Observed Delay Times Research Articles

Shear wave splitting in a young continent‐continent collision: An example from Eastern Turkey

We have determined the shear wave splitting fast polarization direction and delay time using data from the ETSE broadband experiment (Eastern Turkey Seismic Experiment), a deployment of 29 broadband seismic stations across the collision zone of the Arabian, Eurasian, and Anatolian plates. Our results show that the fast polarization directions are relatively uniform and they exhibit primarily NE–SW orientations. No abrupt changes in anisotropy directions are observed across the main tectonic units in the region: the Bitlis Suture (BS) and the North and Eastern Anatolian Fault zones. The fast polarization directions are determined to be sub‐parallel to the Anatolian, Arabian, and Eurasian absolute plate velocities, except for those stations in the northeastern corner of the Anatolian Plateau. Observed delay times range from 0.7 to 2.0 seconds with an average value of 1.0 second; the largest values are within the northern Anatolian Plateau which is underlain by an exceptionally low velocity zone in the uppermost mantle. We interpret shear wave splitting as the vector difference of the Eurasian lithosphere and northeastern or southwestern directed flow of the asthenospheric mantle. Comparisons of the polarization anisotropy with measurements of Pn azimuthal anisotropy suggest vertical anisotropic layering except in those areas which are underlain by partially molten uppermost mantle.

Fission time in the 28Si + natPt reaction

The experimental fission times are analyzed for excited nuclei produced in the 28Si + natPt reaction. Experimental lifetimes obtained by the crystal-blocking technique range between 10−17 and 10−18 s at bombarding energies between 140 and 170 MeV, respectively. Experimental data are analyzed within the statistical theory of nuclear reactions and the double-humped-fission-barrier model with allowance for preequilibrium processes and the nuclear-dissipation phenomenon. It is shown that fission barriers retain their double-humped structure for nuclear temperatures up to about 1.7–1.8 MeV and that the lifetimes of excited strongly deformed states in the second potential well contribute substantially to the observed delay times in the fission decay channel.

Experimental Evidence of Shell Effects in the Fission Time of Heavy Nuclei

The crystal blocking technique has been used to measure delay times in the fission channel for excited nuclei produced in the 28 Si+ n a t Pt reaction. Experimental results range from 10 -17 to 10 -18 s at bombarding energies from 140 to 170 MeV. It is shown that fission barriers keep their double-humped structure for nuclear temperature up to 1.7–1.8 MeV, and that the lifetimes of excited states in the second potential well contribute essentially to the observed delay times in the fission channel.

Phase Separation Induced by Conformational Ordering of Gelatin in Gelatin/Maltodextrin Mixtures

Mixtures of gelatin and maltodextrin in aqueous solution have been quenched to temperatures at which they are initially miscible but where gelatin ordering is initiated. In many cases phase separation was observed to occur after a significant time delay, and the dependence of these delays on quench temperature and biopolymer concentration has been studied in detail using turbidity measurements and confocal laser scanning microscopy (CLSM). Furthermore, by observing the optical rotation (OR) and turbidity of the system simultaneously, the gelatin helix content and the time delay before the onset of phase separation were monitored concurrently. The observed delay times were found to correspond to the time taken for the development of a certain degree of gelatin ordering, which drives the separation process. A further consequence of gelatin ordering is the viscosifying of the solution and, at sufficient concentrations, the formation of a gel. Therefore, rheological measurements have been used in addition to turbidity measurements and CLSM in order to monitor further the structural development of the systems. A comparison of the data obtained from these techniques shows that while the development of a certain elasticity will trap the system morphology, this elasticity is not directly related to that found at the gel point. At low maltodextrin concentrations, where phase separation was not detected by turbidity, transmission electron microscopy (TEM) has been used to examine the microstructure on a smaller length scale.

Investigation of shell effects for heavy fissionable nuclei by the blocking technique

The crystal blocking technique has been used to measure delay times in the fission channel for excited nuclei produced in the 28Si+natPt reaction. Experimental results range from 10−17 to 10−18 s at bombarding energies from 140 to 170 MeV. It is shown that fission barriers keep their double-humped structure for temperature up to 1.7–1.8 MeV, and that the lifetimes of excited states in the second potential well contribute essentially to the observed delay times in the fission channel.

