Frequency Spectral Amplitude Research Articles

Statistical Spectral Characteristics of Three‐Dimensional Winds in the Mesopause Region Revealed by the Andes Lidar

AbstractBy analyzing data recorded at the Andes Lidar Observatory in Cerro Pachon, Chile (30.3°S, 70.7°W) from May 2014 to July 2019, we investigated the fundamental features of three‐dimensional wind and temperature spectra. The vertical wavenumber spectral amplitudes of horizontal winds show obvious seasonal variations that are closely related to the seasonal variations in the source and background winds. The wavenumber spectral slopes of the horizontal winds are systematically less negative than −3, with mean values of −1.96 and −2.18 for zonal and meridional winds, respectively. The zonal and meridional wind frequency spectra have mean slopes of −1.37 and −1.56, respectively; these values are slightly less negative than −5/3. Moreover, the frequency spectral amplitudes show different seasonal variations from those of the wavenumber spectra, possibly because they correspond to different GW spectral components. The vertical wind has obviously different spectral features than the horizontal winds. The vertical wind spectra are notably shallower than the horizontal wind spectra, with mean slopes of −0.82 and −0.91 for the wavenumber and frequency spectra, respectively, departing evidently from those expected under linear instability theory (LIT). Although the vertical wind spectrum is almost always separable, the horizontal wind spectra are separable only at high frequencies. As the frequency increased, the horizontal wind wavenumber spectra become shallower and depart from the spectral slope expected under LIT, likely because high‐frequency GWs are not completely saturated. In general, our results do not support LIT.

Determination of spectral decay parameter and Quality factor in Kermanshah region, NW Iran

Among important parameters in simulation of earthquake data in high frequencies are the high frequency spectral amplitude decay and the Quality factor. Amplitude spectral decay is determined by the Kappa parameter (K) and the Quality factor (Q) which is usually expressed by a power relation of frequency (f) as Q = Q0 f n, where Q0 is Q at 1 Hz. The 2017 Sarpol-e-Zahab earthquake with magnitude Mw = 7.3 in Kermanshah province near the Iran-Iraq border caused extensive destruction and heavy human loss. Thus, the study of different aspects of this event is of high importance. In this paper an attempt is made to partly explain the attenuation properties of this region in Zagros suture zone by determining the Kappa and the Quality factors in this region. In this study, accelerograph records of aftershocks of the above-mentioned earthquake have been analysed. The best linear fit for the Kappa, based on the distance (R) in km, is estimated as: K = 0.0005 R + 0.034 for the horizontal component, which exhibits increase with increasing epicentral distance. The correlation of the Quality factor was also found as Q = 88.6 f 0.8, which is in accordance with an active tectonic region.

Reinforcement effect of polypropylene fiber on dynamic properties of cemented tailings backfill under SHPB impact loading

Dynamic loads such as blasting may lead to diverse levels of failure in cemented tailings backfill (CTB) under diverse strain rates and frequency spectral amplitudes. Hence, changes in dynamic properties of filling due to these loads should be studied as a function of strain rates for CTB mixtures. In this study, the dynamic characteristics of CTB samples containing polypropylene fiber contents were examined with the use of split Hopkinson pressure bar (SHPB) system and high speed photography technology. The experimental results illustrate that the backfill has a damping influence on the elastic wave. The original waveform noise of CTB with fiber is higher than the one without fiber, and the average strain rate changes the reflected waveform. The effect is greater than the impact velocity. Secondly, the dynamic strength of CTB with fiber is higher than the static strength, which obeys a great effect of the strain rate. Meanwhile, the backfill (with fiber) stress-strain characteristics show a ‘double peak’ phenomenon, and the difference between the first and the second peak stresses decreases with increasing fiber content. The failure mode analysis demonstrated that the fiber reinforcement stopped the development of big cracks during the tensile fiber pull-out failure regime. Lastly, the findings of the present study can offer an important reference to the overall design of backfilling regarding the understanding of the good dynamic characteristics and the reduction of operational costs.

Hypersynchrony in MEG spectral amplitude in prospectively-identified 6-month-old infants prenatally exposed to alcohol.

