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1.
In western India during the Bhuj earthquake (Mw 7.6) on January 26, 2001, the Anjar City at ~30 km southwest of Bhuj experienced three types of damage scenario: severely damaged, less damaged and non-damaged. Similar damage patterns were also observed for the 1819 (Mw 7.8) and the 1956 (Mw 6.0) earthquakes. Microtremor array measurements were conducted in and around the Anjar city to examine the strength of soil structures and damage pattern. Significant differences are observed in frequencies and amplitudes in horizontal-to-vertical spectral ratio (HVSR) using microtremor measurements. The severely- damaged site shows two peak amplitudes: 2.8 at 1.2 Hz; and 4.0 at 8.0 Hz. The less-damaged site also shows two amplitudes: 2.5 and 2.1 at 1.4 Hz; and 2.0 Hz, respectively. The non-damaged site, on the other hand, shows that the HVSR curves become almost flatter. Similar results for three types of damage scenario based on analyses of earthquake records are also observed for the study area. The microtremor array measurements has revealed shear wave velocity Vs≥400 m/s at 18 m depth in the non-damaged, at 40 m in the less-damaged and at 60 m depth in the severely-damaged sites. The site amplitudes and the Vs values show a good correlation with the soil characteristics and damage pattern, suggesting that strength of soil layers at varying depths is a dictating factor for the estimate of the earthquake risk evaluation of the area under study.  相似文献   

2.
Characterization of shallow structures was performed by using different approaches analysing both P- and S-wave seismic data with different resolution. The refraction tomography provided P and S velocity models of the first 80 m, while the reflection seismic processing gives a reasonable stacking velocity field until 300 m depth for both P- and S-wave data. So, we estimated the Vp/Vs ratio and an empirical relationship between the two velocities. We characterised the shallow layers using tomographic velocity models and the deeper layers using seismic images with different resolution. The seismic images were obtained by conventional CMP reflection seismic processing and by a novel multi-refractor imaging technique.  相似文献   

3.
We determined crustal structure along the latitude 30°N through the eastern Tibetan Plateau using a teleseismic receiver function analysis. The data came mostly from seismic stations deployed in eastern Tibet and western Sichuan region from 2004 to 2006. Crustal thickness and Vp/Vs ratio at each station were estimated by the H–k stacking method. On the profile, the mean crustal thickness and Vp/Vs ratio were found to be 62.3 km and 1.74 in the Lhasa block, 71.2 km and 1.79 near the Bangong–Nujiang suture, 66.3 km and 1.80 in the Qiangtang block, 59.8 km and 1.81 in the Songpan–Garze block, and 42.9 km and 1.76 in the Yangtze block, respectively. The estimated crustal thicknesses are consistent with predictions based on the topography and the Airy isostasy, except near the Bangong–Nujiang suture and in the Qiangtang block where the crust is 5–10 km thicker than predicted, indicating that the crust may be denser, possibly due to mafic underplating. We also inverted receiver functions for crustal velocity structure along the profile, which reveals a low S-wave velocity zone in the lower crust beneath the eastern Tibetan Plateau, although the extent of the low-velocity zone varies considerably. The low-velocity zone, together with previous results, suggests limited partial melting and localized crustal flow in the lower crust of the eastern Tibetan Plateau.  相似文献   

4.
Inversion of local earthquake travel times and joint inversion of receiver functions and Rayleigh wave group velocity measurements were used to derive a simple model for the velocity crustal structure beneath the southern edge of the Central Alborz (Iran), including the seismically active area around the megacity of Tehran. The P and S travel times from 115 well-located earthquakes recorded by a dense local seismic network, operated from June to November 2006, were inverted to determine a 1D velocity model of the upper crust. The limited range of earthquake depths (between 2 km and 26 km) prevents us determining any velocity interfaces deeper than 25 km. The velocity of the lower crust and the depth of the Moho were found by joint inversion of receiver functions and Rayleigh wave group velocity data. The resulting P-wave velocity model comprises an upper crust with 3 km and 4 km thick sedimentary layers with P wave velocities (Vp) of ～5.4 and ～5.8 km s?1, respectively, above 9 km and 8 km thick layers of upper crystalline crust (Vp ～6.1 and ～6.25 km s?1 respectively). The lower crystalline crust is ～34 km thick (Vp ～ 6.40 km s?1). The total crustal thickness beneath this part of the Central Alborz is 58 ± 2 km.  相似文献   

