The objective determination of the instantaneous predominant frequency of seismic signals and inferences onQ of coda waves     

Antonio Rovelli  Sandro Marcucci  Giuliano Milana 《Pure and Applied Geophysics》1988,128(1-2):281-293

A technique to detect spectrum variations versus time along seismic signals is applied to coda waves of local earthquakes (Friuli, Northern Italy). The technique consists of an autoregressive modeling and utilizes nonlinear spectral analysis where the spectrum of stochastic processes is estimated as the transfer function of the filter that whitens the process under analysis. This approach appears to be particularly well suited to those investigations where automatic measurements of the instantaneous frequency have to be carried out on digital data. The detection of variations of the instantaneous frequency along the coda allows computation of seismic-Q in the lithosphere and its frequency dependence: the result obtained is $$Q = 100f^{0.4} $$ which appears to be strongly consistent with that, based on the estimate of the coda amplitude decay in the band including the most significant frequencies of the signals under analysis.  相似文献   

Geobal post-seismic deformation     

Antonio Piersanti  Giorgio Spada  Roberto Sabadini  Maurizio Bonafede 《Geophysical Journal International》1995,120(3):544-566

  相似文献   

Influence of bottom topography roughness on the jet and inertial recirculation of a mid-latitude gyre     

Bernard Barnier  Christian Le Provost 《Dynamics of Atmospheres and Oceans》1993,18(1-2)

Several numerical experiments are conducted to examine the influence of mesoscale, bottom topography roughness on the inertial circulation of a wind-driven, mid-latitude ocean gyre. The ocean model is based on the quasi-geostrophic formulation, and is eddy-resolving as it features high vertical and horizontal resolutions (six layers and a 10 km grid). An antisymmetrical double-gyre wind stress curl forces the baroclinic modes and generates a strong surface jet. In the case of a flat bottom, inertia and inverse energy cascade force the barotropic mode, and the resulting circulation features strong, barotropic, inertial gyres. The sea-floor roughness inhibits the inertial circulation in the deep layers; the barotropic component of the flow is then forced by eddy-topography interactions, and its energy concentrates at the scales of the topography. As a result, the baroclinicity of the flow is intesified: the barotropic mode is reduced with regard to the baroclinic modes, and the bottom flow (constrained by the mesoscale sea-floor roughness) is decoupled from the surface flow (forced by the gyre-scale wind). Rectified, mesoscale bottom circulation induces an interfacial form stress at the thermocline, which enhances horizontal shear instability and opposes the eastward penetration of the jet. The mean jet is consequently shortened, but the instantaneous jet remains very turbulent, with meanders of large meridional extent. The sea-floor roughness modifies the energy pathways, and the eddies have an even more important role in the establishment of the mean circulation: below the thermocline, rectification processes are dominant, and eddies transfer energy toward permanent mesoscale circulations strongly correlated with topography, whereas above the thermocline mean flow and eddy generation are influenced by the mean bottom circulation through interfacial stress. The topography modifies the vorticity of the barotropic and highest baroclinic modes. Vorticity accumulates at the small topographic scales, and the vorticity content of the highest modes, which is very weak in the flat-bottom case, increases significantly. Few changes occur in surface-intensified modes. In the deep layers of the model, the inverse correlation between relative vorticity and topography at small scales ensures the homogenization of the potential vorticity, which mainly retains the largest scales of the bottom flow and the scale of β.  相似文献   

长波稳定性的季节变化          下载免费PDF全文

吕克利  农尚尧  蒋乐 《热带气象学报》1993,9(2):105-113

利用多层模式导得了多层斜压大气中的斜压不稳定的必要条件,还利用迭代-还原法计算了春夏秋冬实际基流上不稳定波的增长率。结果显示,长波扰动的增长率及其不稳定波的波长范围有明显的季节变化,冬季增长率最大,不稳定波的波长范围也最宽,春季次之,夏季最弱;β参数对长波不稳定起稳定化作用,它减小不稳定波的增长率,缩小不稳定波的波长范围;Froude数的影响与β参数相反,它对长波起不稳定化作用.地形的存在,使不稳定波的波长范围缩小,对增长率的影响比较复杂。  相似文献   

Geothermobarometry in albite-garnet orthogneisses: A case study from the Gran Paradiso nappe (Western Alps)     

Elisabeth Le Goff  Michel Ballvre 《Lithos》1990,25(4):261-280

The aim of this paper is to estimate syntectonic P-T conditions within albite- and garnet-bearing orthogneisses. These rocks are generally characterized by the assemblage quartz + albite + biotite + phengite + CaFe-garnet + epidote + titanite. Garnet contains up to 55 mole per cent of grossular. K-feldspar is a relict magmatic phase.

