Seismic response analysis of NAGRA-Net stations using advanced geophysical techniques (original) (raw)
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Earthquake Source and Local Geology Effects on the Seismic Site Response
NATO Science for Peace and Security Series C: Environmental Security, 2009
Strong shallow and intermediate-depth scenario earthquakes for two major cites in Bulgaria are discussed. The contribution of the earthquake source and the local site geology to the seismic input is illustrated. Due to the lack of strong motion records a neo-deterministic seismic hazard assessment procedure is used to generate synthetic seismic signals. After some parametric analyses the computed signals are validated against the few available data. Prognostic estimates of the dynamic coefficient for the target sites are performed with respect to the defined scenario earthquakes and local site models, corresponding to the Eurocode 8 ground types A, B and C. The obtained results show that: (1) the seismic source influence on the seismic input at a given site is comparable with that of the local site geology; (2) the dynamic coefficients, computed for accelerograms (observed and computed) due to strong intermediate-depth Vrancea earthquakes overestimate significantly the values recommended by the Eurocode 8 (EC8) for periods T > 1s.
Within-site variability in earthquake site response
Geophysical Journal International, 2021
The within-site variability in site response is the randomness in site response at a given site from different earthquakes and is treated as aleatory variability in current seismic hazard/risk analyses. In this study, we investigate the single-station variability in linear site response at K-NET and KiK-net stations in Japan using a large number of earthquake recordings. We found that the standard deviation of the horizontal-to-vertical Fourier spectral ratio at individual sites, that is single-station horizontal-to-vertical spectral ratio (HVSR) sigma σ HV,s , approximates the within-site variability in site response quantified using surface-to-borehole spectral ratios (for oscillator frequencies higher than the site fundamental frequency) or empirical groundmotion models. Based on this finding, we then utilize the single-station HVSR sigma as a convenient tool to study the site-response variability at 697 KiK-net and 1169 K-NET sites. Our results show that at certain frequencies, stiff, rough and shallow sites, as well as small and local events tend to have a higher σ HV,s. However, when being averaged over different sites, the single-station HVSR sigma, that is σ HV , increases gradually with decreasing frequency. In the frequency range of 0.25-25 Hz, σ HV is centred at 0.23-0.43 in ln scales (a linear scale factor of 1.26-1.54) with one standard deviation of less than 0.1. σ HV is quite stable across different tectonic regions, and we present a constant, as well as earthquake magnitude-and distance-dependent σ HV models.
Statistical estimation of earthquake site response from noise recordings
Standard spectral ratio from earthquake recordings (SSR) is considered the reference empirical method for assessing site effects as a function of frequency. However, other estimates can be easily obtained from noise measurements (i.e., Horizontal-to-Vertical Spectral Ratio, HVN), even though their reliability in terms of amplitude is controversial. In the framework of the ToK ITSAK-GR (2006-2010) EC project, Cultrera et al. (2010) analyzed recordings from 64 sites worldwide, founding that it is possible to have linear combinations of the HVN amplitudes significantly correlated to linear combinations of the SSR. In the present paper we show how to estimate the SSR spectral ratios when only noise measurements are available, using the results of the canonical correlation analysis between SSR and HVN recorded at several sites. The SSR evaluation has been tested by a cross validation procedure: the expected SSR at each validation site are in turn estimated by a weighted average of the SSR values measured at the other sites; the weights are properly set to account more for the sites with similar behavior in terms of the canonical correlation between HVN and SSR. To evaluate the goodness of the estimation, we compared all the inferred and original SSR, and we performed a critical analysis on the spectral characteristics of earthquake site response that can be easily recovered from noise measurements.
