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1.
The deep seismic reflection data on profile HY2 are reprocessed by the method of simultaneous inversion of velocity distribution and interface position. By the travel-time inversion with the data of the diving wave Pg and fault plane reflection wave, we determine the geometric form and velocity of Haiyuan fault zone interior and surrounding rock down to 10 km depth. The measured data show that the amplitudes have strong attenuation in the range of stake number 37–39 km, suggesting the fault zone has considerable width in the crustal interior. The results of this paper indicate that to the north of the fault zone the crystalline basement interface upheaves gradually from southwest to northeast and becomes shallow gradually towards northeast, and that to the south of the fault zone, within the basin between Xihua and Nanhua mountains, the folded basement becomes shallow gradually towards southwest. The obliquity of the fault zone is about 70° above the 3 km depth, about 60° in the range of the 3–10 km depths. From the results of this paper and other various citations, we believe that Haiyuan fault zone is in steep state from the Earth’s surface to the depth of 10 km. Foundation item: Joint Seismological Science Foundation of China (201001) and State Key Basic Research Development and Programming Project (95-13-02-02). Contribution No. RCEG200308, Exploration Geophysical Center, China Earthquake Administration.  相似文献   

2.
The Buyuk Menderes Graben is a depression in the Menderes core complex of western Turkey. The region is one of the most rapidly deforming regions of continental crust in the world and has exceptionally high seismic activity. In this study, shallow and deep seismic studies were conducted at the Buyuk Menderes graben. These studies included surface geological mapping and two seismic reflection sections. Detailed modelling was performed with the seismic study. In addition to these, a moving windows power spectrum was applied to the Bouguer gravity profile data of the study area. Since no deep well is available in this area, the geological interpretation of the seismic stratigraphy is based on the correlation with the surface geology, this was combined with the major reflections and the seismic facies observed along the profiles, and, thus, four main seismic units can be distinguished in the basin fill. Structural features of the basin is driven by a complex extensional faults system, consisting of a low-angle, S-dipping Buyuk Menderes detachment and by its synthetic and antithetic splays, bordering the opposite flanks of the basin. As a result of conventional deep seismic reflection sections and gravity data, three layers were defined in the study area. The first layer occurs at a thickness of 6 km, and the second layer is between 13 and 18 km. The third layer is at ~33 km and may also emphasize Moho depth. The Buyuk Menderes graben has three clear reflectors which are base sediments, brittle-ductile transition, Moho and faults that show a half-graben floored by a detachment. The Moho depth is comparable with previous estimates. According to the results obtained, Bouguer gravity and seismic results are very much consistent with each other. It was observed that at the depths determined from seismic and gravity data, the distribution percentage of earthquake focal depths also rises.  相似文献   

3.
Long Valley Caldera is an active volcanic region in east central California. Surface deformation on the resurgent dome within the caldera was an order of magnitude higher for the five-month period September 1997 through January 1998 compared to the previous three-year average. However, the location of the immediate (shallow) source of deformation remained essentially constant, 5–7 km beneath the dome, near the top of a region of probable magma accumulation defined by seismic data. Similarly, although the rate of seismic moment release increased dramatically, earthquake locations remained similar to earlier periods. The rate of deformation increased exponentially between April–May 1997 and late November 1997 with a time constant of ∼55–65 days, after which it decreased exponentially with about the same time constant. We develop a model consistent with these observations and also consistent with independent constraints on sub-surface rheology from thermal, geochemical and laboratory data. Deformation at sites on the resurgent dome most sensitive to the shallow deformation source are well fit by a model with a single pressure source at 6 km depth which experienced a pressure pulse that began in late 1996, peaked in November 1997, close to the time of major seismic moment release, and essentially ended in mid-1999. The pressure source in our model is surrounded by a 1 km thick “shell” of Maxwell viscoelastic material (shell viscosity 1016 Pa s) within an elastic half space, and has peak values that are much lower than corresponding purely elastic half space models. The shell viscosity is characteristic of a weak, deformable solid, e.g. quartz-bearing country rock surrounding the magma chamber at temperatures in the range 500–600°C, i.e. above the brittle–ductile transition, and/or largely crystallized rhyolite near its solidus temperature of ∼670°C, material that probably exists near the top of the zoned magma chamber at Long Valley.  相似文献   

