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1.
The 3-D P- and S-wave velocity models of the upper crust beneath Southwest Iberia are determined by inverting arrival time data from local earthquakes using a seismic tomo~raphy method. We used a total of 3085 P- and 2780 S-wave high quality arrival times from 886 local earthquakes recorded by a per- manent seismic network, which is operated by the Institute of Meteorology （IM）, Lisbon, Portugal. The computed P- and S-wave velocities are used to determine the 3-D distributions of Vp/Vs ratio. The 3-D velocity and Vp/Vs ratio images display clear lateral heterogeneities in the study area. Significant veloc- ity variations up to ~6% are revealed in the upper crust beneath Southwest lberia, At 4 km depth, both P- and S-wave velocity take average to high values relative to the initial velocity model, while at 12 km, low P-wave velocities are clearly visible along the coast and in the southern parts. High S-wave velocities at 12 km depth are imaged in the central parts, and average values along the coast; although some scattered patches of low and high S-wave velocities are also revealed. The Vp/Vs rztio is generally high at depths of 4 and 12 km along the coastal parts with some regions of high Vp/Vs ratio in the north at 4 km depth, and low Vp/Vs ratio in the central southern parts at a depth of 12 km, The imaged low velocity and high Vp/Vs ratios are related to the thick saturated and unconsolidated sediments covering the region; whereas the high velocity regions are generally associated with the Mesozoic basement rocks.  相似文献   

2.
Abstract: The ratio of P- to S-wave velocities (Vp/Vs) is regarded as one of the most diagnostic properties of natural rocks. It has been used as a discriminant of composition for the continental crust and provides valuable constraints on its formation and evolution processes. Furthermore, the spatial and temporal changes in Vp/Vs before and after earthquakes are probably the most promising avenue to understanding the source mechanics and possibly predicting earthquakes. Here we calibrate the variations in Vp/Vs in dry, anisotropic crustal rocks and provide a set of basic information for the interpretation of future seismic data from the Wenchuan earthquake Fault zone Scientific Drilling (WFSD) project and other surveys. Vp/Vs is a constant (Ф0) for an isotropic rock. However, most of crustal rocks are anisotropic due to lattice-preferred orientations of anisotropic minerals (e.g., mica, amphibole, plagioclase and pyroxene) and cracks as well as thin compositional layering. The Vp/Vs ratio of an anisotropic rock measured along a selected pair of propagation-vibration directions is an apparent value (Фij) that is significantly different from the value for its isotropic counterpart (Ф0). The usefulness of apparent Vp/Vs ratios as a diagnostic of crustal composition depends largely on rock seismic anisotropy. A 5% of P- and S-wave velocity anisotropy is sufficient to make it impossible to determine the crustal composition using the conventional criteria (Vp/Vs≤1.756 for felsic rocks, 1.7561.944 fluid-filled porous/fractured or partially molten rocks) if the information about the wave propagation-polarization directions with respect to the tectonic framework is unknown. However, the variations in Vp/Vs measured from borehole seismic experiments can be readily interpreted according to the orientations of the ray path and the polarization of the shear waves with respect to the present-day principal stress directions (i.e., the orientation of cracks) and the frozen fabric (i.e., foliation and lineation).  相似文献   

3.
We have measured P- and S-wave velocities on two amphibolite and two gneiss samples from the Kola superdeep borehole as a function of pressure (up to 600 MPa) and temperature (up to 600 °C). The velocity measurements include compressional (Vp) and shear wave velocities (Vs1, Vs2) propagating in three orthogonal directions which were in general not parallel to inherent rock symmetry axes or planes. The measurements are accompanied by 3D-velocities calculations based on lattice preferred orientation (LPO) obtained by TOF (Time Of Flight) neutron diffraction analysis which allows the investigation of bulk volumes up to several cubic centimetres due to the high penetration depth of neutrons. The LPO-based numerical velocity calculations give important information on the different contribution of the various rock-forming minerals to bulk elastic anisotropy and on the relations of seismic anisotropy, shear wave splitting, and shear wave polarization to the structural reference frame (foliation and lineation). Comparison with measured velocities obtained for the three propagation directions that were not in accordance with the structural frame of the rocks (foliation and lineation) demonstrate that for shear waves propagating through anisotropic rocks the vibration directions are as important as the propagation directions. The study demonstrates that proper measurement of shear wave splitting by means of two orthogonal polarized sending and receiving shear wave transducers is only possible when their propagation and polarization directions are parallel and normal to foliation and lineation, respectively.  相似文献   

