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1.
The first P-arrival-time data from 513 local earthquakes were analyzed to study lateral variation of the depth to the Conrad
and Moho discontinuities beneath the Chugoku and Shikoku districts, southwest Japan, as well as to determine earthquake hypocenters
and P-wave station corrections. The depth to the discontinuity was estimated by minimizing the travel-time residuals of more
than 8700 first P arrivals observed at 55 seismic stations. The Conrad and Moho discontinuities are located within depth ranges
of 15–25 km and 30–40 km, respectively. The Moho is deeper under the mountain area than under the Seto Inland Sea area, and
especially deep under the Pacific Coast of the Shikoku district and the mountain area in the Chugoku district. The depth variation
of the Moho is quite similar to the Bouguer gravity anomaly distribution and the lateral variations of the P-wave velocity.
The deep Moho under the southern Shikoku is located at the portion in which the continental Moho under the island arc meets
the oceanic Moho that is the boundary interface between the oceanic crust and the Philippine Sea (PHS) plate dipping toward
the back arc. Although there are high mountains in the northern and middle Shikoku, the Moho is not so deep because subduction
of the PHS plate prevents the Moho from getting deep, while the Moho is deep due to isostatic balance under the mountain area
in the Chugoku district. In addition, we indicated the possibility that the upper boundary of the oceanic crust just above
the high-velocity PHS plate is in contact with the deep Moho under the western Chugoku. The contact of the Moho with the oceanic
crust can explain the markedly negative gravity anomaly observed in the western Chugoku and the later phase that appears just
after the first P arrival from local earthquakes. 相似文献
2.
研究龙门山及邻区地壳密度结构对于认识该地区地震活动性具有重要意义.根据龙门山及邻区( 100°～105°E,28°～33°N)的布格重力异常资料,选取了跨越龙门山断裂带的6条重力测线,在深地震测深资料约束下,使用Geosoft软件分别反演出了龙门山地区地下的沉积层、康拉德界面和莫霍面的深度分布.研究结果表明:龙门山断裂带两侧的地壳结构明显不同,西面高原地区沉积层较薄,大部分为基岩出露;而东边盆地沉积层明显较厚,多在6km以上.莫霍面和康拉德面在两侧均相对平缓,康拉德面从东部的大约24km增加到青藏高原山区的35km左右;莫霍面深度从东部盆地的大约42km增加到西部青藏高原的67km左右.龙门山断裂带整体表现为一条近SN向的陡变重力梯度带,并在其地壳内各界面均发生错断,莫霍面和康拉德面错断距离分别达6 ～ 7km和3～ 5km.该区地壳的这种陡变和不均匀性是导致地震活动性强烈的主要原因之一. 相似文献
3.
本文对喜马拉雅计划二期部分台站的远震波形数据进行接收函数提取,利用接收函数共转换点叠加方法获得阿拉善地块、鄂尔多斯地块以及银川—河套盆地下方0~80 km深度的速度间断面结构.结果表明:鄂尔多斯地块成层性好,地壳厚度为38~42 km,康拉德界面为18~22 km,阿拉善地区的Moho面深度为38~45 km.河套盆地地壳厚度约52 km,银川断陷盆地和贺兰山下方的Moho面最深为~55 km.鄂尔多斯西缘构造边界下方Moho面变化明显,且黄河断裂为深大断裂直接切割莫霍界面.根据本文的间断面成像结果我们进一步确定阿拉善地块与鄂尔多斯地块分属不同的大地构造单元.与此同时,我们推测贺兰山以西70~80 km范围内和鄂尔多斯地块西缘北段存在地壳增厚变形的可能. 相似文献
4.
Crust and uppermost mantle structure of the Ailaoshan-Red River fault from receiver function analysis 总被引：5，自引：1，他引：5
XU Mingjie WANG Liangshu LIU Jianhua ZHONG Kai LI Hua HU Dezhao XU Zhen 《中国科学D辑(英文版)》2006,49(10):1043-1052
S-wave velocity structure beneath the Ailaoshan-Red River fault was obtained from receiver functions by using teleseismic body wave records of broadband digital seismic stations. The average crustal thickness, Vp/Vs ratio and Poisson’s ratio were also estimated. The results indicate that the interface of crust and mantle beneath the Ailaoshan-Red River fault is not a sharp velocity discontinuity but a characteristic transition zone. The velocity increases relatively fast at the depth of Moho and then increases slowly in the uppermost mantle. The average crustal thickness across the fault is 36―37 km on the southwest side and 40―42 km on the northeast side, indicating that the fault cuts the crust. The relatively high Poisson’s ratio (0.26―0.28) of the crust implies a high content of mafic materials in the lower crust. Moreover, the lower crust with low velocity could be an ideal position for decoupling between the crust and upper mantle. 相似文献
5.