Upper mantle anisotropy beneath the Geoscope stations

Seismic anisotropy has been widely studied this last decade, particularly by measuring splitting of vertically propagating core shear waves. The main interest in this technique is to characterize upper mantle flow beneath seismic stations. On the other hand, the major restriction in this method is that a single station gives a single anisotropy measurement. Alternative methods have been developed in order to avoid this restriction. An accurate determination of upper mantle seismic anisotropy beneath a seismic station may allow one, by doing anisotropy correction, to characterize remote or deeper anisotropy. The Geoscope network is ideal for this purpose because it is composed of a large set (about 26) of high‐quality, broadband seismometers globally distributed and because some of these stations have run for more than 10 years and most of them for more than 5 years. We selected about 100 events at each site, generally of magnitude (mb) > 6.0, and we performed systematic measurements of the splitting parameters (fast polarization direction ϕ and delay time δt) on SKS, SKKS, and PKS phases. Splitting on oceanic islands has been difficult to observe owing to the low quality of the signal but also perhaps owing to complex upper mantle structures beneath the stations. Station KIP (Kipapa, Hawaii) in the Pacific is the only oceanic Geoscope station with a clear anisotropy. We determined well‐constrained splitting parameters for 10 of the 17 continental stations that may be explained by a single anisotropic layer. The poor correlation between fast polarization directions and the absolute plate motion together with the apparent incoherence between the plate velocities and the observed delay times suggest that a simple drag‐induced asthenospheric flow alone fails to explain most of the observations. For some stations located on or near major lithospheric structures (TAM, Tamanrasset, Algeria, for instance), we observe a good correlation between fast polarization directions and regional structures. At station SCZ (Santa Cruz, California), we found clear variations of the splitting parameters as a function of the event backazimuth, compatible with two layers of anisotropy. Three stations (CAN (Canberra), HYB (Hyderabad, India) and SSB (Saint Sauveur Badole, France)) seem to be devoid of detectable anisotropy.

Anisotropy beneath the Iberian Peninsula: The Contribution of the ILIHA-NARS Broad-band Experiment

The presence of anisotropy beneath the Iberian Peninsula and its main distinctive features can be established through the analysis of teleseismic shear-wave splitting observed in the ILIHA-NARS experiment. In this experiment, an homogeneous data set is provided by a network of 14 broad-band stations deployed over the entire peninsula for about one year. Even if technical problems led to an amount of data smaller than expected, significant variations in the inferred fast velocity direction are observed for stations located in different Iberian domains. The stations in Central and East Iberia show a fast velocity direction oriented roughly E-W, coincident with previous results in Toledo. A clearly different NE-SW direction is observed in the Ossa-Morena zone, supporting the image from a previous regional experiment. The observed delay times lie between 0.5 and 1 s. Although large-scale mechanisms, such as the absolute plate motion of Eurasia, can be invoked to explain the origin of anisotropic features in many sites, the regional variations observed in some domains imply that differentiated origins of the anisotropy have to be considered, probably related to the particular tectonics in the area. An interesting example of this fact is provided by the stations in the Betic chain; the fast velocity direction inferred for a station located in the limit of the External Betics (South Iberian domain), oriented N80°E, is clearly different from the N15–35°E direction observed in the Internal Betics (Alboran crustal domain), the origin of which has to be related to the Alpine building of the chain.

Acoustic Edge Modes of the Degenerate Two-Dimensional Electron Gas Studied by Time-Resolved Magnetotransport Measurements.

Edge magnetoplasmons propagating along the smooth boundary of a two-dimensional electron gas are studied by time-resolved magnetotransport experiments. The incident pulse splits up into several pulses due to modal dispersion. The observed delay times are well described by the predictions of Aleiner and Glazman for the fundamental and acoustic modes [Phys. Rev. Lett. 72, 2935 (1994)], if the quantized Hall conductance is taken into account.

Seismic anisotropy beneath Tibet: evidence for eastward extrusion of the Tibetan lithosphere?