Early identification of children who experience developmental delays due to prenatal alcohol exposure (PAE) remains a challenge for individuals who do not exhibit facial dysmorphia. It is well-established that children with PAE may still exhibit the cognitive and behavioral difficulties, and individuals without facial dysmorphia make up the majority of individuals affected by PAE. This study employed a prospective cohort design to capture alcohol consumption patterns during pregnancy and then followed the infants to 6 months of age. Infants were assessed using magnetoencephalography to capture neurophysiological indicators of brain development and the Bayley Scales of Infant Development-III to measure behavioral development. To account for socioeconomic and family environmental factors, we employed a two-by-two design with pregnant women who were or were not using opioid maintenance therapy (OMT) and did or did not consume alcohol during pregnancy. Based on prior studies, we hypothesized that infants with PAE would exhibit broad increased spectral amplitude relative to non-PAE infants. We also hypothesized that the developmental shift from low to high frequency spectral amplitude would be delayed in infants with PAE relative to controls. Our results demonstrated broadband increased spectral amplitude, interpreted as hypersynchrony, in PAE infants with no significant interaction with OMT. Unlike prior EEG studies in neonates, our results indicate that this hypersynchrony was highly lateralized to left hemisphere and primarily focused in temporal/lateral frontal regions. Furthermore, there was a significant positive correlation between estimated number of drinks consumed during pregnancy and spectral amplitude revealing a dose-response effect of increased hypersynchrony corresponding to greater alcohol consumption. Contrary to our second hypothesis, we did not see a significant group difference in the contribution of low frequency to high frequency amplitude at 6 months of age. These results provide new evidence that hypersynchrony, previously observed in neonates prenatally exposed to high levels of alcohol, persists until 6 months of age and this measure is detectable with low to moderate exposure of alcohol with a dose-response effect. These results indicate that hypersynchrony may provide a sensitive early marker of prenatal alcohol exposure in infants up to 6 months of age.

Circadian variation of human ventricular fibrillation dominant frequency

Aim Circadian variation in human ventricular fibrillation (VF) dominant frequency is unknown. If present this would provide evidence of physiological influence on VF. The objective was to quantify the circadian variation in human VF dominant frequency. Methods Eight-lead Holter ECG recordings were obtained from a patient with severe myocarditis and chronic VF who was supported by a biventricular assist device. Recordings of up to 24 h duration were obtained on 6 days with an average interval between recordings of 7 days. Dominant frequency and amplitude were obtained using spectral analysis and assessed for (i) circadian (ii) inter-recording and (iii) inter-lead differences. Results There was a significant circadian variation in amplitude (night: 0.027 ± 0.004 mV Hz vs day: 0.044 ± 0.006 mV Hz, p < 0.0001) but not dominant frequency (night: 7.85 ± 0.62 Hz vs day: 7.93 ± 0.54 Hz, p > 0.05). There were significant differences between recordings in dominant frequency which ranged from 6.80 ± 0.29 Hz to 8.36 ± 0.38 Hz ( p < 0.0001) and dominant frequency spectral amplitude which ranged from 0.033 ± 0.014 mV Hz to 0.043 ± 0.017 mV Hz ( p < 0.0001). Histograms of dominant frequencies in leads exhibited strikingly different distributions, particularly in V2 that was characterised by a bimodal distribution, while the other leads were characterised by predominantly unimodal distributions. Conclusion VF dominant frequency spectral amplitude exhibited circadian variability. In a patient with severe myocarditis, supported with a biventricular assist device and in chronic VF, these results provide evidence for modulation of VF, probably induced by changes in posture and physical activity.

Attenuation of Ground-Motion Spectral Amplitudes in Southeastern Australia

A dataset comprising some 1200 weak- and strong-motion records from 84 earthquakes is compiled to develop a regional ground-motion model for southeastern Australia (sea). Events were recorded from 1993 to 2004 and range in size from moment magnitude 2.0 ≤ M ≤ 4.7. The decay of vertical-component Fourier spectral amplitudes is modeled by trilinear geometrical spreading. The decay of low- frequency spectral amplitudes can be approximated by the coefficient of R −1.3 (where R is hypocentral distance) within 90 km of the seismic source. From approximately 90 to 160 km, we observe a transition zone in which the seismic coda are affected by postcritical reflections from midcrustal and Moho discontinuities. In this hypocentral distance range, geometrical spreading is approximately R +0.1 . Beyond 160 km, low-frequency seismic energy attenuates rapidly with source–receiver distance, having a geometrical spreading coefficient of R −1.6 . The associated regional seismic-quality factor can be expressed by the polynomial: log Q ( f ) = 3.66 − 1.44 log f + 0.768 (log f ) 2 + 0.058 (log f ) 3 for frequencies 0.78 ≤ f ≤ 19.9 Hz. Fourier spectral amplitudes, corrected for geometrical spreading and anelastic attenuation, are regressed with M to obtain quadratic source scaling coefficients. Modeled vertical-component displacement spectra fit the observed data well. Amplitude residuals are, on average, relatively small and do not vary with hypocentral distance. Predicted source spectra (i.e., at R = 1 km) are consistent with eastern North American (ena) models at low frequencies ( f less than approximately 2 Hz) indicating that moment magnitudes calculated for sea earthquakes are consistent with moment magnitude scales used in ena over the observed magnitude range. The models presented represent the first spectral ground-motion prediction equations developed for the southeastern Australian region. This work provides a useful framework for the development of regional ground-motion relations for earthquake hazard and risk assessment in sea.