Ki Young Kim Jung Mo Lee Wooil Moon Chang-Eob Baag Heeok Jung Myung Ho Hong 《Pure and Applied Geophysics》2007,164(1):97-113
In order to investigate the velocity structure of the southern part of the Korean peninsula, seismic refraction profiles were
obtained along a 294-km WNW-ESE line and a 335-km NNW-SSE line in 2002 and 2004, respectively. Seismic waves were generated
by detonating 500–1000 kg explosives in drill holes at depths of 80–150 m. The seismic signals were recorded by portable seismometers
at nominal intervals of 1.5–1.7 km. Separate velocity tomograms were derived from first arrival times using a series expansion
method of travel-time inversion. The raypaths indicate several mid-crust interfaces including those at approximate depths
of 2–3, 15–17, and 22 km. The Moho discontinuity with refraction velocity of 7.8 to 8.4 km/s has a maximum depth of 37–39
km under the southern central portion of the peninsula. The Moho becomes shallower as the Yellow Sea and the East Sea are
approached on the west and east coasts of the peninsula, respectively. The depth of the 7.6 km/s velocity contour varies from
29.4 km to 36.5 km. The discrepancy in depth between the seismological Moho and the interpreted critically refracting interface
may result from the presence of a gradual transition between the crust and mantle. The velocity tomograms show particular
crustal structures including (1) the existence of an over 70-km wide low-velocity zone centered at 6–7 km depth under the
Okchon fold belt and Ryeongnam massif, (2) existence of high-velocity materials under the Gyeongsang basin, and (3) the downward
extension of the Yeongdong fault to depths greater than 10 km. 相似文献
6.
川滇地区地壳上地幔三维速度结构研究 总被引：95，自引：22，他引：95
根据云南和四川地震台网174个台站记录的4625个区域地震初至Ｐ波和Ｓ波走时资料，并结合其它深部地球物理资料，确定了川滇地区地壳上地幔三维速度结构．在上地壳速度异常分布中，四川盆地为正异常，川西高原为负异常，龙门山断裂带为正、负异常的边界．龙门山断裂、鲜水河断裂以及红河断裂等，在下地壳和上地幔的速度异常中仍显示出构造分界特征，说明它们可能穿透了莫霍界面．腾冲火山区和攀西构造带在50km深度上呈现负速度异常，与上地幔温度和物质组成的差异相联系．川滇地区地壳结构的总体特征是:地壳和上地幔的低平均速度，地壳厚度变化剧烈，地壳和（或）上地幔存在高导层、高热流值．这些同印度板块与欧亚板块碰撞的构造背景有关．川滇菱形块体在地壳内总体上为正常或正异常速度，而其边界的深大走滑断裂存在负速度异常，它有助于地壳块体沿断裂的侧向挤出．在主要的地震带上，中下地壳的负速度异常与地震活动性相关．多数强烈地震发生在具有正速度异常或正常速度分布的上中地壳深度上，而其下方则通常是负速度异常带． 相似文献
7.
The spectral study of the Bouguer anomally map of Central India suggests an uplifted crust-mantle interface under the Mahandi graben. This study has delineated three subsurface levels of anomalous masses at the respective depths of 23 km, 8 km, and 2 km apparently representing the Moho, an intermediate discontinuity in the sialic part of the crust and the basement, respectively. Model study of the Bouguer anomaly along a profile suggests a typical continental graben type subsurface structure with a low density depression in the sialic part of the crust between 8 and 18 km supported by an elevated upper mantle of intermediate density (3.4 g/cm3) varying in depth from 25 km to 55 km. The depths of the inferred interfaces in case of Bundelkhand granite are 32 km, 11 km, and 1.5 km, which might represent the Moho, the base of intruded granite massif, and some shallow compositional variation. Similar studies in case of Vindhyan basin have brought out three discontinuities at the respective depths of 16 km, 6–4.5 km, and 2.4 km. The first horizon at the depth of 16 km probably represents the interface between the granitic and the basaltic part of the crust. The 6–4.5 km is the depth of the basement, with the 2.4 km interface separating Bijawar rocks from Vindhyans wherever they are present. A generalized inversion of a profile across a positive belt of Bouguer anomaly representing the subsurface Bijawar rocks support the above result. 相似文献
8.
