisotropy in the upper crust is generally negative, i.e., vertically polarized S-waves (Vsv) are faster than horizontally polarized S-waves (Vsh). Due to the existence of sills, the radial anisotropy in the middle-to-lower crust is generally positive, i.e., horizontally polarized S-waves (Vsh) are faster than vertically polarized S-waves (Vsv). Similar radial anisotropic results to those of the single-station analyses are obtained by the two-station analyses utilizing the cross-correlograms. The widespread volcanic and plutonic rocks in the region are consistent with the current seismic interpretations of the crustal deformations. 相似文献
14.
Three dimensional shear wave velocity structure of the crust and upper mantle beneath China from ambient noise surface wave tomography 下载免费PDF全文
下载免费PDF全文 Xinlei Sun Xiaodong Song Sihua Zheng Yingjie Yang Michael H. Ritzwoller 《地震科学（英文版）》2010,23(5):449-463
We determine the three-dimensional shear wave velocity structure of the crust and upper mantle in China using Green's functions obtained from seismic ambient noise cross-correlation.The data we use are from the China National Seismic Network,global and regional networks and PASSCAL stations in the region.We first acquire cross-correlation seismograms between all possible station pairs.We then measure the Rayleigh wave group and phase dispersion curves using a frequency-time analysis method from 8 s to 60 s.After that,Rayleigh wave group and phase velocity dispersion maps on 1° by 1° spatial grids are obtained at different periods.Finally,we invert these maps for the 3-D shear wave velocity structure of the crust and upper mantle beneath China at each grid node.The inversion results show large-scale structures that correlate well with surface geology.Near the surface,velocities in major basins are anomalously slow,consistent with the thick sediments.East-west contrasts are striking in Moho depth.There is also a fast mid-to-lower crust and mantle lithosphere beneath the major basins surrounding the Tibetan plateau (TP) and Tianshan (Junggar,Tarim,Ordos,and Sichuan).These strong blocks,therefore,appear to play an important role in confining the deformation of the TP and constraining its geometry to form its current triangular shape.In northwest TP in Qiangtang,slow anomalies extend from the crust to the mantle lithosphere.Meanwhile,widespread,a prominent low-velocity zone is observed in the middle crust beneath most of the central,eastern and southeastern Tibetan plateau,consistent with a weak (and perhaps mobile) middle crust. 相似文献
15.
Sverker Olsson Roland G. Roberts R. Böðvarsson 《Physics of the Earth and Planetary Interiors》2007,160(2):157-168
Teleseismic data recorded by stations in the Swedish National Seismic Network (SNSN) are used for a study of upper mantle structure beneath the Baltic Shield using the receiver function technique. The data show very clear conversions from the 410 and 660 km discontinuities. The signals associated with P to S conversions at these discontinuities arrive 1-2 s earlier than predicted by global models such as IASP91 or PREM. We interpret this as a manifestation of higher than average velocities in the mantle beneath the shield, consistent with lower than average global temperatures. For a 1400 km profile along the network, we observe variations of around 1 second in delay times of P410s and slightly less for P660s. Under the assumption that the mantle discontinuities are at a given constant depth, the delay times of the mantle converted phases are tomographically inverted to reveal P and S velocity structure below the stations. Synthetic tests show that this tomographic inversion has the potential to resolve P and S velocity variations at structural scales adequate for upper mantle studies. Results from application to real data appear to be consistent with independently produced mantle velocity structures deduced from normal tomographic arrival time data. For the P velocity model, a north-dipping body of (relatively) low velocity is found for the central part of the profile at 58-64°N. A sharp contrast from low to high velocities that may be associated with the Proterozoic-Archean boundary is found at 66°N. 相似文献
16.
A crustal tomographic image, from the surface down to 35 km depth beneath the Betic Cordillera (southern Spain), is obtained using data on local earthquakes recorded at stations from the National and Andalusian Seismic Networks. The velocity structure and the hypocentre locations are derived from the inversion of P first arrival times, using an iterative simultaneous inversion method. The reliability of the results is assessed using different control parameters. The inverted velocity field in the uppermost layers shows a significant lateral variability which reflects most of the large-scale geological features of the Betic Cordillera. Well determined local surface anomalies allow to constrain the location and geometry of the most prominent Neogene sedimentary basins. The upper crust is well resolved throughout the whole region, and is characterized by relatively high velocities in the Internal Betics and in the South Iberian Massif and lower velocities within the External Betics. A relatively well constrained event cluster displays a NNE–SSW trend, and outlines the contact zone between the Internal and the External domains. The middle and lower crustal levels show reliable results beneath the central part of the Betic Cordillera. High averaged velocities are obtained within the South Iberian and the Alboran domains, in contrast to a relatively low velocity anomaly which characterizes the boundary between them. These findings support the hypothesis of the lack of well differentiated crustal levels below the contact zone, while crustal layering is better defined beneath the Alboran and the Iberian domains. 相似文献
17.