4.
M.G. Kopylova  J. Lo  N.I. Christensen 《Lithos》2004,77(1-4):493-510
Modes and compositions of minerals in Slave mantle xenoliths, together with their pressures and temperatures of equilibrium were used to derive model depth profiles of P- and S-wave velocities (Vp, Vs) for composites equivalent to peridotite, pyroxenite and eclogite. The rocks were modeled as isotropic aggregates with uniform distribution of crystal orientations, based on single-crystal elastic moduli and volume fractions of constituent minerals. Calculated seismic wave velocities are adjusted for in situ pressure and temperature conditions using (1) experimental P- and T- derivatives for bulk rocks' Vp and Vs, and (2) calculated P- and T- derivatives for bulk rocks' elastic moduli and densities. The peridotite seismic profiles match well with the globally averaged IASP91 model and with seismic tomography results for the Slave mantle. In peridotite, an observed increase of seismic wave velocities with depth is controlled by lower degrees of chemical depletion in the deeper upper mantle. In eclogite, seismic velocities increase more rapidly with depth than in peridotite. This follows from contrasting first-order pressure derivatives of bulk isotropic moduli for eclogite and peridotite, and from the lower compressibility of eclogite at high pressures. Our calculations suggest that depletion in cratonic mantle has a distinct seismic signature compared to non-cratonic mantle. Depleted mantle on cratons should have slower Vp, faster Vs and should show lower Poisson's ratios due to an orthopyroxene enrichment. For the modelled Slave craton xenoliths, the predicted effect on seismic wave velocities would be up to 0.05 km/s.  相似文献   

5.
The aim of this study is to define the Vp and Vp/Vs structure of the fault zone ruptured by the ML 5.1 earthquake of October 15, 1996 which occurred near Reggio Emilia (central-northern Italy). A 1-month-long seismic sequence followed the mainshock and occurred in a small region along the outer border of the northern Apenninic belt, at depth ranging between 10 and 17 km. P- and S-wave arrival times from 304 aftershocks recorded by two local dense seismic arrays installed in the epicentral region have been inverted to obtain one- and three-dimensional velocity models by using state of the art local earthquake tomographic techniques. Velocity models and aftershock relocation help us to infer the seismotectonic of the region. Earthquakes originated along a NW-dipping backthrust of a NE-trending main thrust, composing the western part of the broad Ferrara Arc. A main high Vp and high Vp/Vs region delineates a pop-up structure in the center of the area. The high Vp/Vs within the pop-up structure supports the presence of a zone with increased pore pressure. The hypocentral depth of both mainshock and aftershocks is greater than those usually found for the main seismogenic regions of the Apenninic belt. P-wave velocity values in the seismogenic area, obtained by tomography, are compatible with rocks of the Mesozoic cover and suggest that seismicity occurred within the Mesozoic units stack at present by compressional tectonics.  相似文献   

6.
The Peloritani Mountain Belt (north-eastern Sicily) represents the connection between the Southern Appenninic Range and the Appenninic Maghrebid Chain. The lithotypes outcropping in a 36 km long and approximately 8 km wide area in the eastern part of the Peloritani Mountains are considered to represent most properly the composition of the lower crust. We selected 7 representative samples of silicate rocks (amphibolite, paragneisses, augen gneiss, phyllitic quartzite, pegmatitic rock) and 3 samples of calcite rocks (calc-schist, marbles) for the petrophysical measurements. Measurements were done on sample cubes of dry rocks in a multi-anvil apparatus. Raising of pressure gives rise to velocity increase, but the rate is different in the silicate and calcite rocks and closely related to progressive closure of microcracks. Linear behaviour is approached above about 200 MPa. Increasing temperature at 600 MPa decreases velocities in most silicate and in the calcite rocks with almost linear slopes. Substantial anisotropy of P- and S-wave velocities and shear wave splitting is found in both rock types. The residual anisotropy observed above about 200 MPa is attributed to lattice preferred orientations (LPO) of major minerals. 3D velocity calculations for an amphibolite, a paragneiss and a marble sample based on the LPO of hornblende, biotite and calcite, respectively, confirm the experimental findings of a close relationship between velocity anisotropy, shear wave splitting, shear wave polarization, lattice preferred orientation and the structural frame of the rocks (foliation, lineation). In the silicate rocks, the intrinsic (600 MPa) average P-wave velocities and Poisson's (Vp / Vs) ratios exhibit a tendency for a linear increase with densities, whereas the three calcite rocks cluster at markedly higher P-wave velocities and Poisson's (Vp / Vs) ratios, compared to their densities. In the silicate rocks, there is also a linear trend for an inverse relationship between the SiO2 content, density and the Poisson's (Vp / Vs) ratio, respectively.  相似文献   

