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1.
PcP and PmKP travel times are computed for three simple or parametric Earth models, based on free-oscillation and travel-time data B1, PEM-A and HB1 and compared with PcP and PmKP travel times from different sources. This comparison is made only for the region above and below the core-mantle boundary and is of interest because of the current search for a standard Earth model. The comparison shows that only model B1 does not need a correction for its PcP travel times. For the PmKP travel times for the three models, corrections of the form Δt = a + bm were obtained. The models need the following corrections for b: ?1.3 for B1, 2.8 for HB1 and 0.6 for PEM-A. The corrections a are shown to be equal to the observed corrections for PcP at large epicentral distances. The inversions of free-oscillation data to obtain Earth models are most successful when body-wave phases that interact with the core are included.  相似文献   

2.
A high-resolution three-dimensional model of the Clyde Sea and the adjacent North Channel of the Irish Sea is used to compute the major diurnal and semidiurnal tides in the region, the associated energy fluxes and thickness of the bottom boundary layer. Initially, the accuracy of the model is assessed by performing a detailed comparison of computed tidal elevations and currents in the region, against an extensive database that exists for the M2, S2, N2, K1 and O1 tides. Subsequently, the model is used to compute the tidal energy flux vectors in the region. These show that the major energy flux is confined to the North Channel region, with little energy flux into the Clyde Sea. Comparison with the observed energy flux in the North Channel shows that its across-channel distribution and its magnitude are particularly sensitive to the phase difference between elevation and current. Consequently, small changes in the computed values of these parameters due to slight changes of the order of the uncertainty in the open-boundary values to the model, can significantly influence the computed energy flux. The thickness of the bottom boundary layer in the region is computed using a number of formulations. Depending upon the definition adopted, the empirical coefficient C used to determine its thickness varies over the range 0.1 to 0.3, in good agreement with values found in the literature. In the North Channel, the boundary layer thickness occupies the whole water depth, and hence tidal turbulence produced at the sea bed keeps the region well mixed. In the Clyde Sea, the boundary layer thickness is a small fraction of the depth, and hence the region stratifies.Responsible Editor: Phil Dyke  相似文献   

3.
In a tidal channel with adjacent tidal flats, along–channel momentum is dissipated on the flats during rising tides. This leads to a sink of along–channel momentum. Using a perturbative method, it is shown that the momentum sink slightly reduces the M2 amplitude of both the sea surface elevation and current velocity and favours flood dominant tides. These changes in tidal characteristics (phase and amplitude of sea surface elevations and currents) are noticeable if widths of tidal flats are at least of the same order as the channel width, and amplitudes and gradients of along–channel velocity are large. The M2 amplitudes are reduced because stagnant water flows from the flats into the channel, thereby slowing down the current. The M4 amplitudes and phases change because the momentum sink acts as an advective term during the fall of the tide, such a term generates flood dominant currents. For a prototype embayment that resembles the Marsdiep–Vlie double–inlet system of the Western Wadden Sea, it is found that for both the sea surface elevation and current velocity, including the momentum sink, lead to a decrease of approximately 2% in M2 amplitudes and an increase of approximately 25% in M4 amplitudes. As a result, the net import of coarse sediment is increased by approximately 35%, while the transport of fine sediment is hardly influenced by the momentum sink. For the Marsdiep–Vlie system, the M2 sea surface amplitude obtained from the idealised model is similar to that computed with a realistic three–dimensional numerical model whilst the comparison with regard to M4 improves if momentum sink is accounted for.  相似文献   

4.
《Continental Shelf Research》1987,7(10):1181-1209
The quadratic law of bottom friction demands an increased frictional coefficient for the S2 and N2 tides with respect to a dominant M2 tidal signal. A numerical model of the semidiurnal tides in the northeast Atlantic gives an increase in friction of ∼35% for the N2, S2 and K2 tides with respect to the M2 tide and this value is close to a theoretically derived estimate for the region, providing confirmation of the general widescale applicability of the quadratic friction law. Small differences in friction also occur for the N2, S2 and K2 tides and this is attributed to the interaction of the L2 and μ2 tides with the M2 tide in the presence of quadratic friction. Energy dissipation relationships, anomalous K2/S2 amplitude ratios and the role of quadratic friction on 18.6 year variations of semidiurnal forcing are examined.  相似文献   