5.
Serpentinization of the mantle wedge is an important process that influences the seismic and mechanical properties in subduction zones. Seismic detection of serpentines relies on the knowledge of elastic properties of serpentinites, which thus far has not been possible in the absence of single-crystal elastic properties of antigorite. The elastic constants of antigorite, the dominant serpentine at high-pressure in subduction zones, were measured using Brillouin spectroscopy under ambient conditions. In addition, antigorite lattice preferred orientations (LPO) were determined using an electron back-scattering diffraction (EBSD) technique. Isotropic aggregate velocities are significantly lower than those of peridotites to allow seismic detection of serpentinites from tomography. The isotropic VP/VS ratio is 1.76 in the Voigt–Reuss–Hill average, not very different from that of 1.73 in peridotite, but may vary between 1.70 and 1.86 between the Voigt and Reuss bonds. Antigorite and deformed serpentinites have a very high seismic anisotropy and remarkably low velocities along particular directions. VP varies between 8.9 km s? 1 and 5.6 km s? 1 (46% anisotropy), and 8.3 km s? 1 and 5.8 km s? 1 (37%), and VS between 5.1 km s? 1 and 2.5 km s? 1 (66%), and 4.7 km s? 1 and 2.9 km s? 1 (50%) for the single-crystal and aggregate, respectively. The VP/VS ratio and shear wave splitting also vary with orientation between 1.2 and 3.4, and 1.3 and 2.8 for the single-crystal and aggregate, respectively. Thus deformed serpentinites can present seismic velocities similar to peridotites for wave propagation parallel to the foliation or lower than crustal rocks for wave propagation perpendicular to the foliation. These properties can be used to detect serpentinite, quantify the amount of serpentinization, and to discuss relationships between seismic anisotropy and deformation in the mantle wedge. Regions of high VP/VS ratios and extremely low velocities in the mantle wedge of subduction zones (down to about 6 and 3 km.s?1 for VP and VS, respectively) are difficult to explain without strong preferred orientation of serpentine. Local variations of anisotropy may result from kilometer-scale folding of serpentinites. Shear wave splittings up to 1–1.5 s can be explained with moderately thick (10–20 km) serpentinite bodies.  相似文献   

6.
The Pannonian depression is an extensional back-arc basin in central Europe and is an integral part of the Alpine–Carpathian orogenic mountain belts. It can be characterized by thinned lower crust, shallow Moho discontinuity, high surface heat flow and Moho temperature, implying recent active tectonic processes. Imaging the velocity structure of the upper mantle may help us to better understand the structure and formation of the Pannonian region.In this paper, Pn traveltimes from regional earthquakes are used to tomographically image the lateral velocity variations in the uppermost mantle beneath the Pannonian basin. The set of linear tomographic equations, built up of the time term equation for each source–receiver pair, is solved by a truncated singular value decomposition algorithm. The explicit computation of the generalized inverse of the tomographic equations makes it possible to deduce both the resolution matrix and the model covariance matrix, allowing us to estimate the resolution and reliability of the solution.The mean compressional wave velocity in the uppermost mantle beneath the Pannonian basin is 7.9 km/s, substantially lower than the average continental Pn velocity of 8.1 km/s. It is mostly due to the high Moho temperature having values on average 400–500 °C more than those in the surrounding areas. The velocity anomalies range from −0.3 to 0.3 km/s relative to the mean velocity of 7.9 km/s. Due to high Moho temperature, below the North Hungarian range low (7.6–7.7 km/s) velocities can be found. High-velocity anomalies of around 8.1 km/s can be detected along the W-SW boundaries of Hungary and at the junction of the Pannonian basin and the Southern Carpathians. The Great Hungarian Plain shows average (7.9 km/s) Pn velocities.  相似文献   