7.
We determined high-resolution three-dimensional P- and S-wave velocity (Vp, Vs) structures beneath Kyushu in Southwest Japan using 177,500 P and 174,025 S wave arrival times from 8515 local earthquakes. A Poisson's ratio structure was derived from the obtained Vp and Vs values. Our results show that significant low-Vp, low-Vs and high Poisson's ratio zones are extensively distributed along the volcanic front in the uppermost mantle, which extend and dip toward the back-arc side in the mantle wedge. In the crust, low-Vp, low-Vs and high Poisson's ratio anomalies exist beneath the active volcanoes. The subducting Philippine Sea slab is clearly imaged as a high-Vp, high-Vs and low Poisson's ratio zone from the Nankai Trough to the back-arc. A thin low-velocity zone is detected above the subducting Philippine Sea slab in the mantle wedge, and earthquakes in the upper mantle are distributed along the transition zone between this thin low-velocity zone and the high-velocity Philippine Sea slab, which may imply that oceanic crust exists on the top of the slab and the forearc mantle wedge is serpentinized due to the slab dehydration. The seismic velocity of the subducting oceanic crust with basaltic or gabbroic composition is lower than that of the mantle according to the previous studies. The serpentinization process could also dramatically reduce the seismic velocity in the forearc mantle wedge.  相似文献   

8.
About 30 samples representing major lithologies of Sulu ultrahigh-pressure (UHP) metamorphic rocks were collected from surface exposures and exploration wells, and compressional (Vp) and shear wave (Vs) velocities and their directional dependence (anisotropy) were determined over a range of constant confining pressures up to 600 MPa and temperatures ranging from 20 to 600 °C. Samples range in composition from acidic to ultramafic. P- and S-wave velocities measured at 600 MPa vary from 5.08 to 8.64 km/s and 2.34 to 4.93 km/s, respectively. Densities are in the range from 2.60 to 3.68 g/cm3. To make a direct tie between seismic measurements (refraction and reflection) and subsurface lithologies, the experimental velocity data (corresponding to shallow depths) were used to calculate velocity profiles for the different lithologies and profiles of reflection coefficients at possible lithologic interfaces across the projected 5000-m Chinese Continental Scientific Drilling Program (CCSD) crustal segment. Comparison of calculated in situ velocities with respective intrinsic velocities suggests that the in situ velocities at shallow depths are lowered by an increased abundance of open microcracks. The strongly reflective zone beneath the Donghai drill site can be explained by the impedance contrasts between the different lithologies. Contacts between eclogite/peridotite and felsic rocks (gt-gneiss, granitic gneiss), in particular, may give rise to strong seismic reflections. In addition, shear-induced (lattice preferred orientation (LPO)-related) seismic anisotropy can increase reflectivity. For the explanation of the high velocity bodies (>6.4 km/s) around 1000 m and below 3200-m depth, large proportions of eclogite/peridotite (about 40 and 30 vol.%, respectively) are needed.  相似文献   

9.
横波地震基本理论的研究   总被引：10，自引：0，他引：10  
The people have highly attached importance to the technipues of S-wave seismics. It is basic
premise of S一wave exploration or combining S-wave and P-wave study the basic theory for S-wave seismics. In the basic theory，especially，seismic-wave velocities(Vp,Vs) and its ratio(Vp,Vs) must be studied.
  相似文献   

