scholarly journals SITE RESPONSE IN GUWAHATI REGION USING STANDARD SPECTRAL RATIO

2016 ◽  
Vol 05 (04) ◽  
pp. 77-81
Author(s):  
Sasanka Borah .
Keyword(s):  
Site Response ◽  
Spectral Ratio ◽  
Standard Spectral Ratio

A Comparison of Site Response Techniques Using Weak-Motion Earthquakes and Microtremors

2006 ◽  
Vol 22 (1) ◽  
pp. 169-188 ◽  
Author(s):  
Sheri Molnar ◽  
John F. Cassidy
Keyword(s):  
Site Response ◽  
Strong Motion ◽  
Spectral Ratio ◽  
Ratio Method ◽  
Near Surface ◽  
Fundamental Frequencies ◽  
Standard Spectral Ratio ◽  
Impedance Contrast ◽  
Spectral Ratio Method ◽  
Geologic Setting

The applicability of the microtremor spectral ratio method is examined by comparing microtremor and weak-motion earthquake site responses at seven permanent strong-motion sites in Victoria, British Columbia. For each site, a weak-motion earthquake standard spectral ratio (bedrock reference), the average horizontal-to-vertical spectral ratio of up to five weak-motion earthquakes, and the average microtremor (Nakamura method) spectral ratio are compared. The geologic setting of Victoria is ideal for site response studies with a near-surface high impedance contrast between thin geologic layers of Victoria clay (about 11 m maximum in this study) and Pleistocene till or bedrock. Regardless of excitation source (weak-motion earthquakes or microtremors) and spectral ratio method, similar peak amplitudes and fundamental frequencies were found. Thicker material (>10 m) sites displayed higher peak amplitudes (up to six times amplification) at frequencies of 2–5 Hz compared to sites with a thin lens of material (<3 m) over bedrock that showed peak amplitudes at frequencies of >8 Hz.

Methodology to identify the reference rock sites in regions of medium-to-high seismicity: an application in Central Italy

2020 ◽  
Vol 222 (3) ◽  
pp. 2053-2067 ◽  
Author(s):  
Giovanni Lanzano ◽  
Chiara Felicetta ◽  
Francesca Pacor ◽  
Daniele Spallarossa ◽  
Paola Traversa
Keyword(s):  
Ground Motion ◽  
Site Effects ◽  
Site Response ◽  
Central Italy ◽  
Large Data ◽  
Spectral Ratio ◽  
Data Set ◽  
Standard Spectral Ratio ◽  
Resonance Frequencies ◽  
The Impact

SUMMARY To evaluate the site response using both empirical approaches (e.g. standard spectral ratio, ground motion models (GMMs), generalized inversion techniques, etc.) and numerical 1-D/2-D analyses, the definition of the reference motion, that is the ground motion recorded at stations unaffected by site-effects due to topographic, stratigraphic or basin effects, is needed. The main objective of this work is to define a robust strategy to identify the seismic stations that can be considered as reference rock sites, using six proxies for the site response: three proxies are related to the analysis of geophysical and seismological data (the repeatable site term from the residual analysis, the resonance frequencies from horizontal-to-vertical spectral ratios on noise or earthquake signals, the average shear wave velocity in the first 30 m); the remaining ones concern geomorphological and installation features (outcropping rocks or stiff soils, flat topography and absence of interaction with structures). We introduce a weighting scheme to take into account the availability and the quality of the site information, as well as the fulfillment of the criterion associated to each proxy. We also introduce a hierarchical index, to take into account the relevance of the proposed proxies in the description of the site effects, and an acceptance threshold for reference rock sites identification. The procedure is applied on a very large data set, composed by accelerometric and velocimetric waveforms, recorded in Central Italy in the period 2008–2018. This data set is composed by more than 30 000 waveforms relative to 450 earthquakes in the magnitude range 3.2–6.5 and recorded by more than 450 stations. A total of 36 out of 133 candidate stations are identified as reference sites: the majority of them are installed on rock with flat topography, but this condition is not sufficient to guarantee the absence of amplifications, especially at high frequencies. Seismological analyses are necessary to exclude stations affected by resonances. We test the impact of using these sites by calibrating a GMMs. The results show that for reference rock sites the median predictions are reduced down to about 45 per cent at short periods in comparison to the generic rock motions.

scholarly journals Combining recordings of earthquake ground-motion and ambient vibration analysis to estimate site response variability in the city of Lucerne, Switzerland

2021 ◽  
Author(s):  
Paulina Janusz ◽  
Vincent Perron ◽  
Christoph Knellwolf ◽  
Walter Imperatori ◽  
Luis Fabian Bonilla ◽  
...  
Keyword(s):  
Seismic Hazard ◽  
Ground Motion ◽  
Urban Areas ◽  
Ambient Noise ◽  
Site Response ◽  
Spectral Ratio ◽  
Response Variability ◽  
Standard Spectral Ratio ◽  
Hazard And Risk ◽  
The City