5.
The mean tangential stresses at a corrugated interface between a solid, electrically insulating mantle and a liquid core of magnetic diffusivity λ are calculated for uniform rotation of both mantle and core at an angular velocity Ω in the presence of a corotating magnetic field B. The core and mantle are assumed to extend indefinitely in the horizontal plane. The interface has the form z = η(x, y), where z is the upward vertical distance and x, y are the zonal and latitudinal distances respectively. The function η(x, y) has a planetary horizontal length scale (i.e. of the order of the radius of the Earth) and small amplitude and vertical gradient. The liquid core flows with uniform mean zonal velocity U0 relative to the mantle. Ω and B possess vertical and horizontal components.The vertical (poloidal) component Bp is uniform and has a value of 5 G while the horizontal (toroidal) field BT = Bpαz, where α is a constant. When |α| ? 1, the mean horizontal stresses are found to have the same order of magnitude (10?2 N m?2) as those inferred from variations in the decade fluctuations in the length of the day, although the exact numerical values depend on the orientation of Ω as well as on the wavenumbers in the zonal and latitudinal directions.The influence of the steepness (as measured by α) of the toroidal field on the stresses is investigated to examine whether the constraint that the mean horizontal stresses at the core-mantle interface be of the order of 10?2 N m?2 might provide a selection mechanism for the behaviour of the toroidal field in the upper reaches of the outer core of the Earth. The results indicate that the restriction imposed on α is related to the value assigned to the toroidal field deep into the core. For example, if |α| ? 1 then the tangential stresses are of the right order of magnitude only if the toroidal field is comparable with the poloidal field deep in the core.  相似文献   

6.
Two accurately calibrated superconducting gravimeters (SGs) provide high quality tidal gravity records in three central European stations: C025 in Vienna and at Conrad observatory (A) and OSG050 in Pecný (CZ). To correct the tidal gravity factors from ocean loading effects we compared the load vectors from different ocean tides models (OTMs) computed with different software: OLFG/OLMP by the Free Ocean Tides Loading Provider (FLP), ICET and NLOADF. Even with the recent OTMs the mass conservation is critical but the methods used to correct the mass imbalance agree within 0.1 nm/s2. Although the different software agrees, FLP probably provides more accurate computations as this software has been optimised. For our final computation we used the mean load vector computed by FLP for 8 OTMs (CSR4, NAO99, GOT00, TPX07, FES04, DTU10, EOT11a and HAMTIDE). The corrected tidal factors of the 3 stations agree better than 0.04% in amplitude and 0.02° in phase. Considering the weighted mean of the three stations we get for O1 δc = 1.1535 ± 0.0001, for K1 δc = 1.1352 ± 0.0003 and for M2 δc = 1.1621 ± 0.0003. These values confirm previous ones obtained with 16 European stations. The theoretical body tides model DDW99/NH provides the best agreement for M2 (1.1620) and MATH01/NH for O1 (1.1540) and K1 (1.1350). The largest discrepancy is for O1 (0.05%). The corrected phase αc does not differ significantly from zero except for K1 and S2. The calibrations of the two SG's are consistent within 0.025% and agree with Strasbourg results within 0.05%.  相似文献   

7.
A total number of 46 local earthquakes (2.0≤ML≤4.0) recorded in the period 2000–2011 by the Egyptian seismographic network (ENSN) were used to estimate the total (Qt−1), intrinsic (Qi1) and scattering attenuation (Qsc1) in Cairo metropolitan area, Egypt. The multiple lapse time window analysis (MLTWA) under the assumption of multiple isotropic scattering with uniform distribution of scatters was firstly applied to estimate the pair of Le1, the extinction length inverse, and B0, the seismic albedo, in the frequency range 3–24 Hz. To take into account the effect of a depth-dependent earth model, the obtained values of B0 and Le1 were corrected for an earth structure characterized by a transparent upper mantle and a heterogeneous crust. The estimated values of Qt−1, Qsc1 and Qi1 exhibited frequency dependences. The average frequency-dependent relationships of attenuation characteristics estimated for the region are found to be: Qt1=(0.015±0.008)f (−1.02±0.02), Qsc−1=(0.006±0.001)f (−1.01±0.02), and Qi1=(0.009±0.008)f (−1.03±0.02); showing a predominance of intrinsic absorption over scattering attenuation. This finding implies that the pore-fluid contents may have great effect on the attenuation mechanism in the upper crust where the River Nile is passing through the study area. The obtained results are comparable with those obtained in other tectonic regions.  相似文献   

