Non-stationary seismic response of MDOF systems by wavelet transform

A wavelet-based formulation has been presented in this paper for the stochastic analysis of a linear multi-degree-of-freedom (MDOF) classically damped system subjected to earthquake ground motion. The ground motion has been modelled as a non-stationary process (both in amplitude and frequency) using wavelets. Closed-form expressions of the moments of the instantaneous Power Spectral Density Function (PSDF) of the response have been derived and used to predict the statistics of the response peak of any desired order. For illustration of the formulation, an example torsionally coupled multistoried building has been considered along with the twenty synthetically generated time-histories corresponding to an example ground motion process. © 1997 John Wiley & Sons, Ltd.     

On precursors to the Italy earthquake M.6.2 using VLF signal of NSY(45.9 kHz) received at Kiel Longwave Monitor

On August 24, 2016, an earthquake of magnitude 6.2 struck Central Italy (42.706°N and 13.223°E) at 1:36 UT. We present the results obtained from the US Navy VLF Transmitter’s NSY signal of 45.9 kHz transmitted from Niscemi, in the province of Sicily, Italy and received at the Kiel Longwave Monitor, Germany for 2016. We analysed the terminator times and their individual differences. We also analysed trends, dispersion and night time fluctuation which gave us a possible precursor the Italy earthquake. We found ionospheric perturbations in these parameters on 14th, 19th and 22nd of August, few days prior. Moreover, we filtered the possible effects due to lightning, storms and auroras if any.     

A damage-based definition of effective peak acceleration

This paper presents a rational basis for obtaining the Effective Peak Acceleration (EPA) of a given ground motion process. The proposed formulation considers the statistical variability in the ground motion, and is centred on the idea of explicitly linking EPA with expected cumulative damage in the structures due to the inelastic excursions. The structural behaviour has been modelled by a Single-Degree-Of-Freedom (SDOF) bilinear hysteretic oscillator. EPA is considered to be the expected PGA of a scaled ground motion process such that this oscillator undergoes a specified expected damage under the unscaled process if it is linearly designed for the scaled process. For estimation of the damage, the oscillator has been replaced by an equivalent linear oscillator through stochastic averaging. A parametric study has been carried out to investigate the dependence of EPA on several governing parameters, and it has been shown that despite the strong dependence of EPA on oscillator time period, it may be possible to obtain ‘period-averaged’ EPA values for several ground motion processes for engineering applications. © 1998 John Wiley & Sons, Ltd.     

Explosion-triggered star formation in the central region of the Galaxy

The role of shock waves generated by explosions in the nucleus of a galaxy and propagating through the surrounding gas, on the efficiency of star formation in this region, is investigated. It is found that if the gas in the shock-compressed layer follows polytropic changes in the density and temperature, a very wide range of masses of the fragments can be formed. Calculation shows that under suitable physical conditions a burst of star formation is actually realized.     

Heterogeneous agglutinitic glass and the fusion of the finest fraction (F3) model

Abstract— Evidence in favor of the model fusion of the finest fraction (F³) for the origin of lunar agglutinitic glass has been accruing. They include (1) theoretical expectations that shock pulses should engulf and melt smaller grains more efficiently than larger grains, (2) experimental results of impact shock, albeit at lower than presumed hypervelocity impacts of micrometeorites on the lunar regolith, and (3) new analyses confirming previous results that average compositions of agglutinitic glass are biased towards that of the finest fraction of lunar soils from which they had formed. We add another reason in support of the F³ model. Finer grains of lunar soils are also much more abundant. Hence, electrostatic forces associated with the rotating terminator region bring the finest grains that are obviously much lighter than courser grains to the surface of the Moon. This further contributes to the preferential melting of the finest fraction upon micrometeoritic impacts. New backscattered electron imaging shows that agglutinitic glass is inhomogeneous at submicron scale. Composition ranges of agglutinitic glass are extreme and deviate from that of the finest fraction, even by more than an order of magnitude for some components. Additionally, we show how an ilmenite grain upon impact would produce TiO₂‐rich agglutinitic glass in complete disregard to the requirements of fusion of the finest fraction. We propose an addition to the F³ model to accommodate these observations (i.e., that micrometeorite impacts indiscriminately melt the immediate target regardless of grain size or grain composition). We, therefore, suggest that (1) agglutinitic glass is the sum of (a) the melt produced by the fusion of the finest fraction of lunar soils and (b) the microvolume of the indiscriminate target, which melts at high‐shock pressures from micrometeoritic impacts, and that (2) because of the small volume of the melt and incorporating cold soil grains, the melt quenched so rapidly that it did not mix and homogenize to represent any preferential composition, for example, that of the finest fraction.     