7.
《Marine pollution bulletin》2010,60(8-12):221-233
This study assessed foraminiferal assemblages in Biscayne Bay, Florida, a heavily utilized estuary, interpreting changes over the past 65 years and providing a baseline for future comparisons. Analyses of foraminiferal data at the genus level revealed three distinct biotopes. The assemblage from the northern bay was characterized by stress-tolerant taxa, especially Ammonia, present in low abundances (∼2.0 × 103 foraminifers/gram) though relatively high diversity (∼19 genera/sample). The southwestern margin of the bay was dominated by Ammonia and Quinqueloculina, an assemblage characterized by the lowest diversities (∼12 genera/sample) and highest abundances (∼1.1 × 104 foraminifers/gram), influenced by both reduced salinity and elevated organic-carbon concentrations. A diverse assemblage of smaller miliolids and rotaliids (∼26 genera/sample) characterized the open-bay assemblage, which also had a significant component (∼10%) of taxa that host algal endosymbionts. In the past 65 years, populations of symbiont-bearing taxa, which are indicators of normal-marine conditions, have decreased while stress-tolerant taxa, especially Ammonia spp., have increased in predominance.  相似文献   

8.
Prominent postcursors to S/Sdiff waves with delays as large as 26 s are observed in Northern America for Papua New Guinea events. These waves sample the northern side of the Pacific large low shear velocity province revealed by global shear velocity (Vs) tomographic models. The emergence of the postcursors strongly depends on the epicenter-to-station azimuth, indicating that the waveforms are, in general, strongly affected by 3-dimensional (3D) heterogeneities. We limit our focus to an azimuthal range around 60°, measured clockwise from north at the epicenter, where the records show a relatively small azimuthal variation, suggesting a relatively small 3D effect there. In this azimuthal range we attempt 2D structural modelling along the great circle plane towards stations in southern US. First, we use a 2D ray theory to search for a range of models, which generate a postcursor to the main Sdiff phase with a delay time consistent with the observations. Then, for some typical models, we calculated waveforms at periods down to 5 s using the spectral element method. We obtained several models that provide synthetic waveforms in a fair agreement with the observations. The result shows that two types of low Vs regions are required to explain the data. One is a broad and weak anomaly region with a Vs reduction of 5% or so, constituting a part of the Pacific large low shear velocity province at the base of the mantle. The other is a laterally localized strong anomaly region with a more than 25% reduction of Vs within a thickness of at least 80 km and a width on the order of 500 km.  相似文献   

9.
In Ottawa, Canada, unusually high amplification ratios have recently been measured in clayey silts (called ‘Leda Clays’) at low levels of earthquake-induced ground shaking. However, the contribution of seismic Q, or material damping (ξ=1/2Q), to the overall ground motion at soft soil sites across the city is not well understood. This research investigates attenuation measurements in soft soils (Vs<250 m/s) for ongoing seismic hazard evaluation in the Ottawa area. The work focuses on in situ measurements of damping in two deep boreholes drilled into Leda Clay. To investigate the possibility of frequency-dependent dynamic properties of these materials at low strains, a new approach to the spectral ratio technique has been developed for the measurement of Qs in the field using a mono-frequency vibratory source (generating signals between 10 and 100 Hz), and two identical downhole 3-component geophones. Monofrequency signals also allowed for the measurement of dispersion (variation of velocity with frequency). Analysis of the data show that dynamic properties are, for the most part, independent of frequency in the homogenous silty soils, yielding negligible variation in shear wave velocity (<2 m/s) across the frequency test band, and small strain Qs's ranging from 170 to 200 (damping of 0.25–0.30%) over soil thickness intervals ranging from 10 to 60 m. At intervals within 20 m of the ground surface, laminated silt and clay beds of elevated porosity are found to have slight influence on the frequency dependence of damping for frequencies greater than 70 Hz (damping increase to 0.6%).  相似文献   