8.
Stress variation and fluid migration occur in deformation zones, which are expected to affect seismic waves reflected off or propagating across such structures. We developed a basic experimental approach to monitor the mechanical coupling with respect to seismic coupling across a single discontinuity between a granite sample in contact with a steel platen. Piezoceramics located on the platen were used to both generate and record the P and S wave fields reflected off the discontinuity at normal incidence. This way, normal (B n ) and tangential (B t ) compliances were calculated using Schoenberg’s linear slip theory (Schoenberg, J Acoust Soc Am 68:1516–1521, 1980) when the roughness, the effective pressure (P eff, up to 200 MPa), and the nature of the filling (gas or water) vary. We observe that increasing the effective pressure decreases B n and B t , which is interpreted as the effect of the closure of the voids at the interface, permitting more seismic energy to be transmitted across the interface. Values of B n are significantly higher than those of B t at low P eff (<60–80 MPa) in dry conditions, and significantly drop under water-saturated conditions. The water filling the voids therefore helps to transmit the seismic energy of compressional waves across the interface. These results show that the assumption B n  ≈ B t commonly found in some theoretical approaches does not always stand. The ratio B n /B t actually reflects the type of saturating fluids and the effective pressure, in agreement with other experimental studies. However, we illustrate that only the relative variations of this ratio seem to be relevant, not its absolute value as suggested in previous studies. Consequently, the use of B n against B t plots may allow effective pressure variation and the nature of the pore fluid to be inferred. In this respect, this experimental approach at sample scale helps to pave the way for remotely monitoring in the field the hydro-mechanical state of deformation zones, such as seismogenic faults, fractured reservoirs, or lava conduits.  相似文献   

9.
A three-dimensional model covering the northwest European Shelf and part of the adjacent Atlantic Ocean is used to examine the influence of water depth change upon the distribution of maximum tidal bed stress. The direction of bed stress is an indicator of sediment movement as bed load and various regions of convergence and divergence in good agreement with observations are identified. Calculations are performed with water depths reduced by 35 m, corresponding to 10 000 years before present (B.P.). Initially, the model is forced by only the M2 tide, although subsequently five constituents, namely M2, S2, N2, K1 and O1, are used for tidal forcing. Although the distribution of extreme bed stresses computed with only M2 tidal forcing is comparable to that computed with five tides, the additional tidal constituents modify the magnitude of the bed stress. In particular the diurnal tides show regions of local enhanced current amplitude in the shelf-edge region with corresponding changes in bed stress. When water depths are reduced such that the North Sea and English Channel are separated, then there is a significant change in the tidal distribution in the shallow Southern Bight which influences bed-stress distributions and hence bed-load sediment transport in the area. Besides changes in shallow regions, the distribution of tides at the shelf edge is affected. A discussion of the limitations of the present coarse-grid model in shelf-edge regions and how it can be used to provide boundary conditions for limited-area three-dimensional models that can include stratification is presented. Also the importance of stratification for sediment movement at the shelf edge is briefly discussed.Responsible Editor: Phil Dyke  相似文献   

10.
In the present work we analyzed the high-frequency band of crustal tilts at Genoa in order to investigate indirect loading effects by the quarter-diurnal oceanic tides, as well as by an observed oceanic oscillation of 3.66 h which was previously ascribed to the proper oscillation of the Ligurian Sea. With this aim two hourly series of tidal observation recorded by an horizontal pendulum along a NS direction have been submitted to spectral analyses (1 May 1965-28 February 1966; 1 June 1966-31 December 1966).Power spectra revealed two peaks near the periods of 6.15 and 3.66 h and highly resolved Fourier spectra allowed us to detect the loading effects by the oceanic tides M4, MS4 and MK4. As regards to the oscillation of 3.66 h we have found a spreading of spectral lines and non-significant values for the phases. This result provides a decisive verification of the existence of a proper oscillation of the Ligurian Sea regarded as a wide oscillating system with an open boundary from Nice (Côte d'Azur) to Calvi (Corsica).Finally, a spectral analysis performed on sea-level observations at Genoa revealed a complete correspondence between ocean and earth oscillations in the quarter-diurnal band and at the frequency of the longitudinal free oscillation of the Ligurian basin, with a mean amplitude ratio of about 1 mseca/mm.  相似文献   