4.
The interpretation of the Jarrafa magnetic and gravity highs, NW Libyan offshore, suggests that it may be caused by a body of high-density and high magnetization. Analysis of their power spectra indicates two groups of sources at: (1) 2.7 km depth, probably related to the igneous rocks, some of which were penetrated in the JA-1 borehole, (2) 5 km depth, corresponding to the top of the causative body and (3) 10 km depth, probably referring to the local basement depth. The boundary analysis derived from applied horizontal gradient to both gravity and magnetic data reveals lineaments many of which can be related to geological structures (grabens, horsts and faults).The poor correlation between pseudogravity fields for induced magnetization and observed gravity fields strongly suggests that the causative structure has a remanent magnetization (D = −16°, I = 23°) of Early Cretaceous age, fitting with the opening of the Neo Tethys 3 Ocean.Three-dimensional interpretation techniques indicate that the magnetic source of the Jarrafa magnetic anomaly has a magnetization intensity of 0.46 A/m, which is required to simulate the amplitude of the observed magnetic anomaly. The magnetic model shows that it has a base level at 15 km.The history of the area combined with the analysis and interpretation of the gravity and magnetic data suggests that: (1) the source of the Jarrafa anomaly is a mafic igneous rock and it may have formed during an Early Cretaceous extensional phase and (2) the Jarrafa basin was left-laterally sheared along the WNW Hercynian North Graben Fault Zone, during its reactivation in the Early Cretaceous.  相似文献   

5.
The late Triassic to early Tertiary Coast Mountains Batholith (CMB) of British Columbia provides an ideal locale to study the processes whereby accreted terranes and subduction-related melts interact to form stable continental crust of intermediate to felsic composition and complementary ultramafic residuals. Seismic measurements, combined with calculated elastic properties of various CMB rock compositions, provide a window into the deep-crustal lithologies that are key to understanding the processes of continental growth and evolution. We use a combination of seismic observations and petrologic modeling to construct hypothetical crustal sections at representative locations across the CMB, then test the viability of these sections via forward modeling with synthetic seismic data. The compositions that make up our petrologic forward models are based on calculations using the free energy minimization program Perple_X to predict mineral assemblages at depth for the bulk compositions of exposed plutonic rocks collected in the study area. Seismic data were collected along two transects in west-central British Columbia: a southern line that crossed the CMB near the town of Bella Coola (near 52° N), and a northern line centered on the towns of Terrace and Kitimat (near 54° N). Along both transects, seismic receiver functions reveal high Vp/Vs ratios near the Insular/Intermontane terrane boundary and crustal thickness increasing from 26 ± 3 km to 34 ± 3 km (at the 1 sigma certainty level) from west to east across the Coast Shear Zone (CSZ). On the southern line, we observe an anomalous region of complex receiver functions and diminished Moho signals beneath the central portion of the CMB. Our petrologic and seismic profiles show that observed seismic data from much of the CMB can be well-matched in terms of crustal thickness and structure, average Vp/Vs, and amplitude of the Moho converted phase, without including ultramafic residual material in the lower crust.  相似文献   

6.
Seismic anisotropy is a unique observational tool for remotely studying deformation and stress within the Earth. Effects of anisotropy can be seen in seismic data; they are due to mineral alignment, fractures or layering. Seismic anisotropy is linked to local stress and strain, allowing modern geophysics to derive geomechanical properties from seismic data for supporting well planning and fracking. For unravelling anisotropic properties of the crust, the teleseismic receiver functions methodology has started to be widely applied recently due to its ability in retrieving the three-dimensional characteristics of the media sampled by the waves. The applicability of this technique is tested here by a field test carried out around the Kontinental Tiefbohrung site in southeastern Germany. We compare our results to previous investigations of the metamorphic rock pile of the Zone Erbendorf-Vohenstrauss, drilled down to 9 km depth, which sampled an alternating sequence of paragneiss and amphibolite, in which a strong foliation has been produced by ductile deformation. The application of the receiver functions reveals the presence of two distinct anisotropic layers within the metamorphic rock pile at 0–4 km and below 6 km depth, with up to 8% anisotropy; the depth of these two layers corresponds to the location of mica-rich paragneiss which show intense foliation, and finally proves the relation between the signal in the receiver functions, rock texture and presence of cracks. We have now the capability of providing insights from passive seismic data on geomechanical properties of the rocks, useful for geological exploration and engineering purposes, which will help influencing expensive drilling decisions thanks to future application of this seismic technique.  相似文献   