10.
Seismogenesis of aftershocks occurring in the Kachchh seismic zone for more than last 10?years is investigated through modeling of fractal dimensions, b-value, seismic velocities, stress inversion, and Coulomb failure stresses, using aftershock data of the 2001 Bhuj earthquake. Three-dimensional mapping of b-values, fractal dimensions, and seismic velocities clearly delineate an area of high b-, D-, and Vp/Vs ratio values at 15?C35?km depth below the main rupture zone (MRZ) of the 2001 mainshock, which is attributed to higher material heterogeneities in the vicinity of the MRZ or deep fluid enrichment due to the release of aqueous fluid/volatile CO2 from the eclogitisation of the olivine-rich lower crustal rocks. We notice that several aftershocks are occurred near the contacts between high (mafic brittle rocks) and low velocity regions while many of the aftershocks including the 2001 Bhuj mainshock are occurred in the zones of low velocity (low dVp, low dVs and large Vp/Vs) in the 15?C35?km depth range, which are inferred to be the fractured rock matrixes filled with aqueous fluid or volatiles containing CO2. Further support for this model comes from the presence of hydrous eclogitic layer at sub-lithospheric depths (34?C42?km). The depth-wise stress inversions using the P- and T-axes data of the focal mechanisms reveal an increase in heterogeneity (i.e., misfit) with an almost N?CS ??1 orientation up to 30?km depth. Then, the misfit decreases to a minimum value in the 30?C40?km depth range, where a 60o rotation in the ??1 orientation is also noticed that can be explained in terms of the fluid enrichment in that particular layer. The modeling of Coulomb failure stress changes (??CFS) considering three tectonic faults [i.e., NWF, GF, and Allah bund fault (ABF)] and the slip distribution of the 2001 mainshock on NWF could successfully explain the occurrences of moderate size events (during 2006?C2008) in terms of increase in positive ??CFS on GF and ABF. In a nutshell, we propose that the fluid-filled mafic intrusives are acting as stress accentuators below the Kachchh seismic zone, which generate crustal earthquakes while the uninterrupted occurrence of aftershocks is triggered by stress transfer and aqueous fluid or volatile CO2 flow mechanisms. Further, our results on the 3-D crustal seismic velocity structure, focal mechanisms, and b-value mapping will form key inputs for understanding wave propagation and earthquake hazard-related risk associated with the Kachchh basin.  相似文献   

11.
在常温常压条件对中国大陆科学钻CCSD主孔岩心的700样品进行了弹性波速度测量，并建立了主孔2000m的波速(Vp和Vs)连续剖面，为检验地球物理模型的合理解释提供了岩石物理学方面的宝贵资料。主孔中新鲜榴辉岩纵波速度(Vp)最大(7.86km／s)，正副片麻岩波速最小，又分别为5.53km／s和5．71km／s，榴辉岩的波速随着退变质作用的增强而明显减小。主孔2000m总平均Vp速度为6．2km／s，它与地球物理探测方法获得的大别-苏鲁造山带上地壳具有6.2-6．3km／s高速层结论是一致的。大部分岩石具有明显地震波各向异性。水饱和度使岩石纵波(Vp)速度和剪切波速度(Vs)分别增加19％和6％，而使Vp的各向异性降低3％～4％。不同岩性界面的反射系数(Rc)是产生地震反射的主要原因。金红石榴辉岩与片麻岩之间具有很高的反射系数(0.24-0．31)。韧性剪切带中糜棱岩化片麻岩和面理化榴辉岩使岩石各向异性和反射强度明显增加。岩石微裂隙与主孔原位波速变化有密切关系。饱水岩石速度(Vp和Vs)可以代表CCSD主孔原位状态的地震波速度。上述成果为本区地震反射体成因提供了重要的岩石物理性质约束。  相似文献   