11.
The energetic electron distribution dynamics and radioemission have been numerically simulated in order to interpret three characteristic types of the frequency spectrum slope dynamics between 17 and 34 GHz in different parts of solar flaring loops observed with the Nobeyama Radioheliograph. In all types, the spectrum slope decreases at the rise phase of microwave burst. The behavior is qualitatively different at the decay phase of the burst: the spectrum slope increases immediately after the burst maximum in the first type; continues decreasing or stabilizes, not reaching zero, in the second type; and continues decreasing to zero and becomes positive in the third type. The parameters at which the calculated dynamics of the frequency spectrum slope in different parts of the magnetic loop most strictly corresponds to the observed dynamics have been found. It has been indicated that the first type of the spectrum slope dynamics is related to an increase and decrease in the radio source optical thickness at the rise and decay phases, respectively. The second type is caused by the flattening of the electron energy spectrum that continues at the decay phase. The third type is related to a gradual flattening process of the electron energy spectrum, combined with a large ratio of the plasma density to the magnetic field strength (n 0 /B) within a loop, and is explained by the Razin effect.  相似文献   

12.
The water level of five river stages and seven groundwater wells in the Taipei Basin were analysed by spectral analysis in the frequency domain. The diurnal, semi‐diurnal and quarter‐diurnal tidal components of the Tanshui River appear to relate closely to astronomical tides as K1, M2 and M4, respectively. It is also found that the diurnal component reveals a reversed phase angle in the middle section of the Tanshui River; the phase of the quarter‐diurnal component is also found to be reversed at stations upstream in the Tanshui River and Hsintien Stream. It is believed that these phenomena could be caused by local variation in the river channel topography. The autospectrum and cross‐spectrum between groundwater elevation and nearby river stage were observed to correlate highly with the frequency of the astronomical tides K1, M2 and M4. From the study of the phase shift and time lag of water level fluctuations at river stages and groundwater wells, it was found that the tidal effects of diurnal, semi‐diurnal, and quarter‐diurnal components were significantly different. The relationships between phase and the fluctuated range of atmospheric pressure and water level imply that change in atmospheric pressure does not affect water level fluctuation in the river stage and groundwater well. Copyright © 1999 John Wiley & Sons, Ltd.  相似文献   

13.
The purpose of this work is to give a self-consistent model of the magnetic mirrors using a perturbative magnetohydrostatic approach. With the help of this model a number of features have been revealed like geometry, stability and behavior for different temperature anisotropies (A=T/T||). The basic relations we use in order to derive the model for the mirror structures are the magnetohydrostatic equilibrium condition and an expression for the anisotropy in the case of bi-Maxwellian distribution (Lee et al., J. Geophys. Res. 92 (1987) 2343). Based on these equations, we have found analytical expressions for the magnetic field (δB), pressure (δp) and temperature (δT) perturbations. From the investigation of the dependence of the magnetic mirrors on the unperturbed anisotropy (A0), we have found the well-known behavior (opposite phase variations of the magnetic field intensity and number density) for A0>1 (Tsurutani et al., Geophys. Res. 87 (1982) 6060). For A0<1, the behavior is different but the mirror structures still exist. However, if the anisotropy is in a range of values depending on the plasma parameter β0⊥=p0⊥/(B02/2μ0), the magnetic mirrors can no longer exist. From the comparison between the current density deduced from the Ampere law, necessary to sustain the magnetic mirror, and the gradient-curvature drift current density actually being inside the magnetic mirror, we have been able to determine instability regions in the (A0,β0⊥)-plane.  相似文献   