7.
Introduction The deep faults in the crust have direct relation to the occurrence of earthquakes and the dis-tribution of active seismic zones, so the researches on the geometric form and physical parametersof deep crustal faults are always an important problem in seismology. The researches are not onlysignificant to knowledge the deep tectonic background of strong earthquake and seismogenicmechanism, but also play a very important role in earthquake hazard estimation and earthquakeprevent…  相似文献   

8.
中朝地台东北缘3km深度地震层析成像的地质解释   总被引：1，自引：0，他引：1  
利用本区丰富的地质,人工地震探测资料,对３ｋｍ深度层析成象成果进行了分析对比,结果表明：（１）３Ｋｍ深度速度值与人工地震探测所获地震波速值吻合较好;（２）３ｋｍ深度速度分布与该深度处的岩石介质分布有较好的对应关系,高速区对应地层主要为太古界－下元古界变质岩和地槽区的古生界浅变质岩,低速区对应主要为中生界火山,碎屑岩,燕山期花岗岩体和盖层性质的中元古－古代界沉积岩。（３）层析成象结果可划分深度不同的  相似文献   

9.
Seismic reflection data were acquired across volcanic hosted massive sulfides (VHMS) of Vihanti in order to improve the understanding of the regional geological setting. Commercially processed seismic data from Vihanti are of good quality, but reprocessing can be used to extract additional information about geological structures. Especially, careful velocity analysis influences the quality of seismic images. Differentiating reflections caused by fractures from those caused by lithological contacts is very important for exploration and geological modeling. Reflections from fracture zones known from drilling stack with lower velocity (~ 5100 m/s) compared to typical stacking velocities of the Vihanti area (> 5500 m/s). The reprocessing also indicated that fracture zones are better imaged with low frequencies due to the better overall continuity of the fault zones at scales of hundreds of meters rather than at shorter seismic wavelengths.In full stacks, long offset data can mask structure close to the surface. More detailed seismic images of the shallow subsurface emerged by preferentially stacking short offset data wherever acquisition and processing lines lay close together and were nearly straight. Long offset data remains valuable for imaging deeper structures as well as dipping reflectors. Cross-dip-analysis revealed a bright diffractor located near the base of the Vihanti volcanic basin at 1.5 km depth. The seismic data allow a geological interpretation in which the Vihanti structure has developed through significant thrust faulting and displacement of the lithological contacts. Gentle folds that were formed prior to faulting are visible as undulating reflectivity in seismic sections. The reprocessed seismic section indicates a potential deep extension of the ore-hosting altered volcanic and calc-silicate rocks previously unexplored.  相似文献   

10.
The Ningdu basin, located in southern Jiangxi province of southwest China, is one of the Mesozoic basin groups which has exploration prospects for geothermal energy. A study on the detailed velocity structure of the Ningdu basin can provide important information for geothermal resource exploration. In this study, we deployed a dense seismic array in the Ningdu basin to investigate the 3D velocity structure and discuss implications for geothermal exploration and geological evolution. Based on the dense seismic array including 35 short-period (5 s-100 ?Hz) seismometers with an average interstation distance of ～5 ?km, Rayleigh surface wave dispersion curves were extracted from the continuous ambient noise data for surface wave tomographic inversion. Group velocity tomography was conducted and the 3D S-wave velocity structure was inverted by the neighborhood algorithm. The results revealed obvious low-velocity anomalies in the center of the basin, consistent with the low-velocity Cretaceous sedimentary rocks. The basement and basin-controlling fault can also be depicted by the S-wave velocity anomalies. The obvious seismic interface is about 2 ?km depth in the basin center and decreases to 700 ?m depth near the basin boundary, suggesting spatial thickness variations of the Cretaceous sediment. The fault features of the S-wave velocity profile coincide with the geological cognition of the western boundary basin-controlling fault, which may provide possible upwelling channels for geothermal fluid. This study suggests that seismic tomography with a dense array is an effective method and can play an important role in the detailed investigations of sedimentary basins.  相似文献   