P-T conditions are estimated using several independent methods. First, it is shown that exchange reactions based on the Fe---Mg partitioning between garnet and biotite or garnet and phengite cannot be used to estimate temperatures in these rocks, due to the high grossular content of garnet. Second, maximum and minimum pressures are constrained, respectively, by the occurrence of albite instead of jadeite + quartz and by the assemblage phengite + biotite + quartz. Third, phase equilibria in albite- and garnet-bearing metagranites are modelled in the system K₂O---CaO---FeO---Al₂O₃---SiO₂---H₂O. Equilibrium curves are calculated for the observed phase compositions. Uncertainties in P-T estimates mainly result from the choice of appropriate non-ideal solution models for the garnet.

An application is developed for granites from the Gran Paradiso nappe (Western Alps). These granites show an heterogeneous deformation of Alpine age expressed by mylonitic shear zones cutting across weakly deformed domains. Estimated P-T conditions for the synkinematic assemblages are 10–16 kbar at 550±50°C.  相似文献   

Probabilistic compressive strength of sound dry granite     

P. Kittl  M. León  G. Díaz  A. Lillo 《Rock Mechanics and Rock Engineering》1990,23(1):21-28

Summary The different functions of cumulative probability of fracture that can be used in the Probabilistic Strength of Materials in the case of constant uniaxial compression are described. Sound fine-grained granite was used to study volume influence by fracturing rectangular prisms, and then no noticeable influence was observed. Since this is showing that all the fracture stresses are belonging to a single set they were included in a single group that exhibited two functions of specific risk of fracture. The population with the lesser fracture stress has no critical zone while the other population does have it and a critical zone in the order of 10^–6 m³ is the minimum size exhibiting a complete fracture of the specimen when the same collapses. All the statistical functions were found to be acceptable according to theX ² criterion.  相似文献   

Volatile fluxes from volcanoes   总被引：1，自引：0，他引：1  

Marie-Françoise Le Cloarec  Bernard Marty 《地学学报》1991,3(1):17-27

Volatile fluxes from Mid Ocean Ridge (MOR) and subaerial volcanism have been estimated or re-evaluated using several natural tracers-³He, ²¹⁰Po, SO₂-and chemical ratios of volatile species in lavas and volcanic gases.
These estimates confirm the net predominance of anthropogenic fluxes over volcanic fluxes for CO₂, SO₂ and trace metals.
They also suggest that, while most of the volatiles transferred during MOR volcanism come from the mantle, volatiles stored at the surface of the Earth supply an appreciable fraction of subaerial fluxes and can be the dominant source for some of them.
The surface inventory of volatile species cannot result from steady-state degassing with constant rate and needs much greater fluxes in the past or other volatile supply processes. This inventory is the result of several of the following processes: capture of the solar nebula and its subsequent partial escape, impact degassing of accreting bodies, and, from Archean to present mantle, degasssing through volcanism and associated phenomena, with recycling into the mantle through subduction.  相似文献   

Association of subsurface radon changes in Alaska and the northeastern United States with earthquakes     

Robert L. Fleischer  Antonio Mogro-Campero 《Geochimica et cosmochimica acta》1985,49(4):1061-1071

Changes of radon concentration in the ground have been reported to correlate with seismic activity in many places over widely varying distances from the earthquake epicenters, sometimes over 100's and occasionally over 1000's of km depending on the magnitude M of the earthquakes. For proper use of such correlations in forecasting the location and size of impending earthquakes, it would be useful to establish a relation between M and the maximum distance x_M at which radon can be significantly altered by preseismic effects.We have monitored radon in the ground continuously at Blue Mountain Lake, NY starting Dec. 3. 1975 and at three locations in Alaska—Icy Bay, Yakataga, and Sand Point (starting Nov. 12, 1979; March 5, 1980; and June 29, 1980 respectively). Sample correlations from visual examination of the recent radon record and tentative use of a proposed scaling relation show a number of possible earthquake-related signals at these locations. The data are compatible with scaling relations that were derived from two separate models of premonitory elastic strains. In this work x_M = 10^0.48M, where x_M is in units of km and M ≥ 3. Since tilt and strain signals correlate similarly with magnitude and distance, it is likely that most earthquake-related radon signals are mechanically induced.  相似文献   