收集了南北地震带区域地震台网中292个地震台站2008年1月至2011年3月期间的地震波形数据,由频时分析方法提取了Love波相速度频散曲线,经过反演得到了研究区内的Love波相速度分布.根据Love波纯路径频散,采用线性反演方法对0.25°×0.25°的网格点进行了一维S波速度结构反演,利用线性插值获取了南北地震带地区的三维S波速度结构.结果显示了松潘—甘孜地体和川滇菱形块体地区的下地壳具有明显的S波低速层分布,该异常分布特征支持解释青藏高原隆升及其地壳物质运移的下地壳流模型.在100至120km深度上,川滇菱形块体西北部呈现较强的S波高速异常,这可能是印度岩石圈板块沿喜马拉雅东构造结下插至该区域所致,该区域下地壳的低速软弱物质与上地幔的高速强硬物质形成了鲜明对比,暗示了地壳和上地幔可能具有不同的构造运动和变形方式,这为该区域的壳幔动力学解耦提供了条件. 相似文献
18.
19.
The three-dimensional P-wave velocity structure of Mount Spurr is determined to depths of 10 km by tomographic inversion
of 3,754 first-arriving P-wave times from local earthquakes recorded by a permanent network of 11 seismographs. Results show
a prominent low-velocity zone extending from the surface to 3–4 km below sea level beneath the southeastern flank of Crater
Peak, spatially coincident with a geothermal system. P-wave velocities in this low-velocity zone are approximately 20% slower
than those in the shallow crystalline basement rocks. Beneath Crater Peak an approximately 3-km-wide zone of relative low
velocities correlates with a near-vertical band of seismicity, suggestive of a magmatic conduit. No large low-velocity zone
indicative of a magma chamber occurs within the upper 10 km of the crust. These observations are consistent with petrologic
and geochemical studies suggesting that Crater Peak magmas originate in the lower crust or upper mantle and have a short residence
time in the shallow crust. Earthquakes relocated using the three-dimensional velocity structure correlate well with surface
geology and other geophysical observations; thus, they provide additional constraints on the kinematics of the Mount Spurr
magmatic system.
Received: 4 December 1997 / Accepted: 27 February 1998 相似文献
20.
Hüseyin G?kalp 《Pure and Applied Geophysics》2012,169(10):1749-1776
The high level of seismic activity in eastern Turkey is thought to be mainly associated with the continuing collision of the Arabian and Eurasian tectonic plates. The determination of a detailed three-dimensional (3D) structure is crucial for a better understanding of this on-going collision or subduction process; therefore, a body wave tomographic inversion technique was performed on the region. The tomographic inversion used high quality arrival times from earthquakes occurring in the region from 1999 to 2001 recorded by a temporary 29 station broadband IRIS-PASSCAL array operated by research groups from the Universities of Bo?azi?i (Turkey) and Cornell (USA). The data was inverted and consisted of 3,114 P- and 2,298 S-wave arrival times from 252 local events with magnitudes (M D) ranging from 2.5 to 4.8. The stability and resolution of the results were qualitatively assessed by two synthetic tests: a spike test and checkerboard resolution test and it was found that the models were well resolved for most parts of the imaged domain. The tomographic inversion results reveal significant lateral heterogeneities in the study area to a depth of ~20?km. The P- and S-wave velocity models are consistent with each other and provide evidence for marked heterogeneities in the upper crustal structure beneath eastern Turkey. One of the most important features in the acquired tomographic images is the high velocity anomalies, which are generally parallel to the main tectonic units in the region, existing at shallow depths. This may relate to the existence of ophiolitic units at shallow depths. The other feature is that low velocities are widely dispersed through the 3D structure beneath the region at deeper crustal depths. This feature can be an indicator of the mantle upwelling or support the hypothesis that the Anatolian Plateau is underlain by a partially molten uppermost mantle. 相似文献