11.
A remarkable earthquake struck Yutian, China on June 26th, 2020. Here, we use Sentinel-1 images to investigate the deformation induced by this event. We invert the InSAR observations using a two-step approach: a nonlinear inversion to constrain fault geometries with uniform slip based on the rectangular plane dislocation in an elastic half-space, followed by a linear inversion to retrieve the slip distribution on the fault plane. The results show that the maximum LOS displacement is 22.6 ​cm, and the fault accessed to the ruptured characteristics of normal faults with the minor left-lateral strike-slip component. The fault model indicates a 210° strike. The main rupture zone concentrates in the depth of 5–15 ​km, and the fault slip peaks at 0.89 ​m at the depth of 9 ​km. Then, we calculate the variation of the static Coulomb stress based on the optimal fault model, the results suggest that the Coulomb stress of the Altyn Tagh fault and other neighboring faults has increased and more attention should be paid to possible seismic risks.  相似文献   

12.
From an analysis of many seismic profiles across the stable continental regions of North America and northern Europe, the crustal and upper mantle velocity structure is determined. Analysis procedures include ray theory calculations and synthetic seismograms computed using reflectivity techniques. TheP wave velocity structure beneath the Canadian Shield is virtually identical to that beneath the Baltic Shield to a depth of at least 800 km. Two major layers with a total thickness of about 42 km characterize the crust of these shield regions. Features of the upper mantle of these region include velocity discontinuities at depths of about 74 km, 330 km, 430 km and 700 km. A 13 km thickP wave low velocity channel beginning at a depth of about 94 km is also present.A number of problems associated with record section interpretation are identified and a generalized approach to seismic profile analysis using many record sections is described. TheS wave velocity structure beneath the Canadian Shield is derived from constrained surface wave data. The thickness of the lithosphere beneath the Canadian and Baltic Shields is determined to be 95–100 km. The continental plate thickness may be the same as the lithospheric thickness, although available data do not exclude the possibility of the continental plate being thicker than the lithosphere.  相似文献   

13.
碳酸盐岩复杂的孔隙结构如何影响其弹性性质一直是地球物理研究的难点问题,在此基础上如何半定量甚至是定量地对碳酸盐岩储层预测,特别是如何有效地获取孔隙结构参数相关的地震属性体一直是油气工业界追求的目标,本研究从数字岩心角度入手,联合测井以及地震数据尝试探究这一问题的解决方案.首先针对代表不同孔隙结构类型的有限数目的碳酸盐岩样品获得其对应的高精度数字岩心数据体,为了获得更加可靠的具有地球物理含义的弹性性质随孔隙度变化的统计规律,我们通过子网格的技术,在有限数目的碳酸盐岩数字岩心数据体上获得了大量的数字岩心子网格样本,对于每个子网格样本可以分别获得其对应的数字岩心图像孔隙度、表征孔隙软硬程度的孔隙结构参数(γ)、以及基于有限元法模拟的弹性性质,由此基于数字岩心的研究思路,我们最终获得了基于孔隙结构因子表征与分类下的弹性性质与孔隙度的定量化解释量版.与此同时,在地震尺度上通过叠前地震资料获取的纵横波及密度属性体后,基于如上获得的定量化解释量版,我们最终获得了针对碳酸盐岩储层的新的属性体——孔隙结构参数(γ)属性体,这使得在地震尺度上预测碳酸盐岩储层的孔隙结构类型成为可能,也使利用地震数据在孔隙结构参数表征与分类下的碳酸盐岩储层反演精度的提高成为可能.  相似文献   

14.
A refined digital model of the Moho depth is constructed for the Antarctica on a uniform grid with resolution of 1° × 1°. The model is based on seismic data. Results are presented as a digital table that defines the Moho depth (the Moho, or M) at each point. A large volume of new data on reflection, refraction, converted and surface waves, as well as receiver functions and data on subglacial relief, were analyzed. The new model provides far more precise and detailed information about the Moho than the previous model. The difference in the crustal thickness between these two models may amount up to −10–±24 km.  相似文献   