14.
The viscosity of natural rhyolitic melt from Lipari, Aeolian Islands and melt-bubble emulsions (30–50 vol% porosity) generated from Lipari rhyolite have been measured in a concentric cylinder rheometer at temperatures and shear rates in the range 925–1150°C and 10−3–10−1.2 s−1, respectively, in order to better understand the dependence of emulsion shear viscosity on temperature and shear rate in natural systems. Bubble-free melt exhibits Newtonian–Arrhenian behavior in the temperature range 950–1150°C with an activation energy of 395±30 kJ/mol; the shear viscosity is given by log ηm=−8.320+20624/T. Suspensions were prepared from natural rhyolite glass to which small amounts of Na2SO4 were added as a ‘foaming agent’. Reasonably homogeneous magmatic mixtures with an approximate log-normal distribution of bubbles were generated by this technique. Suspension viscosity varied from 106.1 to 108.37 Pa s and systematically correlates with temperature and porosity in the shear stress range (104.26–105.46 Pa) of the experiments. The viscosity of melt-bubble emulsions is described in terms of the relative viscosity, ηr=ηe/ηm where ηe is the emulsion viscosity and ηm is the viscosity of melt of the same composition and temperature. The dependence of relative viscosity on porosity for magmatic emulsions depends on the magnitude of the capillary number Ca≡G/(σrb−1ηm−1), the ratio of viscous forces acting to deform bubbles to interfacial forces resisting bubble deformation. For inviscid bubbles in magmatic flows three regimes may be identified. For Ca<0.1, bubbles are nearly spherical and relative viscosity is an increasing function of porosity. For dilute systems, ηr=1+φ given by the classical result of Taylor [Proc. R. Soc. London A 138 (1932) 41–48]. For Ca in the range 0.1<Ca<10, emulsions behave as power law fluids and the relative viscosity depends on shear rate (or Ca) as well as porosity. At high Ca (Ca>10) an asymptotic regime is reached in which relative viscosity decreases with increasing porosity and is independent of Ca. Our experiments were carried out for 30<Ca<925 in order to quantify the maximal effect of bubbles in reducing the viscosity of magmatic emulsions relative to single-phase melt at identical conditions of shear rate and temperature. The viscosity of a 50 vol% emulsion is a factor of five smaller than that of melt alone. Rheometric measurements obtained in this study are useful in constraining models of magma transport and volcanic eruption mechanics relevant to transport of volatile-saturated magma in the crust and upper mantle.  相似文献   

15.
This paper studies the ionospheric and geomagnetic response to an X6.2 solar flare recorded at 14:30 UT on December 13, 2001, in quiet geomagnetic conditions which allow the variations in the geomagnetic field and ionosphere measurements to be easily related to the solar flare radiation.By using measurements from the global positioning system (GPS) and geomagnetic observatories, the temporal evolution of ionospheric total electron content variation, vTECV, and geomagnetic field variations, δB, as well as their rates of variation, were obtained around the subsolar point at different solar zenith angles. The enhancement of both parameters was recorded one to three minutes later than the Geostationary Operational Environmental Satellite (GOES) programme recording; such delay tends to depend on the latitude, longitude, and solar zenith angle of the observatory's observations.The vTECV is related to the local time and the δB to the intensity and position of the ionospheric currents.The vTECV′s maximum value is always recorded later than the maximum values reached by δB and the X-ray intensity. The maximum δB is larger in the local morning than in the afternoon.The rates of vTECV and δB have two maximum values at the same time as the maximum values recorded by Hα (for each ribbon).This work shows the quantitative and qualitative relations between a solar flare and the ionospheric and geomagnetic variations that it produces.  相似文献   

16.
Non-Maxwellian ion velocity distribution functions have been theoretically predicted and confirmed by observations, to occur at high latitudes. These distributions deviate from Maxwellian due to the combined effect of the E×B drift and ion-neutral collisions. The majority of previous literature, in which the effect of ion self-collisions was neglected, established a clear picture for the ion distribution under a wide range of conditions. At high altitudes and/or for solar maximum conditions, the ion-to-neutral density ratio increases and, hence, the role of ion self-collisions becomes appreciable. A Monte Carlo simulation was used to investigate the behavior of O+ ions that are E×B-drifting through a background of neutral O, with the effect of O+ (Coulomb) self-collisions included. Wide ranges of the ion-to-neutral density ratio n i /n n and the electrostatic field E were considered in order to investigate the change of ion behavior with solar cycle and with altitude. For low altitudes and/or solar minimum (n i /n n \leq10?5), the effect of self-collisions is negligible. For higher values of n i /n n , the effect of self-collisions becomes significant and, hence, the non-Maxwellian features of the O+ distribution are reduced. For example, the parallel temperature T i\Vert increases, the perpendicular temperature T i decreases, the temperature anisotropy approaches unity and the toroidal features of the ion distribution function become less pronounced. Also, as E increases, the ion-neutral collision rate increases, while the ion-ion collision rate decreases. Therefore, the effect of ion self-collisions is reduced. Finally, the Monte Carlo results were compared to those that used simplified collision models in order to assess their validity. In general, the simple collision models tend to be more accurate for low E and for high n i /n n .  相似文献   