10.
Modeling of multimode surface wave group velocity dispersion data sampling the eastern and the western Ganga basins, reveals a three layer crust with an average Vs of 3.7 km s?1, draped by ～2.5 km foreland sediments. The Moho is at a depth of 43 ± 2 km and 41 ± 2 km beneath the eastern and the western Ganga basins respectively. Crustal Vp/Vs shows a felsic upper and middle crust beneath the eastern Ganga basin (1.70) compared to a more mafic western Ganga basin crust (1.77). Due to higher radiogenic heat production in felsic than mafic rocks, a lateral thermal heterogeneity will be present in the foreland basin crust. This heterogeneity had been previously observed in the north Indian Shield immediately south of the foreland basin and must also continue northward below the Himalaya. The high heat producing felsic crust, underthrust below the Himalayas could be an important cause for melting of midcrustal rocks and emplacement of leucogranites. This is a plausible explanation for abundance of leucogranites in the east-central Himalaya compared to the west. The uppermost mantle Vs is also significantly lower beneath the eastern Ganga basin (4.30 km s?1) compared to the west (4.44 km s?1).  相似文献   

11.
Two-dimensional crustal velocity models are derived from passive seismic observations for the Archean Karelian bedrock of north-eastern Finland. In addition, an updated Moho depth map is constructed by integrating the results of this study with previous data sets. The structural models image a typical three-layer Archean crust, with thickness varying between 40 and 52 km. P wave velocities within the 12–20 km thick upper crust range from 6.1 to 6.4 km/s. The relatively high velocities are related to layered mafic intrusive and volcanic rocks. The middle crust is a fairly homogeneous layer associated with velocities of 6.5–6.8 km/s. The boundary between middle and lower crust is located at depths between 28 and 38 km. The thickness of the lower crust increases from 5–15 km in the Archean part to 15–22 km in the Archean–Proterozoic transition zone. In the lower crust and uppermost mantle, P wave velocities vary between 6.9–7.3 km/s and 7.9–8.2 km/s. The average Vp/Vs ratio increases from 1.71 in the upper crust to 1.76 in the lower crust.The crust attains its maximum thickness in the south-east, where the Archean crust is both over- and underthrust by the Proterozoic crust. A crustal depression bulging out from that zone to the N–NE towards Kuusamo is linked to a collision between major Archean blocks. Further north, crustal thickening under the Salla and Kittilä greenstone belts is tentatively associated with a NW–SE-oriented collision zone or major shear zone. Elevated Moho beneath the Pudasjärvi block is primarily explained with rift-related extension and crustal thinning at ∼2.4–2.1 Ga.The new crustal velocity models and synthetic waveform modelling are used to outline the thickness of the seismogenic layer beneath the temporary Kuusamo seismic network. Lack of seismic activity within the mafic high-velocity body in the uppermost 8 km of crust and relative abundance of mid-crustal, i.e., 14–30 km deep earthquakes are characteristic features of the Kuusamo seismicity. The upper limit of seismicity is attributed to the excess of strong mafic material in the uppermost crust. Comparison with the rheological profiles of the lithosphere, calculated at nearby locations, indicates that the base of the seismogenic layer correlates best with the onset of brittle to ductile transition at about 30 km depth.We found no evidence on microearthquake activity in the lower crust beneath the Archean Karelian craton. However, a data set of relatively well-constrained events extracted from the regional earthquake catalogue implies a deeper cut-off depth for earthquakes in the Norrbotten tectonic province of northern Sweden.  相似文献   

12.
We investigate the crustal seismic structure of the Adria plate using teleseismic receiver functions (RF) recorded at 12 broadband seismic stations in the Apulia region. Detailed models of the Apulian crust, e.g. the structure of the Apulian Multi-layer Platform (AMP), are crucial for assessing the presence of potential décollements at different depth levels that may play a role in the evolution of the Apenninic orogen. We reconstruct S-wave velocity profiles applying a trans-dimensional Monte Carlo method for the inversion of RF data. Using this method, the resolution at the different depth level is completely dictated by the data and we avoid introducing artifacts in the crustal structure. We focus our study on three different key-elements: the Moho depth, the lower crust S-velocity, and the fine-structure of the AMP. We find a well defined and relatively flat Moho discontinuity below the region at 28–32 km depth, possibly indicating that the original Moho is still preserved in the area. The lower crust appears as a generally low velocity layer (average Vs = 3.7 km/s in the 15–26 km depth interval), likely suggestive of a felsic composition, with no significant velocity discontinuities except for its upper and lower boundaries where we find layering. Finally, for the shallow structure, the comparison of RF results with deep well stratigraphic and sonic log data allowed us to constrain the structure of the AMP and the presence of underlying Permo–Triassic (P–T) sediments. We find that the AMP structure displays small-scale heterogeneities in the region, with a thickness of the carbonates layers varying between 4 and 12 km, and is underlain by a thin, discontinuous layer of P–T terrigenous sediments, that are lacking in some areas. This fact may be due to the roughness in the original topography of the continental margins or to heterogeneities in its shallow structure due to the rifting process.  相似文献   

