Variation of Site Response at the UCSB Dense Array of Portable Accelerometers

1993 ◽  
Vol 9 (2) ◽  
pp. 289-302 ◽  
Author(s):  
Jamison H. Steidl
Keyword(s):  
Fourier Transform ◽  
Spectral Density ◽  
Horizontal Plane ◽  
Site Response ◽  
Alluvial Soil ◽  
Spectral Ratio ◽  
Complex Signal ◽  
Cross Spectrum ◽  
Rock Site ◽  
The Cross

Ten Kinemetrics model SSA-2 Solid State Accelerographs were deployed in two dense arrays following the Landers-Big Bear earthquake sequence. The two arrays were separated by approximately three km, the first at a shallow alluvial soil site and the second at a rock site. We examine the soil and rock sites in terms of spectral ratio and cross-spectrum estimates of the site response. In order to construct an accurate representation of the motion in the horizontal plane, we treat the two horizontal components simultaneously as a complex signal. The Fourier transform of this complex signal represents the true motion in the horizontal plane as expressed in the frequency domain. The spectral ratio estimate is the ratio of this Fourier transform at the soil sites to the rock sites. The cross-spectrum estimate is the ratio of the cross-spectral density between the soil and rock sites to the power spectral density of the rock site. Spectral ratio estimates of site amplification are consistently higher than cross-spectrum estimates. On average the soil sites show amplification factors on the order of 2 to 4 relative to the rock sites between the frequencies of 4 to 15 Hz. There are, however, large variations in the ground motion recorded at sites with separations as small as 80 m. These variations demonstrate that site response studies can be biased by the choice of location of the sensor at distances of 80 m. We conclude that in the analysis of site-specific amplification both cross-spectrum and spectral ratio techniques should be used along with ensemble averages over many events.

Earthquake site response estimation: A weak-motion case study

1992 ◽  
Vol 82 (6) ◽  
pp. 2283-2307
Author(s):  
E. H. Field ◽  
K. H. Jacob ◽  
S. E. Hough
Keyword(s):  
Site Response ◽  
Spectral Ratio ◽  
Large Degree ◽  
One Dimensional ◽  
Loma Prieta Earthquake ◽  
Cross Spectrum ◽  
Downward Bias ◽  
Dimensional Models ◽  
The Cross ◽  
Input Motion

Abstract Using weak-motion recordings of aftershocks of the 1989 Loma Prieta earthquake recorded in Oakland, California, near the failed Nimitz Freeway, two methods have been applied to estimate the site response of an alluvium site and three mud-over-alluvium sites. The first estimate is the traditional spectral ratio, and the second utilizes the cross spectrum. Recordings obtained at a nearby bedrock site are used as estimates of the sediment site input motions. While the two site response estimates produce similar peaks and troughs, there is an approximate factor of 2 difference in amplitudes. This discrepancy is evidence that there is a much greater level of noise than would be expected from the pre-event ambient noise. We interpret this as signal-generated noise produced by scattering from heterogeneities, which causes the true sediment site input to differ significantly from the bedrock site recording. Given this level of noise, the cross-spectrum estimate suffers a severe downward bias (by a factor of 2 in this study) and should probably not be used when the input motion is estimated from a bedrock site recording. The spectral-ratio estimates are relatively unbiased, but the level of noise introduces a large degree of uncertainty. Therefore, inferences about site response from individual spectral ratios should probably be avoided. On the other hand, ensemble averages of the estimates significantly reduce the scatter to reveal resonances that agree quite well in frequency and overall shape with those of one-dimensional models whose parameters were determined independently. A discrepancy of higher observed amplitudes than predicted by theory remains unexplained but most likely results from the effects of boundary layer topography, which are not accounted for by the simple one-dimensional models.

scholarly journals The Time-Robustness Analysis of Individual Identification Based on Resting-State EEG

2021 ◽  
Vol 15 ◽  
Author(s):  
Yang Di ◽  
Xingwei An ◽  
Wenxiao Zhong ◽  
Shuang Liu ◽  
Dong Ming
Keyword(s):  
Spectral Density ◽  
Power Spectral Density ◽  
Resting State ◽  
Individual Identification ◽  
Frequency Range ◽  
Cross Spectrum ◽  
Power Spectral ◽  
Phase Lags ◽  
Individual Features