ZHANG Zhongjie BAI Zhiming WANG Chunyong TENG Jiwen Lü Qingtian LI Jiliang LIU Yifeng & LIU Zhenkuan . Institute of Geology Geophysics Chinese Academy of Sciences Beijing China . Institute of Geophysics China Seismological Bureau Beijing China . Institute of Deposition Resource Chinese Academy of Geosciences Beijing China . School of Exploration Information China University of Geosciences Beijing China 《中国科学D辑(英文版)》2005,48(9):1329-1336
The Sanjiang area in southwest China is considered as a tectonic intersection belt between the Tethys-Alps and the western Pacific, and has endured three-phase evolution processes: Proto-Tethys,Paleo-Tethys and Meso-Tethys[1―4]. In this area, its tectonics and struc- ture are extremely complicated, and intensively extru-sive deformation and faults are widely developed[1―3]. For that, the area is considered as the ideal na- ture-laboratory to study the evolution of Paleo-Tethys and also … 相似文献
9.
Salah Saleh 《Pure and Applied Geophysics》2013,170(12):2037-2074
Crustal and lithospheric thicknesses of the southeastern Mediterranean Basin region were determined using 3D Bouguer and elevation data analysis. The model is based on the assumption of local isostatic equilibrium. The calculated regional and residual Bouguer anomaly maps were employed for highlighting both deep and shallow structures. Generally, the regional field in the area under study is considered to be mainly influenced by the density contrast between the crust and upper mantle. Use of the gravity and topographic data with earthquake focal depths has improved both the geometry and the density distribution in the 3-D calculated profiles. The oceanic-continental boundary, the basement relief, Moho depth and lithosphere-asthenosphere boundary maps were estimated. The results point to the occurrence of thick continental crust areas with a thickness of approximately 32 km in northern Egypt. Below the coastal regions, the thickness of crust decreases abruptly (transition zone). An inverse correlation between sediment and crustal thicknesses shows up from the study. Furthermore, our density model reveals the existence of a continental crustal zone below the Eratosthenes Seamount block. Nevertheless, the crustal type beneath the Levantine basin is typically oceanic; this is covered by sedimentary sequences more than 14 km thick. The modeled Moho map shows a depth of 28–30 km below Cyprus and a depth of 26–28 km beneath the south Florence Rise in the northern west. However, the Moho lies at a constant shallow depth of 22–24 km below the Levantine Basin, which indicates thinning of the crust beneath this region. The Moho map reveals also a maximum depth of about 33–35 km beneath both the northern Egypt and northern Sinai, both of which are of the continental crust. The resulting mantle density anomalies suggest important variations of the lithosphere-asthenosphere boundary (LAB) topography, indicating prominent lithospheric mantle thinning beneath south Cyprus (LAB ~90 km depth), followed by thickening beneath the Eratosthenes seamount, Florence Rise, Levantine Basin and reaching to maximum thickness below Cyprian Arc (LAB ~115–120 km depth), and further followed by thinning in the north African margin plate and north Sinai subplate (LAB ~90–95 km depth). According to our density model profiles, we find that almost all earthquakes in the study area occurred along the western and central segments of the Cyprian arc while they almost disappear along the eastern segment. The active subduction zone in the Cyprian Arc is associated with large negative anomalies due to its low velocity upper mantle zone, which might be an indication of a serpentinized mantle. This means that collision between Cyprus and the Eratosthenes Seamount block is marked by seismic activity. Additionally, this block is in the process of dynamically subsiding, breaking-up and being underthrusted beneath Cyprus to the north and thrusted onto the Levantine Basin to the south. 相似文献
10.
A compiled gravity anomaly map of the Western Himalayan Syntaxis is analysed to understand the tectonics of the region around
the epicentre of Kashmir earthquake of October 8, 2005 (Mw = 7.6). Isostatic gravity anomalies and effective elastic thickness
(EET) of lithosphere are assessed from coherence analysis between Bouguer anomaly and topography. The isostatic residual gravity
high and gravity low correspond to the two main seismic zones in this region, viz. Indus–Kohistan Seismic Zone (IKSZ) and
Hindu Kush Seismic Zones (HKSZ), respectively, suggesting a connection between siesmicity and gravity anomalies. The gravity
high originates from the high-density thrusted rocks along the syntaxial bend of the Main Boundary Thrust and coincides with
the region of the crustal thrust earthquakes, including the Kashmir earthquake of 2005. The gravity low of HKSZ coincides
with the region of intermediate–deep-focus earthquakes, where crustal rocks are underthrusting with a higher speed to create
low density cold mantle. Comparable EET (∼55 km) to the focal depth of crustal earthquakes suggests that whole crust is seismogenic
and brittle. An integrated lithospheric model along a profile provides the crustal structure of the boundary zones with crustal
thickness of about 60 km under the Karakoram–Pamir regions and suggests continental subduction from either sides (Indian and
Eurasian) leading to a complex compressional environment for large earthquakes. 相似文献
11.