&lt;p&gt;Estimation of site effects is an essential part of local seismic hazard and risk assessment, especially in densely populated urban areas. The goal of this study is to assess the site response variability in the city of Lucerne (Central Switzerland), located in a basin filled with unconsolidated deposits. Even though it is a low-to-moderate seismicity area, the long-term seismic risk cannot be neglected, in particular, because the region was struck by strong earthquakes in the past (i.e. Mw 5.9 in 1601).&lt;/p&gt;&lt;p&gt;To determine the spatial distribution of the soil response in the test area, we combined earthquake and ambient noise recordings using the Hybrid Standard Spectral Ratio method (SSRh) introduced by Perron et al. (2018). In the first step, we installed a temporary seismic network to record ground-motion from low-magnitude or distant earthquakes. At selected urban sites inside the sedimentary basin, the dataset was used to estimate the amplification factors with respect to a rock site using the Standard Spectral Ratio approach (SSR - Borcherdt, 1970). Then, a survey including several dozens of densely distributed single-station ambient noise measurements was performed which enabled us to estimate the basin response variability relative to the seismic stations of the temporary seismic network. Finally, we corrected the noise-based evaluation using the SSR amplification functions. To verify the useability of the presented technique in the Lucerne area, we applied the SSRh method also to the temporary stations, the resulting amplification functions largely coincide with the SSR curves. However, the daily variability of the noise wavefield due to human activities can slightly affect the results. We will also discuss the influence of the station distribution and density of the temporary network deployment.&lt;/p&gt;&lt;p&gt;The amplification model for the Lucerne area estimated using the SSRh method shows consistency with geological data. The results indicate that seismic waves can be amplified up to 10 times in some parts of the basin compared to the rock site. The highest amplification factors are observed for frequencies between 0.8 and 2Hz. This means a local significant increase in seismic hazard.&lt;/p&gt;&lt;p&gt;The presented work is a part of a detailed site response analysis study for the Lucerne area, considering 2D and 3D site effects and potential non-linear soil behaviour. This PhD project is performed in the framework of the Horizon 2020 ITN funded project URBASIS-EU, which focuses on seismic hazard and risk in urban areas.&lt;/p&gt;&lt;p&gt;REFERENCES&lt;/p&gt;&lt;p&gt;Borcherdt, R.D., 1970. Effects of local geology on ground motion near San Francisco Bay. Bull. Seismol. Soc. Am. 60, 29&amp;#8211;61.&lt;/p&gt;&lt;p&gt;Perron, V., G&amp;#233;lis, C., Froment, B., Hollender, F., Bard, P.-Y., Cultrera, G., Cushing, E.M., 2018. Can broad-band earthquake site responses be predicted by the ambient noise spectral ratio? Insight from observations at two sedimentary basins. Geophys. J. Int. 215, 1442&amp;#8211;1454.&lt;/p&gt;

scholarly journals Near-Source Simulation of Strong Ground Motion in Amatrice Downtown Including Site Effects

Geosciences ◽  
2021 ◽  
Vol 11 (5) ◽  
pp. 186
Author(s):  
Alessandro Todrani ◽  
Giovanna Cultrera
Keyword(s):  
Ground Motion ◽  
Site Effects ◽  
Central Italy ◽  
Strong Motion ◽  
Seismic Source ◽  
Spectral Ratio ◽  
Intensity Measures ◽  
Standard Spectral Ratio ◽  
Heavy Damage ◽  
Strong Ground

On 24 August 2016, a Mw 6.0 earthquake started a damaging seismic sequence in central Italy. The historical center of Amatrice village reached the XI degree (MCS scale) but the high vulnerability alone could not explain the heavy damage. Unfortunately, at the time of the earthquake only AMT station, 200 m away from the downtown, recorded the mainshock, whereas tens of temporary stations were installed afterwards. We propose a method to simulate the ground motion affecting Amatrice, using the FFT amplitude recorded at AMT, which has been modified by the standard spectral ratio (SSR) computed at 14 seismic stations in downtown. We tested the procedure by comparing simulations and recordings of two later mainshocks (Mw 5.9 and Mw 6.5), underlining advantages and limits of the technique. The strong motion variability of simulations was related to the proximity of the seismic source, accounted for by the ground motion at AMT, and to the peculiar site effects, described by the transfer function at the sites. The largest amplification characterized the stations close to the NE hill edge and produced simulated values of intensity measures clearly above one standard deviation of the GMM expected for Italy, up to 1.6 g for PGA.

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.

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

&lt;p&gt;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.&lt;/p&gt;&lt;p&gt;&amp;#160;&lt;/p&gt;

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.

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