An ongoing interest towards identification based on biosignals, such as electroencephalogram (EEG), magnetic resonance imaging (MRI), is growing in the past decades. Previous studies indicated that the inherent information about brain activity may be used to identify individual during resting-state of eyes open (REO) and eyes closed (REC). Electroencephalographic (EEG) records the data from the scalp, and it is believed that the noisy EEG signals can influence the accuracies of one experiment causing unreliable results. Therefore, the stability and time-robustness of inter-individual features can be investigated for the purpose of individual identification. In this work, we conducted three experiments with the time interval of at least 2 weeks, and used different types of measures (Power Spectral Density, Cross Spectrum, Channel Coherence and Phase Lags) to extract the individual features. The Pearson Correlation Coefficient (PCC) is calculated to measure the level of linear correlation for intra-individual, and Support Vector Machine (SVM) is used to obtain the related classification accuracy. Results show that the classification accuracies of four features were 85–100% for intra-experiment dataset, and were 80–100% for fusion experiments dataset. For inter-experiments classification of REO features, the optimized frequency range is 13–40 Hz for three features, Power Spectral Density, Channel Coherence and Cross Spectrum. For inter-experiments classification of REC, the optimized frequency range is 8–40 Hz for three features, Power Spectral Density, Channel Coherence and Cross Spectrum. The classification results of Phase Lags are much lower than the other three features. These results show the time-robustness of EEG, which can further use for individual identification system.

scholarly journals Centrifuge Model Tests of the Effect of the Ground Motion Coherence Function on the Amplification of a Saturated Soil–Water Site

Geofluids ◽  
2021 ◽  
Vol 2021 ◽  
pp. 1-10
Author(s):  
Xijun Song ◽  
Juan Liu ◽  
Jingyan Lan ◽  
Ting Wang
Keyword(s):  
Soil Water ◽  
Site Response ◽  
Coherence Function ◽  
Spectral Ratio ◽  
Saturated Soil ◽  
Order Mode ◽  
Overlying Water ◽  
First Order ◽  
Site Model ◽  
Frequency Phase

Two sets of dynamic centrifugal model tests were designed and implemented in this study: the overlying waterless surface and the water-covered surface. Based on the use of the El Centro waves with different intensities as the base input, the seismic time history at the surface of two sets of free site models was obtained. According to the results of the site response at two sets of the free site surface obtained with a traditional spectral ratio, the coherence functions at the surface and the base were used to modify the traditional spectral ratio for analysis and to evaluate the effect of the ground motion coherence function for site amplification. The modal characteristics and the amplification effect of a typical saturated soil water free site were summarized at the same time. The results showed that the ground response results of the two groups of typical free site centrifugal models were greatly influenced by the coherence function. In the low frequency phase, the coherence function of the amplification spectrum of the site response decreased significantly, while in the high frequency phase, the decrease trend decreased. The coherence function had a significant effect on the first-order mode of the free site. The first-order mode frequency and the amplification factor of a typical free site could be identified effectively. Compared with the saturated land free site model, the saturated soil water free site model had higher-order modes due to the overlying water. It was shown that the overlying water, as part of a complex medium system, could be ignored in the site response and basic cycle estimation.

scholarly journals APPLICATION OF WEIGHTING WINDOWS FOR SPECTRAL DENSITY ESTIMATION

2021 ◽  
Author(s):  
Vera Meerson ◽  
O. Khorolsky
Keyword(s):  
Fourier Transform ◽  
Correlation Function ◽  
Spectral Density ◽  
Density Estimation ◽  
Time Windows ◽  
Spectral Density Estimation ◽  
Important Time

The article discusses the application of some of the most important time windows for spectral density estimation determined by the correlogram method (from correlation function) and the periodogram method (from direct fourier transform).

Feasibility of the use of Microtremors in Estimating Site Response during Earthquakes: Some Test Cases in Italy

1991 ◽  
Vol 7 (4) ◽  
pp. 551-561 ◽  
Author(s):  
Antonio Rovelli ◽  
Shri K. Singh ◽  
Luca Malagnini ◽  
Alessandro Amato ◽  
Massimo Cocco
Keyword(s):  
Ground Motion ◽  
Seismic Waves ◽  
Site Response ◽  
Spectral Ratio ◽  
Test Cases ◽  
Spectral Ratios ◽  
Cultural Noise ◽  
Maximum Amplification

We explore the feasibility of the use of microtremors in estimating the amplification of seismic waves at soft sites in Italy. Microtremors were measured at three soft sites and nearby hard sites at night when the cultural noise was minimum. These soft sites were selected as those showing the largest amplifications of ground motion during earthquakes as compared to the records on the hard sites or with respect to the predicted spectra. We compare the soft-to-hard site microtremor spectral ratios with the corresponding acceleration spectral ratios. A rough estimate of the shape and level of spectral amplification is obtained from the microtremor data in all three cases. However, the details of the soft-to-hard site spectral ratio are not reproduced and some differences appear in (a) the frequency at which the maximum amplification occurs, and (b) the bandwidth of the significant amplification. More testing of the method is needed before its wider use for microzonation in Italy can be recommended.