12.
Introduction The gravity anomaly is an indicator of the density distribution of the underground material. Therefore the gravity anomalies have been important data used for studying the deep crustal struc-ture for a long time. Many people have made detailed researches on the regional crustal structure inverted by Bouguer anomalies. In particular some empirical formulae and practical algorithms about the crustal thickness were brought forward, and a series of results were obtained (MENG, 1996)… 相似文献
13.
Lupei Zhu Ying Tan Donald V. Helmberger Chandan K. Saikia 《Pure and Applied Geophysics》2006,163(7):1193-1213
We use the recordings from 51 earthquakes produced by a PASSCAL deployment in Tibet to develop a two-layer crustal model for
the region. Starting with their ISC locations, we iteratively fit the P-arrival times to relocate the earthquakes and estimate mantle and crustal seismic parameters. An average crustal P velocity of 6.2–6.3 km/s is obtained for a crustal thickness of 65 km while the P velocity of the uppermost mantle is 8.1 km/s. The upper layer of the model is further fine-tuned by obtaining the best synthetic
SH waveform match to an observed waveform for a well-located event. Green's functions from this model are then used to estimate
the source parameters for those events using a grid search procedure. Average event relocation relative to the ISC locations,
excluding two poorly located earthquakes, is 16 km. All but one earthquake are determined by the waveform inversion to be
at depths between 5 and 15 km. This is 15 km shallower, on average, than depths reported by the ISC. The shallow seismicity
cut-off depth and low crustal velocities suggest high temperatures in the lower crust. Thrust faulting source mechanisms dominate
at the margins of the plateau. Within the plateau, at locations with surface elevations less than 5 km, source mechanisms
are a mixture of strike-slip and thrust. Most events occurring in the high plateau where elevations are above 5 km show normal
faulting. This indicates that a large portion of the plateau is under EW extension. 相似文献
14.
Ashutosh Chamoli Anand K. Pandey V. P. Dimri P. Banerjee 《Pure and Applied Geophysics》2011,168(5):827-844
The gravity response and crustal shortening in the Himalayan belt are modeled in detail for the first time in the NW Himalaya.
The Bouguer gravity anomaly along a ~450-km-long (projected) transect from the Sub-Himalaya in the south to the Karakoram
fault in the north across the Indus-Tsangpo Suture Zone is modeled using spectral analysis, wavelet transform and forward
modeling. The spectral analysis suggests three-layer interfaces in the lithosphere at 68-, 34- and 11-km depths corresponding
to the Moho, the Conrad discontinuity and the Himalayan decollement thrust, respectively. The coherence, admittance and cross
spectra suggest crustal shortening because of convergence compensated by lithospheric folding at 536- and 178-km wavelength
at the Moho and the upper-crustal level. An average effective elastic thickness of around 31 km is calculated using the coherence
method. The gravity data are modeled to demarcate intracrustal to subcrustal regional thrust/fault zones. The geometrical
constraints of these faults are obtained in the space scale domain using the wavelet transform, showing good correlation with
the major tectonic boundaries. The crustal configuration along the transect shows how the Moho depth increases from 45 to
80 km towards the north with the locus of flexure of the Indian crust beneath the Higher Himalayan zone. The combination of
forward modeling and wavelet analysis gives insight into the subsurface extent and geometry of regional structures across
the NW Himalaya. 相似文献
15.
利用2014年完成的穿过银川盆地人工源宽角反射与折射剖面的3炮长观测距资料,采用基于地震波走时反演方法的Rayinvr算法得到了研究区地壳和上地幔的速度结构.结果表明:研究区地壳厚度为42—48 km,莫霍面沿剖面展布形态呈现出东西两侧浅、中部较深的特征,莫霍面最深的区段位于贺兰山下方. P波速度沿剖面随着深度的增加呈正梯度增大,然而在深度约为90—103 km的岩石圈地幔中,识别出两组较明显的反射界面,两组界面之间并未发现P波速度随深度而显著增加,表明研究区下方存在与地球平均模型中速度随深度增加而增大不相符的速度结构,推测银川盆地下方岩石圈与软流圈之间可能存在速度过渡带. 相似文献
16.