13.
In a typical seismic dam safety evaluation, standard penetration, cone penetration, Becker penetration, or shear wave velocity (Vs) tests are often first conducted near the toe of an earth dam to infer if any liquefiable soil exists in the foundation of the dam footprint. In current practice, a level-ground condition is commonly assumed when normalizing penetration resistance and Vs, and may be assumed (particularly in preliminary assessments) in applying the cyclic stress method (with or without the Kα correction) to evaluate liquefaction. However, the presence of an earth dam, or any other large embankment or structure, significantly alters the normal and shear stresses in the foundation. This paper identifies and quantifies potential errors in ignoring altered stresses near heavy structures, and presents a methodology to incorporate these effects within the framework of the simplified procedure. Specifically, the effects of these altered stresses (in comparison to the level-ground assumption with and without Kα correction) on the: (1) normalization of field measurements such as penetration resistance and Vs; (2) cyclic stress ratio (CSR); (3) cyclic resistance ratio (CRR); and (4) factor of safety against liquefaction triggering (FSliq), are evaluated by considering static and dynamic analyses of a generic earthen embankment (60 m high) resting on a saturated, cohesionless foundation (30 m deep). Our analyses indicated that ignoring the presence of induced static shear stresses can result in potentially unconservative errors in overburden correction factors of 30% to 60% at shallow depth (although this error is greatly muted at depths exceeding about 15 m), while errors in CSR potentially can range from about 20% too conservative to 40% unconservative. Potential errors in CRR can approach 50% unconservative at shallow depths, but again, this error is muted at depths exceeding about 15 m. Combining these factors, potentially unconservative errors in computing FSliq could exceed 100% at shallow depths (less than 15 m to 20 m) while at greater depth (exceeding 20 m) errors approach 20% on the conservative side.  相似文献   

14.
We present a catalog of moment tensor (MT) solutions and moment magnitudes, Mw, for 119 shallow (h ≤ 40 km) earthquakes in Greece and its surrounding lands (34°N–42°N, 19°E–30°E) for the years 2006 and 2007, computed with the 1D Time-Domain Moment Tensor inversion method (TDMT_INV code of Dreger, 2003). Magnitudes range from 3.2 ≤ Mw ≤ 5.7. Green's functions (GF) have been pre-computed to build a library, for a number of velocity profiles applicable to the broader Aegean Sea region, to be used in the inversion of observed broad band waveforms (10–50 s). All MT solutions are the outcome of a long series of tests of different reported source locations and hypocenter depths. Quality factors have been assigned to each MT solution based on the number of stations used in the inversion and the goodness of fit between observed and synthetic waveforms. In general, the focal mechanisms are compatible with previous knowledge on the seismotectonics of the Aegean area. The new data provide evidence for strike-slip faulting along NW–SE trending structures at the lower part of Axios basin, close to the heavily industrialized, and presently subsiding, region of the city of Thessaloniki. Normal faulting along E–W trending planes is observed at the Strimon basin, and in Orfanou Gulf in northern Greece. A sequence of events in the east Aegean Sea close to the coastline with western Anatolia sheds light on an active structure bounding the north coastline of Psara–Chios Islands about 20–25 km in length exhibiting right lateral strike-slip faulting.  相似文献   