17.
We investigate the origin of diversity of eruption styles in silicic volcanoes on the basis of a 1-dimensional steady conduit flow model that considers vertical relative motion between gas and liquid (i.e., vertical gas escape). The relationship between the assemblage of steady-state solutions in the conduit flow model and magma properties or geological conditions is expressed by a regime map in the parameter space of the ratio of liquid-wall friction force to liquid–gas interaction force (non-dimensional number ε), and a normalized conduit length Λ. The regime map developed in the companion paper shows that when ε is smaller than a critical value εcr, a solution of explosive eruption exists for a wide range of Λ, whereas an effusive solution exists only when Λ ~ 1. On the other hand, when ε > εcr, an effusive solution exists for a wide range of Λ. Diversity of eruption styles observed in nature is explained by the change in ε accompanied by the change in magma viscosity during magma ascent. As magma ascends, the magma viscosity increases because of gas exsolution and crystallization, leading to the increase in ε. For the viscosity of hydrous silicic magma at magma chamber, ε is estimated to be smaller than εcr, indicating that an explosive solution exists for wide ranges of geological parameters. When magma flow rate is small, the viscosity of silicic magma drastically increases because of extensive crystallization at a shallow level in the conduit. In this case, ε can be greater than εcr; as a result, a stable effusive solution co-exists with an explosive solution.  相似文献   

18.
We study the frictional and viscous effects on earthquake nucleation, especially for the nucleation phase, based on a one-degree-of-freedom spring-slider model with friction and viscosity. The frictional and viscous effects are specified by the characteristic displacement, U c, and viscosity coefficient, η, respectively. Simulation results show that friction and viscosity can both lengthen the natural period of the system and viscosity increases the duration time of motion of the slider. Higher viscosity causes a smaller amplitude of lower velocity motion than lower viscosity. A change of either U c (under large η) or η (under large U c) from a large value (U ch for U c and η h for η) to a small one (U cl for U c and η l for η) in two stages during sliding can result in a clear nucleation phase prior to the P-wave. The differences δU c = U ch ? U cl and δη = η h ? η l are two important factors in producing a nucleation phase. The difference between the nucleation phase and the P-wave increases with either δU c or δη. Like seismic observations, the peak amplitude of P-wave, which is associated with the earthquake magnitude, is independent upon the duration time of nucleation phase. A mechanism specified with a change of either η or U c from a larger value to a smaller one due to temporal variations in pore fluid pressure and temperature in the fault zone based on radiation efficiency is proposed to explain the simulation results and observations.  相似文献   

19.
In the present study, the deterministic chaotic behaviour of interplanetary magnetic field (IMF) under various geomagnetic conditions of low and high solar active periods was analyzed, using the time series of IMF |B| and Bz, by employing chaotic quantifiers like, Lyapunov exponent, Tsallis entropy, correlation dimension, and non-linear prediction error. We have investigated whether the chaotic behaviour of interplanetary magnetic field would modify, when it produces major geomagnetic storms, and how it depends on the phase of solar activity. The yearly average values of Lyapunov exponent for the time series of IMF |B| and Bz, show solar flux dependence, whereas those values of entropy, correlation dimension and non-linear prediction error had no significant solar flux dependence. The yearly average values of entropy for quiet periods are higher compared to those values for major storm periods belonging to low/high solar active conditions, for both the time series |B| and Bz.  相似文献   

20.
The three components of the indirect oceanic effect are calculated for the M 2, K 1 and O 1 waves and compared with the observed earth tide. The vertical component of the near Arctic oceanic load explains fairly well the large 45° observed phase lag of M 2. The results for the horizontal components are satisfactory; the discrepancies between different tiltmeters are not due to the oceanic perturbations (unless some local or cavity effects are supposed).  相似文献   

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