15.
In this paper we discuss characteristic features of subduction zone seismicity at depths between about 100 km and 700 km, with emphasis on the role of temperature and rheology in controlling the deformation of, and the seismic energy release in downgoing lithosphere. This is done in two steps. After a brief review of earlier developments, we first show that the depth distribution of hypocentres at depths between 100 km and 700 km in subducted lithosphere can be explained by a model in which seismic activity is confined to those parts of the slab which have temperatures below a depth-dependent critical valueT cr.Second, the variation of seismic energy release (frequency of events, magnitude) with depth is addressed by inferring a rheological evolution from the slab's thermal evolution and by combining this with models for the system of forces acting on the subducting lithosphere. It is found that considerable stress concentration occurs in a reheating slab in the depth range of 400 to 650–700 km: the slab weakens, but the stress level strongly increases. On the basis of this stress concentration a model is formulated for earthquake generation within subducting slabs. The model predicts a maximum depth of seismic activity in the depth range of 635 to 760 km and, for deep earthquake zones, a relative maximum in seismic energy release near the maximum depth of earthquakes. From our modelling it follows that, whereas such a maximum is indeed likely to develop in deep earthquake zones, zones with a maximum depth around 300 km (such as the Aleutians) are expected to exhibit a smooth decay in seismic energy release with depth. This is in excellent agreement with observational data. In conclusion, the incoroporation of both depth-dependent forces and depth-dependent rheology provides new insight into the generation of intermediate and deep earthquakes and into the variation of seismic activity with depth.Our results imply that no barrier to slab penetration at a depth of 650–700 km is required to explain the maximum depth of seismic activity and the pattern of seismic energy release in deep earthquake zones.  相似文献   

16.
Receiver functions are widely employed to detect P-to-S converted waves and are especially useful to image seismic discontinuities in the crust. In this study we used the P receiver function technique to investigate the velocity structure of the crust beneath the Northwest Zagros and Central Iran and map out the lateral variation of the Moho boundary within this area. Our dataset includes teleseismic data (M b ≥ 5.5, epicentral distance from 30° to 95°) recorded at 12 three-component short-period stations of Kermanshah, Isfahan and Yazd telemetry seismic networks. Our results obtained from P receiver functions indicate clear Ps conversions at the Moho boundary. The Moho depths were firstly estimated from the delay time of the Moho converted phase relative to the direct P wave beneath each network. Then, we used the P receiver function inversion to find the properties of the Moho discontinuity such as depth and velocity contrast. Our results obtained from PRF are in good agreement with those obtained from the P receiver function modeling. We found an average Moho depth of about 42 km beneath the Northwest Zagros increasing toward the Sanandaj-Sirjan Metamorphic Zone and reaches 51 km, where two crusts (Zagros and Central Iran) are assumed to be superposed. The Moho depth decreases toward the Urmieh-Dokhtar Cenozoic volcanic belt and reaches 43 km beneath this area. We found a relatively flat Moho beneath the Central Iran where, the average crustal thickness is about 42 km. Our P receiver function modeling revealed a shear wave velocity of 3.6 km/s in the crust of Northwest Zagros and Central Iran increasing to 4.5 km/s beneath the Moho boundary. The average shear wave velocity in the crust of UDMA as SSZ is 3.6 km/s, which reaches to 4.0 km/s while in SSZ increases to 4.3 km/s beneath the Moho.  相似文献   

17.
1-D and 2-D calculation and interpretation are carried out with the DSS data from the western section of Heze-Changzhi profile and the southern section of Zhengzhou-Jinan profile. 2-D velocity structure is determined in Tangyin graben and its adjacent area. The result shows that velocity structure of the crust and upper mantle is obviously different in vertical and lateral directions. Crustal thickness varies apparently in this area, and there are local low velocity blocks in the interior crust. The swelling M-discontinuity corresponds to Tangyin graben and Moho depth at the highest swelling position is 31 km. Toward the east, its depth gradually increases to 32 km in Xunxian swelling; toward the west, M-discontinuity becomes a steep zone at the piedmont uplift of Taihang Mountain and reaches 40 km at depth near Changzhi. Through analyzing the relationship between historical earthquakes and deep structure in North China, we infer that seismic risk exists in Tangyin graben and its adjacent area.  相似文献   