A Global Model of MHD Wave Support of a Stratified Molecular Cloud

We perform numerical simulations of nonlinear MHD waves in a gravitationally stratified molecular cloud that is bounded by a hot and tenuous external medium, within a 1.5-dimensional approximation. Under the influence of a driving source of Alfvénic disturbances, the cloud is lifted up by the pressure of MHD waves and reaches a steady state characterized by oscillations about a new time-averaged equilibrium state. The nonlinear effect results in the generation of longitudinal motions and many shock waves. Models of an ensemble of clouds show that, for various strengths of the input energy, the velocity dispersion in the cloud σ ∝ Z ^0.5, where Z is a characteristic size of the cloud. Furthermore, σ is always comparable to the mean Alfvén velocity of the cloud, consistent with observational results.     

Variability of extragalactic objects in relation to redshift, color, radio spectral index and absorption lines

Optical variability of extragalactic objects, viz., QSOs, BL Lacs and Seyfert galaxies has been monitored systematically over an appreciable period of time and a large amount of data have accumulated. The present work reports results of investigations involving statistical analysis of updated data on relationships between variability and various observed properties of the objects, viz., redshift, color indices, radio spectral index and absorption lines. It is found that at high frequencies (rest frame) radio spectral index does not change significantly with the degree of variability. However, the degree of variability depends on redshifts. On the other hand, presence or absence of absorption lines is significantly associated with variability for QSOs with larger redshifts (z > 1.0), while no such relationship exists for QSOs at smaller redshifts (z < 1.0) or other objects. Correlation between color indices and redshifts depends on the degree of variability and the sample chosen for the color index.     

Some trends in the red-shift distribution of quasi-stellar objects and related peculiar galaxies

Gaps in the red-shift distribution of quasi-stellar objects and related peculiar galaxies have been studied using 205 sources. The result indicates certain definite trends in the distribution of the gaps but does not suggest any periodicity when the entire sample is considered.     

An index of impulsiveness for 2800 MHz impulsive solar noise bursts

The initial growth of intensity, I, of the impulsive solar noise burst observed at 2800 MHz has been fitted with a parabolic curve of the form I=ct ²and the quantity c taken as the index of impulsiveness. Two groups of bursts comprising 85% of all impulsive bursts observed in 1962–63 and 1966 were selected for study. A good fit has been obtained for bursts having peak flux density up to 20 flux units, while for more intense bursts, the average observed growth is more rapid than the parabolic rate. The distribution of the index in the range 0.1 to more than 100 shows two peaks, one for c values 1–10 and another apparent peak for those with c greater than 100. The index is independent of the peak intensity of the burst and its position on the solar disc, while there is a small trend, indicating that shorter bursts are more impulsive than longer duration events. The more easily derived linear rate of rise, b = Peak Flux/Interval from start to peak is related to the parabolic impulsive index by b = 1.86 c ^0.57. The non-linear rate of expansion of a flaring volume suggested by Pneuman when applied to explain the parabolic rise of microwave bursts indicates that the impulsiveness of bursts is inversely related to the contained magnetic field.     

Solar Cycle Variations of Large-Scale Flows in the sun

Using data from the Michelson Doppler Imager (MDI) instrument on board the Solar and Heliospheric Observatory (SOHO), we study the large-scale velocity fields in the outer part of the solar convection zone using the ring diagram technique. We use observations from four different times to study possible temporal variations in flow velocity. We find definite changes in both the zonal and meridional components of the flows. The amplitude of the zonal flow appears to increase with solar activity and the flow pattern also shifts towards lower latitude with time.