15.
Electrical conductivity and seismic velocity are studied for plausible pore geometries in the Earth's interior for reliable quantitative analysis of experimental data such as seismic tomography and magnetotelluric explorations. Electrical conductivity of a two-phase system with equilibrium, interfacial energy-controlled phase geometry is calculated for the dihedral angles θ = 40°–100° that are typical for rock–aqueous fluid and θ = 20°–60° for rock–melt systems of lower crust and upper mantle for the case of tetrakaidecahedral grains. Electrical conductivity vs. seismic velocity correlations are acquired by combining of the simulated electrical conductivities with the seismic velocity calculated with the help of equilibrium geometry model Takei [Takei, Y., Effect of pore geometry on VP/VS: From equilibrium geometry to crack. J. Geophys. Res. 107 (2002): 10.1029/2001JB000522.] for the same pore geometries. The results show that electrical conductivity gradually decreases reaching zero when seismic velocities reach seismic velocities of intact rock for rock–melt systems, while for rock–aqueous fluid systems with θ ≥ 60° conductivity drops to zero at velocities up to 10% smaller. This can explain the seeming discrepancy of the low seismic velocity region, attributed to the high fluid fraction, and the low electrical conductivity of the same region, which is sometimes faced at collocated electromagnetic and seismic experiments.  相似文献   

16.
In a companion paper local transfer functions were estimated at Tecoman using earthquake and microtremor data. In this paper, the subsoil structure at this city is investigated using seismic refraction and cross-correlation of noise records as a case study. P- and S-wave refraction profiles were measured at five sites within the city. Standard analysis constrained only very shallow layers. The P-wave refraction deployment was also used to record ambient vibration. These data were processed using an extension of the SPAC (SPatial AutoCorrelation (Aki, 1957) [1]) method; cross-correlation is computed between station pairs and the results are inverted to obtain a phase velocity dispersion curve. Penetration depth was larger than that from the refraction experiments but the shear-wave velocity of the basement could not be determined. For this reason, additional microtremor measurements were made using broad band seismometers with a larger spacing between stations. The results allowed to constrain the shear-wave velocity of the basement. Site amplification computed for the final profiles compare well with observed ground motion amplification at Tecoman. The case of Tecoman illustrates that even a simple subsoil structure may require crossing data from different experiments to correctly constrain site effects.  相似文献   

17.
A temporary seismological network of broadband three-component stations has been deployed N–S to investigate the crust and upper mantle structure across the Ordos Block and the Yinshan Mountains. P wave receiver functions reveal the Moho depth to be about 41 km beneath the central Ordos Block and down to 45 km beneath the northern Ordos Block, a slight uplifting to 42–43 km beneath the Hetao Graben, increasing to 47–48 km beneath the Yinshan Mountains and then decreasing to 44 km beneath the northern Yinshan Mountains along the profile. In the Ordos Block, the crustal Vp/Vs ratio (about 1.80) south to the Hetao Graben differs from that (about 1.75) beneath the center Ordos Block. The crustal Vp/Vs ratio is significantly lower (about 1.65–1.70) beneath the Yinshan Mountains. The P wave receiver function migration imaging suggests relatively flat discontinuities at 410 and 660 km, indicating the lack of a strong thermal anomaly beneath this profile at these depths, and a low S wave velocity anomaly in the upper mantle beneath the Hetao Graben. We suggest that the low S wave velocity anomaly may be attributable to heat and that the thermal softening advances the evolution of the Hetao Graben, while the lower-crustal ductile flows transfer from the Hetao Graben to the northern Ordos Block, resulting in crustal thickening.  相似文献   

