The Importance of Distinguishing Pseudoresonances and Outcrop Resonances in Downhole Array Data

2019 ◽  
Vol 110 (1) ◽  
pp. 288-294 ◽  
Author(s):  
Yumeng Tao ◽  
Ellen Rathje
Keyword(s):  
Transfer Functions ◽  
Site Response ◽  
Short Note ◽  
Spectral Ratio ◽  
Site Response Analysis ◽  
Response Analysis ◽  
Frequency Range ◽  
Wave Effect ◽  
Array Data ◽  
Downhole Array

ABSTRACT This short note examines the downgoing wave effect and the appearance of pseudoresonances in downhole array data. It is demonstrated that pseudoresonances, distinct from the resonances associated with outcrop conditions, occur for sites with a shallow velocity contrast (VC) or with little to no VC. An approach is outlined to distinguish pseudoresonances from outcrop resonances using the theoretical 1D transfer functions for within and outcrop boundary conditions, as well as the horizontal-to-vertical spectral ratio. This approach is applied to hypothetical shear-wave velocity profiles, as well as three downhole array sites. We establish the importance of distinguishing pseudoresonances from outcrop resonances when using downhole array data to evaluate the accuracy of the 1D site response. For the example downhole array sites shown, the pseudoresonances are not captured well by 1D analysis, whereas the outcrop resonances are captured well. We propose that when evaluating the accuracy of 1D site-response analysis using downhole array data, the comparisons of the empirical and theoretical responses only consider the frequency range associated with outcrop resonances.

2D Geotechnical site-response analysis including soil heterogeneity and wave scattering

2021 ◽  
pp. 875529302110566
Author(s):  
Christopher A de la Torre ◽  
Brendon A Bradley ◽  
Christopher R McGann
Keyword(s):  
Ground Motion ◽  
High Frequency ◽  
Wave Scattering ◽  
Transfer Functions ◽  
Site Response ◽  
Soil Heterogeneity ◽  
Site Response Analysis ◽  
Response Analysis ◽  
Intensity Measures ◽  
Scattering Attenuation

This study describes an approach for modeling wave scattering and the spatial variability of ground motion in geotechnical site-response analysis by modeling soil heterogeneity through 2D correlated random fields. Importantly, the required site-specific inputs to apply the proposed approach in a practical setting are the same as those associated with conventional 1D site-response analysis. The results, which are affected by wave scattering attenuation, are compared to those from conventional laterally homogeneous 1D site-response analyses and 1D analyses with randomized velocity profiles extracted from heterogeneous 2D velocity model realizations. A sensitivity study, involving 5400 2D model realizations, investigates the influence of random field input parameters on wave scattering and site response. The computed ground surface acceleration waveforms and transfer functions show that this method is capable of scattering seismic waves. Multiple ground-motion intensity measures are analyzed to quantify this influence and distinguish between the effects of 1D vertical heterogeneities and averaging across many nodes and realizations, from the effects of wave scattering and 2D ground-motion phenomena. The redistribution of ground-motion energy across wider frequency bands and scattering attenuation of high-frequency waves in the 2D analyses resemble features observed in empirical transfer functions computed in other studies. While analyses with 1D randomized velocity profiles are able to replicate median results from 2D analyses for some low-frequency intensity measures (e.g. transfer functions at [Formula: see text] Hz, and spectral acceleration at the fundamental period), medians and standard deviations of high-frequency intensity measures (e.g. transfer function at [Formula: see text] Hz, [Formula: see text], and Arias intensity), which are influenced by wave scattering, are not appropriately captured. Given the equivalent input information requirements as conventional 1D analysis, and the availability of large computational resources, we advocate that the proposed 2D (and eventually 3D) approach is a fruitful path forward to improve the modeling of site-response physics and realize improved predictive capabilities.

scholarly journals Assessment of seismic vulnerability index of RAJUK area in Bangladesh using microtremor observations

2021 ◽  
Vol 44 (2) ◽  
pp. 1-13
Author(s):  
Abdul L. Helaly ◽  
Mehedi A. Ansary
Keyword(s):  
Resonance Frequency ◽  
Seismic Vulnerability ◽  
Site Response ◽  
Spectral Ratio ◽  
Vulnerability Index ◽  
Peak Amplitude ◽  
Site Response Analysis ◽  
Response Analysis ◽  
Amplitude Parameter ◽  
Significant Damage

Microtremor Horizontal to Vertical spectral ratio technique, also known as the Nakamura’s method is growing in status for site response analysis. 500 locations in RAJUK area (1530 km2) have been selected for microtremor observations. Microtremor data have been compiled and studied to estimate the predominant resonance frequency and H/V peak amplitude following the SESAME (2004) guideline. Finally, seismic vulnerability index of site soil using Nakamura’s technique has been determined from predominant resonance frequency and H/V peak amplitude parameter. The calculated seismic vulnerability index for the studied 500 locations varies between 0.16 and 7.28. The low seismic vulnerability index (Kg) value means that the areas are relatively stiff and underlain by substantial deposit of sediments. The relatively higher Kg values are spread in the soft alluvial deposit areas. The areas with high Kg values are considered as fragile zones that may initiate significant damage to infrastructure situated in those areas during an earthquake.

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