Characterization of site conditions for selected seismic stations in eastern part of Romania
Acta Geophysica, 2018
Strong motion data are essential for seismic hazard assessment. To correctly understand and use this kind of data is necessary to have a good knowledge of local site conditions. Romania has one of the largest strong motion networks in Europe with 134 real-time stations. In this work, we aim to do a comprehensive site characterization for eight of these stations located in the eastern part of Romania. We make use of a various seismological dataset and we perform ambient noise and earthquake-based investigations to estimate the background noise level, the resonance frequencies and amplification of each site. We also derive the Vs 30 parameter from the surface shear-wave velocity profiles obtained through the inversion of the Rayleigh waves recorded in active seismic measurements. Our analyses indicate similar results for seven stations: high noise levels for frequencies larger than 1 Hz, well defined fundamental resonance at low frequencies (0.15-0.29 Hz), moderate amplification levels (up to 4 units) for frequencies between 0.15 and 5-7 Hz and same soil class (type C) according to the estimated Vs 30 and Eurocode 8. In contrast, the eighth station for which the soil class is evaluated of type B exhibits a very good noise level for a wide range of frequencies (0.01-20 Hz), a broader fundamental resonance at high frequencies (* 8 Hz) and a flat amplification curve between 0.1 and 3-4 Hz.
Geophysical Journal International, 2014
The recent growth of seismic monitoring networks allows for systematic studies of local seismic effects at sites with pronounced topography. We applied a terrain classification method to identify such sites within Swiss and Japanese networks and compiled a data set of highquality earthquake recordings. As a number of recent studies have found local effects to be directional at sites with strong topographic features, polarization analysis of particle motion was performed and azimuthally dependent resonant frequencies were estimated. The same procedure was also applied for available ambient vibration recordings. Moreover, average residuals with respect to ground motion prediction models for a reference bedrock were calculated to estimate the average amplification or deamplification for each station. On one hand, observed amplifications are found to be tightly linked with ground motion directionality as estimated by polarization analysis for both earthquake and ambient vibration recordings. On the other hand, we found no clear relation between local topographic features and observed amplification, so the local subsurface properties (i.e. shear wave velocity structure) seem to play the key role and not the geometry itself.
Assessing site response through ambient noise measurements in a seismic prone area
Physics & Astronomy International Journal, 2019
Ambient noise measurements are carried out at 70 sites in different parts of Shillong City, NER, and India. The minimum duration of noise recording is about 60 minutes. We estimate the H/V ratio from these recordings. The fundamental resonant frequency for Shillong City emerges out to be in the range of 3 to 8 Hz. Variation in the resonant frequencies suggests that heterogeneity is prominent in the soil layers of this region. Simultaneously, a non-linear earthquake site response analysis is also attempted using available geotechnical data. A good correlation is observed between site response analysis and HVSR results constrained by wide variation of resonant frequency at short distances in the H/V ratio which indicate a lateral heterogeneity prevailing in the soil layers of the region.
Seismic, geotechnical, and earthquake engineering site characterization
SEG Technical Program Expanded Abstracts 2006, 2006
We determined the seismic model of the soil column within a residential project site in Istanbul, Turkey. Specifically, we conducted refraction seismic survey at 20 locations and estimated the P-and S-wave velocity-depth profiles down to a depth of 30 m. We then combined the seismic velocities with the geotechnical borehole information regarding the lithology of the soil column and determined the site-specific geotechnical earnthquake engineering parameters for the site. Specifically, we computed the maximum soil amplification ratio, maximum surfacebedrock acceleration ratio, depth interval of significant acceleration, maximum soil-rock response ratio, and design spectrum periods TA-TB.
Within the NATO Science for Peace Project 981882 "Site-effect analyses for the earthquake-endangered metropolis Bucharest, Romania" we obtain a unique, homogeneous dataset of seismic, soil-mechanic and elasto-dynamic parameters. Ten 50 m deep boreholes are drilled in the metropolitan area of Bucharest in order to obtain cores for dynamic tests and vertical seismic profiles for an updated microzonation map related to earthquake wave amplification. The boreholes are placed near former or existing seismic station sites to allow a direct comparison and calibration of the borehole data with actual seismological measurements. A database is assembled which contains P-and S-wave velocity, density, geotechnical parameters measured at rock samples and geological characteristics for each sedimentary layer. All the V S-30 computed values belong to type C of soil after this classification (Romanian Code for the seismic design for buildings -P100-1/2006). Approximately 250 samples were gathered from the 10 drill sites. These samples were mostly not disturbed (samples as they were recovered from the tube of the drilling rig) and partly disturbed (those which had no proper consistency). Results obtained by the down-hole method in the 10 boreholes drilled in Bucharest City as well as from laboratory measurements will be used as input data in the program SHAKE2000 in order to obtain the seismic site effects due to the shallow soft layers.