Seismic Q for the lithosphere of the Montenegro region (Yugoslavia): frequency,depth and time windowing effects     

Antonio Rovelli 《Physics of the Earth and Planetary Interiors》1984,34(3):159-172

The availability of accelerometric data for the Montenegro earthquake of 15th April 1979 makes it possible to investigate seismic Q of the lithosphere in that region, in particular, its dependence on frequency, on the depth reached by seismic waves, and on the length of time windows in which signals are processed. Two different spectral methods, S phase energy ratio and coda envelope decay, are applied, respectively, to direct and scattered shear waves. Similar results are obtained using different portions of the recordings, i.e., coda waves for the envelope decay fit and the S wave train, with a significant duration of ～ 10 s, for the energy ratios. The same apparent Q (Q ～ 40 f, where f is the frequency expressed in Hz) that is found for other neighbouring central Mediterranean regions (e.g., Ancona, on the central Italian Adriatic coast; Valnerina, in the central Apennines; Irpinia, in the southern Apennines) is also found for the southern Yugoslavian coast, in the band 1–25 Hz up to a maximum range of ～ 120 km from the focus. This strong frequency dependence is probably connected with the type of small-scale heterogeneity and the same geological age and level of tectonic activity peculiar to all these seismotectonic areas.In order to compare the apparent Q of the whole S wave train, ～ 10 s long, with the (intrinsic) apparent Q of the single direct S wave (usually 1 s or less), the maximum entropy method is applied in the energy spectrum computation for shorter wave trains. The use of shorter time windows does not reveal any significant variation in the tendency of Q to increase linearly with frequency as the length of the time window containing the sample of the S waves decreases. This seems to indicate that scattering-dependent Q is generally inseparable from intrinsic Q in the lithosphere when estimates based on variations with distance of the seismic signal spectrum are used. While the type of linear growth with frequency does not seem to undergo any variations (it remains of the Q = qf type), the data show there are a considerable decrease in the coefficient of proportionality Q with decreasing duration of the window of S waves analysed, probably as a result of variations in seismic attenuation with depth.  相似文献   

Carbonate platform depositional sequences, Upper Cretaceous, south-central Pyrenees (Spain)     

Antonio Sim 《Tectonophysics》1986,129(1-4)

The Upper Cretaceous (Cenomanian-Maastrichtian) of the south-central Pyrenees shows five carbonate platform sequences where the major parameters are tectonism, relative sea-level fluctuations and inherited depositional profile. Depositional geometries and basin analysis permit an understanding of the depositional history.Five depositional sequences have been recognized: (1) The Santa Fe sequence (Middle-Upper Cenomanian), a ramp to a skeletal rimmed shelf with an escarpment bypass margin. The lower boundary is an angular unconformity and the upper one records a sea-level drop. The platform location of the margin was determined by a listric normal fault. (2) An abrupt sea-level rise drowned the former platform. The Congost sequence (Turonian-Lower Coniacian), a distally steepened ramp with erosional distal deep slope. The depositional model was largely controlled by pre-existing basin morphology. Cessation of platform development was due to a relative sea-level drop. (3) The Sant Corneli sequence (Upper Coniacian-Lower Santonian), a mixed terrigenous-skeletal homoclinal ramp with upright margin, deep slope and dysaerobic basin. The slope results from the backstepping by 24 km of the previous margin and gentle basin tilting. The platform margin remained more or less at the same position, and relief between platform and slope increased indicating continuous relative sea-level rise. The upper boundary is an angular unconformity at the platform margin produced by an abrupt sea-level rise and platform drowning, and by listric normal faulting. (4) The Vallcarga sequence (Upper Santonian-Campanian), a distal-steepened skeletal homoclinal ramp, erosional escarpment and turbidite basin, which corresponds to the Mesozoic maximum marine expansion. A listric normal fault created two depositional areas: a more or less flat footwall block with a north-northwest prograding carbonate ramp.  相似文献