12.
汶川5月12日8.0级地震在构造上起因于印度板块与欧亚板块以每年约5 cm的速度聚敛,并因此而引起青藏高原的地壳物质向四川盆地及中国东南大陆运移.主震震源及余震活动集中于以龙门山为中轴的一条长约350 km、宽约100 km的地震活动带.震源深度一般分布丁地壳脆性-韧性转换边界以上约10～20 km区间的地壳震源层之中,属浅源构造地震.主要震源机制与龙门山构造运动方式密切相关,以其地壳厚度向西急剧加厚、重力梯度带、高波速比(Vp/Vs～2.2)等深部异常及逆冲断层兼具走滑性质的地质构造为特征.在震源辐射、路径传播和场地效应研究的基础上,分别计算并比较了岩石和土壤条件下的地震响应谱,特别强调了土壤条件下的场地放大效应;同时对与地震安全性有关的一些问题如地质灾害、地震频谱设计、地震早期预警系统及中、长期至短期地震预报等进行了探讨;特别提供了一个由加权平均计算、以岩石条件下震波衰减模式为基础的地震频谱设计参考实例.地震构造与动力学研究可融人工程地质与环境工程等学科发展.经历汶川地震考验的一些新近设计和建设的工程项目可为今后改进工程建筑规范与标准提供重要而有益的参考.地震预报是当今一大难题,但需探索研究,不可懈怠.地震减灾与预防足目前比较切合实际的安全举措.  相似文献   

13.
本文总结了榴辉岩的高温高压弹性波速测量结果,并将其应用于苏鲁超高压变质带地震资料的解释。由于榴辉岩具有高密度和高波速,它们和长英质片麻岩、大理岩、石英岩、角闪岩、麻粒岩、蛇纹石化橄榄岩的界面可以产生强反射。如果俯冲的陆壳物质以榴辉岩与围岩互层的形式在上地幔保留下来,就可能在造山带的上地幔产生地震反射。根据CCSD孔区地震剖面所建立的地壳成分模型表明:苏鲁超高压带地壳浅部的高速层可归因于夹在花岗质片麻岩、副片麻岩、角闪岩等岩石中的榴辉岩和超基性岩;中地壳主要由中酸性片麻岩、斜长角闪岩和副片麻岩组成;下地壳以中基性麻粒岩为主。在该超高压变质带现今的深部地壳,榴辉岩含量很少或几乎没有。因此,折返的超高压变质岩是以构造岩片的形式沿一系列剪切带逆冲并覆盖在正常的中下地壳之上,深部榴辉岩的缺乏可能与下地壳拆沉作用无关。  相似文献   

14.
We present new results on the structure resulting from Palaeoproterozoic terrane accretion and later formation of one of the aulacogens in the East European Platform. Seismic data has been acquired along the 530-km-long, N–S-striking EUROBRIDGE'97 traverse across Sarmatia, a major crustal segment of the East European Craton. The profile extends across the Ukrainian Shield from the Devonian Pripyat Trough, across the Palaeoproterozoic Volyn Block and the Korosten Pluton, into the Archaean Podolian Block. Seismic waves from chemical explosions at 18 shot points at approximately 30-km intervals were recorded in two deployments by 120 mobile three-component seismographs at 3–4 km nominal station spacing. The data has been interpreted by use of two-dimensional tomographic travel time inversion and ray trace modelling. The high data quality allows modelling of the P- and S-wave velocity structure along the profile. There are pronounced differences in seismic velocity structure of the crust and uppermost mantle between the three main tectonic provinces traversed by the profile: (i) the Pripyat Trough is a ca. 4-km-deep sedimentary basin, fully located in the Osnitsk–Mikashevichi Igneous Belt in the northern part of the profile. The velocity structure is typical for a Precambrian craton, but is underlain by a ca. 5-km-thick lowest crustal layer of high velocity. The development of the Pripyat Trough appears to have only affected the upper crust without noticeable thinning of the whole crust; this may be explained by a rheologically strong lithosphere at the time of formation of the trough. (ii) Very high seismic velocity and Vp/Vs ratio characterise the Volyn Block and Korosten Pluton to a depth of 15 km and probably also the lowest crust. The values are consistent with an intrusive body of mafic composition in the upper crust that formed from bimodal melts derived from the mantle and the lower crust. (iii) The Podolian Block is close to a typical cratonic velocity structure, although it is characterised by relatively low seismic velocity and Vp/Vs ratio. A pronounced SW-dipping mantle reflector from Moho to at least 70 km depth may represent the Proterozoic suture between Sarmatia and Volgo–Uralia, the structure from terrane accretion, or a later shear zone in the upper mantle. The sub-Moho P-wave seismic velocity is high everywhere along the profile, with the exception of the area above the dipping reflector. This velocity change further supports a plate tectonic origin of the dipping mantle reflector. The profile demonstrates that structure from Palaeoproterozoic plate tectonic processes are still identifiable in the lithosphere, even where younger metamorphic equilibration of the crust has taken place.  相似文献   