Strong seismic anisotropy beneath Tibet has recently been reported from the study of SKS shear wave splitting. The fast split waves are generally polarized in an easterly direction, close to the present day direction of motion of the Tibetan crust relative to stable Eurasia, as deduced from Holocene slip rates on the major active faults in and around Tibet. This correlation may be taken to suggest that the whole Tibetan lithosphere is being extruded in front of indenting India and that the anisotropic layer is the deforming asthenosphere, that accommodates the motion of the Tibetan lithosphere relative to the fixed mantle at depth. Uncertainties about this motion are at present too large to bring unambiguous support to that view. Assuming that this view is correct however, a simple forward model is used to compute theoretical delay times as a function of the thickness of the anisotropic layer. The observed delay times would require a 50–100 km thick anisotropic layer beneath south-central Tibet and an over 200 km thick layer beneath north-central Tibet, where particularly hot asthenosphere has been inferred. This study suggests that the asthenospheric anisotropy due to present absolute block motion might be dominant under actively deforming continents.

Disturbance propagation times to the far tail

Simultaneous data collection by IMP 8 in the near‐Earth solar wind and Geotail in the distant tail make possible the correlation of solar wind disturbances with their effects in the distant tail. This paper discusses some of the possible propagation times between the two spacecrafts and compares them with observed times for a particular disturbance when Geotail was near the magnetopause. The best fit matches very closely with the solar wind velocity. Several other propagation mechanisms also match the observed delay times within the observational error. The conversion processes operating across the bow shock, through the magnetosheath, and through the tail tend to maintain the net propagation speed constant, which accounts for the empirical finding that the direct solar wind transit time between IMP 8 and Geotail works pretty well. There is little if any effect of a significant delay resulting from the flow slowing down in the subsonic dayside magnetosheath. The event cannot discriminate between a windsock response of the tail to the solar wind disturbance and a standing wave response without further quantitative modeling.

Shear Wave Splitting Observed in the Southwestern Part of Fukushima Prefecture, Notheastern Japan.

We have investigated shear wave polarization anisotropy in the southwestern part of Fukushima Prefecture, northeastern Japan. Waveform data we analyzed are obtained by a local seismic networks of Tohoku University and Utsunomiya University. We used a cross correlation method to detect the shear wave splitting. For the shallow events beneath the southwestern part of Fukushima Prefecture, observed that fast shear wave oscillation directions (FSODs) are oriented to the NW-SE. The observed delay times are less than 0.1 s and tend to have large values in high seismicity regions. For the intermediate-depth events, observed FSODs are oriented parallel to the dip direction of the subducted slab and most of the observed delay times are less than 1 s. We estimated the location and the cause of anisotropy based on these observations. In the upper crust beneath the southwestern part of Fukushima Prefecture anisotropy is estimated to be caused mainly by preferredly oriented cracks controlled by the tectonic stress.

Quantitative measurements of shear wave polarizations at the Anza Seismic Network, southern California: Implications for shear wave splitting and earthquake prediction

We analyze shear wave polarizations from local earthquakes recorded by the Anza network in southern California, using an automated method which provides unbiased and quantitative measurements of the polarization and the duration of linear motion following the shear wave arrival (the linearity interval). Initial shear wave particle motions are strongly aligned at four stations, a feature that is not predicted by focal mechanisms. The particle motion alignment is most likely caused by shear wave splitting due to anisotropy beneath these stations, a result supported by the clear shear wave splitting seen in a borehole recording near one of the Anza stations. These results are consistent with an earlier analysis of these data by Peacock et al. [1988]. However, our analysis does not support claims by Crampin et al. [1990] that shear wave splitting delay times at station KNW exhibit temporal variations which can be correlated with the occurrence of the North Palm Springs earthquake (ML=5.6) of July 8, 1986. Automatically determined linearity intervals scatter widely from 0.02 to 0.15 s and exhibit no clear temporal trends. We find a correlation between earthquake moment and the linearity interval, possibly a result of longer effective source time functions for the larger events. The inability to identify a distinct slow quasi‐shear wave pulse for the vast majority of these events indicates that scattering strongly affects the particle motion, even in the very early shear wave coda. Analysis of earthquake clusters with similar waveforms recorded at KNW shows that seismic Green functions are stable throughout the observational period and that most linearity interval variation is due to source and ray path differences between events. If shear wave splitting is causing the observed delay times between horizontal components, the waveform stability for events in these clusters restricts any temporal changes in shear wave splitting delay times to less than 5–10%.