18.
A three-dimensional model for the central Fennoscandian Shield was constructed for analysing the thermal, the rheological and the structural conditions in the lithosphere. The mesh covers a rectangular area in the southern Finland with horizontal dimensions of 500 km × 400 km and a depth extent of 100 km. Structural boundaries are derived from the several deep seismic soundings carried out in the area. Constructed model is first used in the calculation of the thermal and the rheological models and secondly in analysing the stress and the deformational conditions with the obtained rheology. Thermal and structural models are solved with the finite element method. The calculated surface HFD is between 40 and 48 mW m−2 in the Proterozoic southern part and below 40 mW m−2 in the older and northern Archaean part of the model. The calculated rheological strength shows a layered structure with two individual rheologically weak layers in the crust and strong layer in the upper part of the lower crust. The minimum brittle–ductile transition (BDT) depth is around 10 km in the southern part of the model while in the north and north-eastern parts the BDT depth is around 45–50 km. Comparison with the focal depth data shows that as most of the earthquakes occur no deeper than the depth of 10 km are they located in the brittle regime. Resulting stress conditions and possible regions of deformation after the model is subjected to pressure of 50 MPa reveals that the stress field is quite uniformly distributed in different crustal layers and that the elastic parameters control more the state of the stress than the applied rheological structure. In the upper crust, the stress intensity has values between 42 and 45 MPa whereas in the middle crust the values are around 50 MPa. Comparison of the 3-D model with earlier 2-D models shows that some differences in the results are to be expected.  相似文献   

19.
We use interferometric synthetic aperture radar (InSAR) observations to investigate the coseismic deformation and slip distribution of the 1997 Mw7.5 Manyi earthquake, a left-lateral strike-slip earthquake occurred on the west portion of the Kunlun fault in the northern Tibet, China. The fault trace is constrained by the combination of interferometric coherence image and azimuth offset image. The total length of the identified fault is about 170 km. We estimate the source parameters using a seven-segment fault model in a homogeneous elastic half-space. We first use a uniform slip model to estimate the slip, width, dip and rake for each segment, resulting in a maximum slip of 5.5 m with a depth of 11 km on the fourth segment. The average dip of the uniform slip model is about 93° northward and the average rake is about −2°. We then use a distributed slip model to estimate the pure strike-slip and oblique slip distribution, respectively. In the distributed slip model, the fault plane is discretized into 225 patches, each of them 4 km × 4 km. We fix the optimal geometric parameters and solve for the slip distribution using a bounded variable least-squares (BVLS) method. We find a geodetic moment of 1.91 × 1020 Nm (Mw7.5), of which almost 68% released in the uppermost 8 km and 82% in the uppermost 12 km. For all the models used in this study, the synthetic profiles along strike show asymmetric displacements on the opposite sides of the fault, which are in agreement with the observations. This suggests that a linear elastic model with variable and non-vertical dips is also reasonable for the mechanism of the Manyi earthquake.  相似文献   

20.
The Experimental platform of Tournemire (Aveyron, France) developed by IRSN (French Institute for Radiological Protection and Nuclear Safety) is located in a tunnel excavated in a clay–rock formation interbedded between two limestone formations. A well-identified regional fault crosscuts this subhorizontal sedimentary succession, and a subvertical secondary fault zone is intercepted in the clay–rock by drifts and boreholes in the tunnel at a depth of about 250 m. A 2D electrical resistivity survey was carried out along a 2.5 km baseline, and a takeout of 40 m was used to assess the potential of this method to detect faults from the ground surface. In the 300 m-thick zone investigated by the survey, electrical resistivity images reveal several subvertical low-resistivity discontinuities. One of these discontinuities corresponds to the position of the Cernon fault, a major regional fault. One of the subvertical conductive discontinuities crossing the upper limestone formation is consistent with the prolongation towards the ground surface of the secondary fault zone identified in the clay–rock formation from the tunnel. Moreover, this secondary fault zone corresponds to the upward prolongation of a subvertical fault identified in the lower limestone using a 3D high-resolution seismic reflection survey. This type of large-scale electrical resistivity survey is therefore a useful tool for identifying faults in superficial layers from the ground surface and is complementary to 3D seismic reflection surveys.  相似文献   

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