15.
A dense seismic network (～100 stations) was operated in the Koyna-Warna region from January 2010 to May 2010, that allow us to collect 400 high-quality local earthquake data of magnitude less than 4. In this region, the fault structure and tectonic setting that accommodate the induced seismicity is not well understood. To investigate the seismotectonics of the region, we have inverted 7826 P- and 7047 S-P arrival times for 3-D Vp and Vp/Vs tomographic models along with hypocenters parameters in the region. Although, Dixit et al. (2014) have performed 3-D local earthquake tomography with double-difference tomography code using catalog differential time data. In this paper, Simulps14 code on the same data set is applied. For better approach P arrival time and S-P travel times are inverted directly for Vp, Vp/Vs variations and earthquake locations. High Vp ～5.9 to 6.5 and low to high Vp/Vs ～1.69-1.74 imaged in the hypocenter region. These features interpreted as a fluid bearing rock mass under high pore pressure. It is also observed that below the trap basement form a local topography depression between the Koyna and Warna Reservoirs. To the South of the Warna reservoir, intense seismic activity defines a major cluster of ～ 5 km width at 3 to7 km deep, located under the trap, where the basement is deepening. Such regions are inferred to be associated with the seismically active faults zones. The obtain velocity anomalies are reliable down to a depth of 10 km. This is also confirmed by the analysis of three resolution parameters viz. Hit count, Derivative Weight sum (DWS) and Resolution Diagonal Elements (RDE).  相似文献   

16.
A unique attempt is made to understand the genesis of intraplate seismicity in the Latur-Killari and Koyna seismogenic regions of India, through derived crustal structure by synthesizing active and passive seismic, magnetotelluric, gravity and heat flow data. It has indicated presence of relatively high velocity/density intermediate granulite (and amphibolite) facies rocks underneath the Deccan volcanic cover caused mainly due to a continuous geodynamic process of uplift and erosion since Precambrian times. These findings have been independently confirmed by detailed borehole geological, geochemical and mineralogical investigations. The crystalline basement rock is found to contain 2 wt% of carbon-di-oxide fluid components. The presence of geodynamic process, associated with thermal anomalies at subcrustal depths, is supported by a high mantle heat flow (29–36 mW/m2) beneath both regions, although some structural and compositional variations may exist as evidenced by P- and S-wave seismic velocities. We suggest that the stress, caused by ongoing uplift and a high mantle heat flow is continuously accumulating in this denser and rheologically stronger mafic crust within which earthquakes tend to nucleate. These stresses appear to dominate over and above those generated by the India–Eurasia collision. The role of fluids in stress generation, as advocated through earlier studies, appears limited.  相似文献   

17.
In order to better constrain the interpretation and the nature of the seismic reflectors, experimental measurements at high confining pressure (up to 300 MPa) and room temperature of the compressional wave velocity (Vp) on 10 samples representative of the most common lithologies along the Aurina (Ahrntal), Tures (Tauferer Tal), and Badia (Abtei Tal) Valleys profile (Eastern Alps, Italy) have been performed. For each sample, the speed of ultrasonic waves was measured in three mutually perpendicular directions, parallel and normal to the rock foliation and lineation.The main results are:(a) Good agreement between the calculated vs. measured modal compositions of the considered rocks, indicating that they were presumably equilibrated at the estimated P–T conditions; therefore, the seismic properties are representative of the crustal level indicated by the thermobarometry.(b) Measured and calculated average Vp are in good agreement, and are typical of mid-crustal level (6.0–6.5 km/s). Only the amphibolites show Vp typical of the lower crust (7.2 km/s).(c) The seismic anisotropy of metapelites is very high (12–27%), both with orthorhombic and transverse isotropy symmetry; amphibolites are transversely isotropic with an anisotropy of 8%; orthogneisses and granitoids are isotropic or weakly anisotropic.(d) The contacts between amphibolites and all other rock types may generate good reflections, provided they are not steeply inclined. Although the metamorphic foliation remains steeply inclined, discordant buried sub-horizontal igneous contacts may be detected.  相似文献   