Teleseismic investigation of the East African Rift - Kenya

We recorded teleseismic events with 19 digital seismic recorders along a 600 km long profile across the East African rift in Kenya. At 5 stations three-component intermediate period seismometers were installed to investigate the presence of partial melt by comparing P- and S-delays. We find an average P-delay greater than 1.5 sec centered on the eastern flank of the rift. The delay pattern can be correlated with the upwarp of the asthenosphere/lithosphere boundary and resembles the Bouguer gravity along the profile. Structural models estimated from 2d-inversions (ACH-algorithm) of the observed delay times suggest a low-velocity zone is present down to large depths beneath the rift, dipping to the west. To the east of the rift LVZ's are located in the crust and upper mantle.

Time-resolving experiments on Dicke superfluorescence ofO2−centers in KCl. Two-color superfluorescence

Single crystals of KCl containing $\mathrm{O}_{2}^{}{}_{}{}^{\ensuremath{-}}$ centers (8\ifmmode\times\else\texttimes\fi{}${10}^{16}$-7\ifmmode\times\else\texttimes\fi{}${10}^{17}$ ${\mathrm{cm}}^{\ensuremath{-}3}$) were excited at low temperatures ($T<30$ K) using frequency-quadrupled single pulses of a mode-locked Nd-doped yttrium aluminum garnet laser. At excitation intensities above 20 GW/${\mathrm{cm}}^{2}$ a spontaneous emission of coherent light was observed which could be identified as Dicke superfluorescence. This superfluorescence was observed on two of the zero-phonon lines in the vibrational progression of the $\mathrm{O}_{2}^{}{}_{}{}^{\ensuremath{-}}$ fluorescence spectrum (0-10 and 0-11 at 592.8 and 629.1 nm, respectively). We have studied the time dependence, the statistics of the delay times, and the intensities. The observed delay times vary between 0.5 and 10 ns, the observed pulse widths between 0.5 and 6 ns, and the coherence time was found to be longer than 100 ps. The observed linewidths of the superfluorescence ($0.016\mathrm{nm}\ensuremath{\cong}14\mathrm{GHz}$) is a factor of 2.5 smaller than that of the spontaneous fluorescence (0.042 nm at 4.2 K). The superfluorescence is observed only below a critical temperature which depends on the $\mathrm{O}_{2}^{}{}_{}{}^{\ensuremath{-}}$ concentration (8 to 28 K for concentrations between 8\ifmmode\times\else\texttimes\fi{}${10}^{16}$ and 7\ifmmode\times\else\texttimes\fi{}${10}^{17}$ ${\mathrm{cm}}^{\ensuremath{-}3}$).

Bilayer silicide formation during the interaction of thin chromium, nickel and platinum films with silicon

Silicide formation in the Si/Ni/Cr and Si/Pt/Cr tertiary systems was investigated. NiSi and PtSi formation was found to take place at about 350 °C, whilst subsequent CrSi 2 growth on top of these silicides started at approximately 440 °C for NiSi and 500 °C for PtSi. The CrSi 2 formation was found to be linear with time, indicating a reaction-limited process. In both cases the silicide phases stayed well separated, with no evidence of tertiary compound formation. The growth rate of CrSi 2 on epitaxial PtSi was found to be nearly 30% slower than that on non-epitaxial PtSi, whilst the activation energy for CrSi 2 formation on NiSi and PtSi was found to be the same, namely 1.8±0.2 eV for both Si〈100〉 and Si〈111〉 substrates. Auger depth profile measurements indicated that the observed delay times before CrSi 2 formation started to take place, and the large difference in the temperature at which CrSi 2 starts to grow on NiSi and PtSi, could probably be ascribed to oxygen contamination.