Sealing law of far-field spectra based on observed parameters of the specific barrier model

A set of acceleration source spectra is constructed using the observed parameters of the specific barrier model of Papageorgiou and Aki. The spectra show a significant departure from the ‘ω2-model’ at the high frequency range. Specifically, the high frequency spectral amplitudes of seismic excitation are higher as compared to the level predicted by the ‘ω2-model’. This is also supported by other observational evidence. The high frequency amplitudes of acceleration scale proportionally to the square root of the rupture areaS, to the rupture spreading velocityv, and to the local strain drop (Δσ/μ) (=strain drop in between barriers). The local strain drop in between barriers is not related in a simple way to the global strain drop, which is the strain drop estimated by assuming that it is uniform over the entire rupture area. Consequently, the similarity law does not apply. Using the source spectra which we constructed, we derive expressions for high frequency amplitudes of acceleration such asa rms anda max. Close to the fault both are independent of fault dimensions and scale as (Δσ/µ)(Δf)1/2, while away from the fault plane they scale asW 1/2(Δσ/µ)(Δf)1/2, whereW is the width of the fault and Δf is the effective bandwidth of the spectra.

Preliminary empirical model for scaling fourier amplitude spectra of strong ground acceleration in terms of modified mercalli intensity and recording site conditions

AbstractThis paper presents an empirical model for scaling Fourier amplitude spectra of ground acceleration during strong earthquake shaking in terms of the reported Modified Mercalli Intensity (MMI) and the simplified characteristics of the geologic environment at the recording station. This analysis shows that (i) for the intermediate and high‐frequency motions the spectral amplitudes approximately double for every level of the MMI; that (ii) the uncertainties associated with estimation of Fourier spectral amplitudes in terms of MMI are not greater than the uncertainties associated with similar estimation in terms of earthquake magnitude and epicentral distance; that (iii) the high frequency spectral amplitudes tend to be greater on basement rock sites relative to alluvium sites, with this trend being reversed for the low‐frequency spectral amplitudes; and that (iv) the spectral amplitudes of very high‐frequency vertical shaking are equal to or higher than the corresponding spectral amplitudes for horizontal shaking.

A simple model for estimating fourier amplitude spectra of horizontal ground acceleration

abstract A simple model for estimating Fourier amplitude spectra ( FS ) is calibrated using the horizontal components of 70 strong-motion records from California, chosen so that the results are not biased by the effects of one earthquake nor by the effects of a single site. An exponential dependence of FS on magnitude M , a geometric dependence of FS on source-to-site distance R , and a soil amplification term Y s are included in the model, using the form F S ( T ) = exp ⁡ ( b 1 + b 2 M + b 4 Y s ) R b 3 where the coefficients b 1 , b 2 , b 3 , and b 4 are determined for each period T of interest. The trends indicate that: (1) larger magnitudes imply relatively more energy content at long periods; (2) source-to-site distance has an important effect on spectral shape, because high frequency spectral amplitudes attenuate faster with distance than do long-period spectral amplitudes; and (3) alluviumsite conditions amplify long-period (>1 sec) amplitudes over those recorded at rock sites by a factor of about 1.6, but decrease short-period (≃0.1 sec) amplitudes. Values of FS calculated from strong-motion records are lognormally distributed about the values predicted from this model. The accuracy of the model, as measured by the dispersion in observed values, is as good as the accuracy implied by the model proposed by Trifunac (1976). Moreover, the model used here reflects a realistic change in frequency content with distance, which the Trifunac model does not; hence, extrapolation of this model to estimate spectral amplitudes for large magnitudes at close distances is more reliable than is extrapolation of the Trifunac model. The model estimates approximately the horizontal spectral amplitudes of the records obtained at Pacoima Dam in 1971 and El Centro in 1940, although these records were not used in the calibration procedure and hence the estimation of their spectral amplitudes constitutes an independent prediction. Investigation of possible nonlinear dependence of In FS on M , using a form similar to that proposed by Trifunac F S ( T ) = exp ⁡ ( b 1 + b 2 M + b 5 M 2 + b 4 Y s ) R b 3 reveals that the M 2 term does not appreciably improve the accuracy of the model, nor does it greatly change values of predicted spectra for the magnitude range of available data (5 to 7.7). Significance of the M 2 term depends solely on records obtained during the 1952 Kern County earthquake; thus, use of models of this type, calibrated with available California strong-motion records, implies the assumption that the Kern County records are typical of a magnitude 7.7 shock.