18.
M. Faccenda  G. Bressan  L. Burlini   《Tectonophysics》2007,445(3-4):210-226
The compressional and shear wave velocities have been measured at room temperature and pressure up to 450 MPa on 5 sedimentary rock samples, representative of the most common lithologies of the upper crust in the central Friuli area (northeastern Italy). At 400 MPa confining pressure the Triassic dolomitic rock shows the highest velocities (Vp  7 km/s, Vs  3.6 km/s), the Jurassic and Triassic limestones samples intermediate velocities (Vp  6.3 /s, Vs  3.5 km/s) and the Cenozoic and Paleozoic sandstones the lowest velocities (Vp  6.15 km/s, Vs  3.35 km/s). The Paleozoic sandstone sample is characterized by the strongest anisotropy (10%) and significant birefringence (0.2 km/s) is found only on the Cenozoic sandstone sample. We elaborated the synthetic profiles of seismic velocities, density, elastic parameters and reflection coefficient, related to 4 one-dimensional geological models extended up to 22 km depth. The synthetic profiles evidence high rheological contrasts between Triassic dolomitic rocks and the soft sandstones and the Jurassic limestones. The Vp profiles obtained from laboratory measurements match very well the in-situ Vp profile measured by sonic log for the limestones and dolomitic rocks, supporting our one-dimensional modelling of the calcareous-carbonatic stratigraphic series. The Vp and Vs values of the synthetic profiles are compared with the corresponding ones obtained from the 3-D tomographic inversion of local earthquakes. The laboratory Vp are generally higher than the tomographic ones with major discrepancies for the dolomitic lithology. The comparison with the depth location of seismicity reveals that the seismic energy is mainly released in correspondence of high-contrast rheological boundaries.  相似文献   

19.
Seismic anisotropy of the crystalline crust: what does it tell us?   总被引：2，自引：0，他引：2  
The study of the directional dependence of seismic velocities (seismic anisotropy) promises more refined insight into mineral composition and physical properties of the crystalline crust than conventional deep seismic refraction or reflection profiles providing average values of P-and S-wave velocities. The alignment of specific minerals by ductile rock deformation, for instance, causes specific types of seismic anisotropy which can be identified by appropriate field measurements.
Vice versa , the determination of anisotropy can help to discriminate between different rock candidates in the deep crust. Seismic field measurements at the Continental Deep Drilling Site (KTB, S Germany) are shown as an example that anisotropy has to be considered in crustal studies. At the KTB, the dependence of seismic velocity on the direction of wave propagation in situ was found to be compatible with the texture, composition and fracture density of drilled crustal rocks.  相似文献   

20.
In this paper we show the seismicity and velocity structure of a segment of the Alpine retro-belt front along the continental collision margin of the Venetian Alps (NE Italy). Our goal is to gain insight on the buried structures and deep fault geometry in a “silent” area, i.e., an area with poor instrumental seismicity but high potential for future earthquakes, as indicated by historical earthquakes (1695 Me = 6.7 Asolo and 1936 Ms = 5.8 Bosco del Cansiglio). Local earthquakes recorded by a dense temporary seismic network are used to compute 3-D Vp and Vp/Vs tomographic images, yielding well resolved images of the upper crust underneath the south-Alpine front. We show the presence of two main distinct high Vp S-verging thrust units, the innermost coincides with the piedmont hill and the outermost is buried under a thick pile of sediments in the Po plain.Background seismicity and Vp/Vs anomalies, interpreted as cracked fluid-filled volumes, suggest that the NE portion of the outermost blind thrust and its oblique/lateral ramps may be a zone of high fluid pressure prone to future earthquakes.Three-dimensional focal mechanisms show compressive and transpressive solutions, in agreement with the tectonic setting, stress field maps and geodetic observations. The bulk of the microseismicity is clustered in two different areas, both in correspondence of inherited lateral ramps of the thrust system. Tomographic images highlight the influence of the paleogeographic setting in the tectonic style and seismic activity of the region.  相似文献   

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