《Journal of Geodynamics》1999,27(4-5):567-583
Upper mantle P and S wave velocities in the western South America region are obtained at depths of foci from an analysis of travel time data of deep earthquakes. The inferred velocity models for the Chile–Peru–Ecuador region reveal an increase of P velocity from 8.04 km/s at 40 km to 8.28 km/s at 250 km depth, while the S velocity remains almost constant at 4.62 km/s from 40 to 210 km depth. A velocity discontinuity (probably corresponding to the L discontinuity in the continental upper mantle) at 220–250 km depth for P and 200–220 km depth for S waves, with a 3–4% velocity increase, is inferred from the velocity–depth data. Below this discontinuity, P velocity increases from 8.54 km/s at 250 km to 8.62 km/s at 320 km depth and S velocity increases from 4.81 km/s at 210 km to 4.99 km/s at 290 km depth. Travel time data from deep earthquakes at depths greater than 500 km in the Bolivia–Peru region, reveal P velocities of about 9.65 km/s from 500 to 570 km depth. P velocity–depth data further reveal a velocity discontinuity, either as a sharp boundary at 570 km depth with 8–10% velocity increase or as a broad transition zone with velocity rapidly increasing from 560 to 610 km depth. P velocity increases to 10.75 km/s at 650 km depth. A comparison with the latest global average depth estimates of the 660 km discontinuity reveals that this discontinuity is at a relatively shallow depth in the study region. Further, a velocity discontinuity at about 400 km depth with a 10% velocity increase seems to be consistent with travel time observations from deep earthquakes in this region. 相似文献
17.
Seismic data recorded in the upper mantle triplication distance range between 10° and 30° are generated by wave propagation through complex upper mantle structure. They can be used to place constraints on seismic velocity structures in the upper mantle, key seismic features near the major discontinuities, and anisotropic structure varying with depth. In this paper, we review wave propagation of the upper mantle triplicated phases, how different key seismic features can be studied using upper mantle triplicated data, and the importance of those seismic features to the understanding of mantle temperature and composition. We present two examples of using array triplicated phases to constrain upper mantle velocity structures and detailed features of a certain discontinuity, with one for a shallow event and the other for deep events. For the shallow event, we present examples of how the array triplication data can be used to constrain several key properties of the upper mantle: existence of a lithospheric lid, existence of a low velocity zone beneath the lithospheric lid, and P/S velocity ratio as a function of depth. For deep events, we show examples of how array triplication data can be used to constrain the detailed structures of a certain discontinuity: velocity gradients above and below the discontinuity, velocity jumps across the discontinuity and depth extents of different velocity gradients. We discuss challenges of the upper mantle triplication study, its connection to other approaches, and its potential for further studying some other important features of the mantle: the existence of double 660-km discontinuities, existence of low-velocity channels near major discontinuities and anisotropy varying with depth. 相似文献
18.
19.
Study on distribution characteristics of strong earthquakes in Sichuan-Yunnan area and their geological tectonic background 总被引：2，自引：0，他引：2
Introduction Both Sichuan and Yunnan are provinces with more earthquakes. Based on catalogue of strong earthquakes in China compiled by the Prediction Department of China Earthquake Administration, there are 639 M5.0 earthquakes during 26 B.C.~A.D. 2001. Among them, 475 are M=5.0~5.9 events, 124 are M=6.0~6.9 events, 39 are M=7.0~7.9 events, and one is M=8 event occurred in Sichuan and Yunnan area. Here is one of the areas where seismic activities are most active in China. Sichuan-Yun… 相似文献
20.
In this work we have tried to detect and collect later phases associated with the Moho discontinuity and use them to study the lateral variations of the crustal thickness in southwest (SW) Japan. We first compute synthetic seismograms for local earthquakes taking into account the focal mechanism solutions estimated from first motion polarity data. Then we compare the synthetics with the observed waveforms to detect the major later phases in seismograms of shallow crustal earthquakes in SW Japan. Taking the advantage of the high quality and great quantity of Hi-net waveform data now available throughout the entire Japan Islands, we have detected 1659 Moho-reflected phases (PmP and SmS) from shallow events in SW Japan. We estimated the crustal thickness in the study area using travel time differences between these later phases and the first P and S arrivals. Our results show that the Moho is deep in Chubu district and becomes shallower along the coastlines of the Japan Sea and the Pacific Ocean. A thinner crust appears in Osaka Bay and Awajii Island where the 1995 Kobe earthquake (M7.2) occurred. 相似文献