18.
Three shallow basins in Huizhou West Lake, China, were compared with respect to phosphorus (P) cycling between sediment and water, binding forms of P in sediment, and macrophyte biomass. The basins had similar sediments and similar depths, but two of the basins were restored by carp fish removal and macrophyte transplantation. These two basins have had clear water, low Chl.a and high macrophyte coverage for seven and ten years, whilst the unrestored control basin had turbid water and higher Chl.a. Judged by diffusive ammonium efflux, sediments in restored basins had higher mineralization rates than the unrestored basin, but the release of total dissolved P were more similar. However, sediments of restored basins released primarily dissolved organic P, while the sediment from the unrestored basin only released dissolved inorganic P. One third of the P release in the unrestored basin occurred from resuspended sediment, while this pathway contributed less than 3% in restored basins where resuspension rates were 10 times lower and the surface sediments affinity for phosphate higher. Besides from the presence of carps in the unrestored basin, the main differences were a large pool of P (700–850 mg P m−2) in macrophyte biomass and a smaller pool (∼150 mg m−2) as loosely adsorbed P in the sediment of restored basins than in the unrestored (0 in macrophytes and 350 mg P m−2 as loosely adsorbed). Also, a tendency of higher concentrations of oxidized iron was observed in the surface sediment from restored basins. The study underlines the potential of trophic structure changes to alter internal nutrient cycling in shallow lakes.  相似文献   

19.
We study the solar dependence of the thermospheric dynamics based on more than 20 years Fabry–Perot interferometer O 6300 Å emission observation of polar cap thermospheric wind from three stations: Thule (76.53°N, 68.73°W, MLAT 86N), Eureka (80.06°N, 86.4°W, MLAT 89N), and Resolute (74.72°N, 94.98°W, MLAT 84N) in combination with the National Center for Atmospheric Research Thermosphere Ionosphere Electrodynamics General Circulation Model (NCAR-TIEGCM). All three stations showed a dominant diurnal oscillation in both the meridional and zonal components, which is a manifestation of anti-sunward thermospheric wind in the polar cap. The three-station observations and the TIEGCM simulation exhibit varying degree of correlations between the anti-sunward thermospheric wind and solar F10.7 index. The diurnal oscillation is stronger at Eureka (∼150 m/s) than that at Resolute (∼100 m/s) according to both observations and TIEGCM simulation. The semidiurnal oscillation is stronger at Resolute (∼20 m/s) than that at Eureka based (∼10 m/s) on data and model results. These results are consistent with a two-cell convection pattern in the polar cap thermospheric winds. The Thule results are less consistent between the model and observations. The simulated meridional wind diurnal and semidiurnal oscillations are stronger than those observed.  相似文献   

20.
The Toarcian Oceanic Anoxic Event (OAE) in the Early Jurassic (∼ 183 Ma ago) was characterized by widespread near-synchronous deposition of organic-rich shales in marine settings, as well as perturbations to several isotopic systems. Characteristically, two positive carbon-isotope excursions in a range of materials are separated by an abrupt negative shift. Carbon-isotope profiles from Toarcian fossil wood collected in England and Denmark have previously been shown to exhibit this large drop (∼ − 7‰) in δ13C values, interpreted as due to an injection of isotopically light CO2 into the ocean–atmosphere system. However, the global nature of this excursion has been challenged on the basis of carbon-isotope data from nektonic marine molluscs (belemnites), which exhibit heavier than expected carbon-isotope values. Here we present new data, principally from fossil wood and bulk carbonate collected at centimetre scale from a hemipelagic section at Peniche, coastal Portugal. This section is low in organic carbon (average TOC = ∼ 0.5%), and the samples should not have suffered significant diagenetic contamination by organic carbon of marine origin. The carbon-isotope profile based on wood shows two positive excursions separated by a large and abrupt negative excursion, which parallels exactly the profile based on bulk carbonate samples from the same section, albeit with approximately twice the amplitude (∼ − 8‰ in wood versus ∼ − 3.5‰ in carbonate). These data indicate that the negative carbon-isotope excursion affected the atmosphere and, by implication, the global ocean as well. The difference in amplitude between terrestrial organic and marine carbonate curves can be explained by greater water availability in the terrestrial environment during the negative excursion, for which there is independent evidence from marine osmium-isotope records and, plausibly, changes in atmospheric CO2 content, for which independent evidence is also available. The Peniche succession is also notable for the occurrence of re-deposited sediments: their lowest occurrence coincides with the base of the negative excursion and their highest occurrence coincides with its top. Thus, slope instability and sediment supply could have been strongly linked to the global environmental perturbation, an association that may misleadingly simulate the effects of sea-level fall.  相似文献   

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