Hard as a rock? Looking for typical and atypical reference sites in the Greek network

2021 ◽  
Author(s):  
Olga-Joan Ktenidou ◽  
Faidra Gkika ◽  
Erion-Vasilis Pikoulis ◽  
Christos Evangelidis
Keyword(s):  
Ground Motion ◽  
Site Response ◽  
Time Budget ◽  
Strong Motion ◽  
General Rule ◽  
Spectral Ratio ◽  
Reference Sites ◽  
Motion Models ◽  
Ground Motion Variability

<p>Although it is nowadays desirable and even typical to characterise site conditions in detail at modern recording stations, this is not yet a general rule in Greece, due to the large number and geographical dispersion of stations. Indeed, most of them are still characterised merely through geological descriptions or proxy-based parameters, rather than through in-situ measurements. Considering: 1. the progress made in recent years with sophisticated ground motion models and the need to define region-specific rock conditions based on data, 2. the move towards large open-access strong-motion databases that require detailed site metadata, and 3. that Greek-provenance recordings represent a significant portion of European seismic data, there are many reasons to improve our understanding of site response at these stations. Moreover, it has been shown recently in several regions that even sites considered as rock can exhibit amplification and ground motion variability, which has given rise to more scientific research into the definition of reference sites. For Greece, in-situ-characterisation campaigns for the entire network would impose unattainable time/budget constraints; so, instead, we implement alternative empirical approaches using the recordings themselves, such as the horizontal-to-vertical spectral ratio technique and its variability. We present examples of 'well-behaved', typical rock sites, and others whose response diverges from what is assumed for their class.</p><p> </p>

Design Wind Speeds Using Fast Fourier Transform

2011 ◽  
pp. 1576-1591
Author(s):  
Z.. Ismail ◽  
N. H. Ramli ◽  
Z.. Ibrahim ◽  
T. A. Majid ◽  
G. Sundaraj ◽  
...  
Keyword(s):  
Time Series ◽  
Fourier Transform ◽  
Spectral Density ◽  
Fast Fourier Transform ◽  
Time Series Data ◽  
Absolute Magnitude ◽  
Stationary Pattern ◽  
Wind Speeds ◽  
Hidden Periodicities ◽  
Using Data

In this chapter, a study on the effects of transforming wind speed data, from a time series domain into a frequency domain via Fast Fourier Transform (FFT), is presented. The wind data is first transformed into a stationary pattern from a non-stationary pattern of time series data using statistical software. This set of time series is then transformed using FFT for the main purpose of the chapter. The analysis is done through MATLAB software, which provides a very useful function in FFT algorithm. Parameters of engineering significance such as hidden periodicities, frequency components, absolute magnitude and phase of the transformed data, power spectral density and cross spectral density can be obtained. Results obtained using data from case studies involving thirty-one weather stations in Malaysia show great potential for application in verifying the current criteria used for design practices.

Turbulent Wall-Pressure Fluctuations: A New Model for Off-Axis Cross-Spectral Density

1997 ◽  
Vol 119 (2) ◽  
pp. 277-280 ◽  
Author(s):  
B. A. Singer
Keyword(s):  
Boundary Layer ◽  
Turbulent Boundary Layer ◽  
Spectral Density ◽  
Flow Direction ◽  
Wall Pressure ◽  
Mean Flow ◽  
New Approach ◽  
Cross Spectral Density ◽  
The Mean ◽  
The Cross

Models for the distribution of the wall-pressure under a turbulent boundary layer often estimate the coherence of the cross-spectral density in terms of a product of two coherence functions. One such function describes the coherence as a function of separation distance in the mean-flow direction, the other function describes the coherence in the cross-stream direction. Analysis of data from a large-eddy simulation of a turbulent boundary layer reveals that this approximation dramatically underpredicts the coherence for separation directions that are neither aligned with nor perpendicular to the mean-flow direction. These models fail even when the coherence functions in the directions parallel and perpendicular to the mean flow are known exactly. A new approach for combining the parallel and perpendicular coherence functions is presented. The new approach results in vastly improved approximations for the coherence.

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