The morphology of widespread electron participation at high latitudes

The morphology of widespread energetic electron participation processes at high latitudes has been studied using an extensive set of satellite Bremsstrahlung X-ray measurements. Long term averages of the X-ray (> 50 keV) intensities and energy spectra observed at 750 km altitude over a broad field of view are presented. These data provide for the first time long term averages of Bremsstrahlung X-rays produced by electrons precipitating into the atmosphere over wide local time and latitude intervals. The average X-ray fluxes were found to be ~30% larger near local noon than near local midnight. The net intensities were generally higher at larger values of the AE index but wide variations in this pattern were observed. For AE > 800 the average flux was a factor of ~100 higher than the flux when AE < 40. The average fluxes of Bremsstrahlung X-rays during a very active 9 day period (19–27 March, 1973) were larger by a factor of 2.2 than the 6 month long-term average from December 1972 to May 1973. A study of the spatial-temporal variations in electron precipitation events was made on the basis of closely co-ordinated measurements between electron spectrometers on two different polar orbiting satellites. Even when the longitude and universal time differences were small, major deviations in the flux versus latitude profiles were generally evident. The time profiles of widespread electron precipitation events have also been investigated using the Bremsstrahlung data. The time correlation coefficients between pairs of X-ray measurements emitted from a large area were significant for delay times of ~40 if both members of the pair were either near midnight or noon. For noon-midnight or midnight-noon pairs the correlations were not significant even at the shortest observed delay times of ~20 min. These co-ordinated satellite observations indicate a general lack of simultaneous electron precipitation in the noon and midnight sectors.

Yet another approach to the travel-time inversion problem

A new technique for the inversion of travel-times is being developed in which it is hoped to combine all the good features of the other known techniques. Since the inversion of T − Δ data provides difficulties with triplications and shadow zones, we have chosen to invert p − τ data derived from the T − Δ data using the Bessonova method. Many computer subroutines capable of minimising very general non-linear functions, including linear and/or nonlinear constraints, are now available. Using this “black box” technique we can decide on the type of model we wish to allow, construct the model with or without low-velocity zones, incorporate smoothness considerations, and specify which velocities are already known sufficiently accurately and thus do not need to be varied. All we need to provide is a function subroutine to calculate the delay τ for a set of ray parameters p, and a suitable function to be minimised such as the weighted sum of the squares of the difference between the calculated and observed delay times τ. We can use spherical layers or use a transformation to flat layers and transform back at the end of the calculation. We can choose a model having layers of constant velocities, having linear velocity distribution through a layer, or having exponential-type spherical layers whose velocities are represented by such relationships as ν = a r b , where ν is the velocity and r is the radius, both of which vary through the layer, and a and b are constants determined from the velocity at the inner and outer radii of the layer. At present the method uses a great deal of computing time, but it is hoped that this can be improved.

DELINEATING LOW‐VELOCITY LENSES

The end of a low‐velocity lens diffracts energy emerging after reflection at a lower level. Conventional interpretive treatment of such energy as an undiffracted reflection could lead to serious inferences of nonexistent structure. Constant moveout down the record is the main criterion for the diffracted reflection situation. The diffractive source, assuming the velocity in the surrounding material to be known, is found as the envelope of emergent travel paths. Reflected energy from the top of the feature, if recognized, can be analyzed for delineating the upper limit of the body. The thickness can be estimated from observed delay times. For discovering and delineating low‐velocity lenses, long reflection spreads should be used.

Delineating Low-Velocity Lenses: ABSTRACT

The end of a low-velocity lens diffracts energy emerging after reflection at a lower level. Conventional interpretive treatment of such energy as an undiffracted reflection could lead to serious inferences of non-existent structure. Constant moveout down the record is the main criterion for the diffracted reflection situation. The diffractive source, assuming the velocity in the surrounding material to be known, is found as the envelope of emergent travel paths. Reflected energy from the top of the feature, if recognized, can be solved for delineating the upper limit of the body. The thickness can be estimated from observed delay times. For discovering and delineating low-velocity lenses, long reflection spreads should be used. End_of_Article - Last_Page 1775------------

Studies on the sunrise effect in regions E and F

The characteristics of an ionospheric layer are known to undergo variations during sunrise in the layer (sunrise-effect). The sunrise-effect begins not when the solar rays first strike the layer, but later. This delay phenomenon in the “sunrise-effect” has been studied for the E- and F-regions of the ionosphere at Calcutta during the period January 1947 to June 1952. It is shown that the observed delay times can be explained as due to the difference between the night-time and daytime heights of an ionospheric layer. The early-morning rays which strike the night side of the ionospheric layer and increase its ionization, must pass above the daytime height of the layer, in order that the active wavelengths do not get absorbed by the thickness of the air through which the rays pass. The measured daytime height of the layer (after the occurrence of the sunrise-effect) agrees reasonably well with the height grazing which the solar rays strike the nighttime height of the layer at the time of commencement of the sunrise-effect.