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>

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 How well can we predict earthquake site response so far? Site-specific approaches

2022 ◽  
pp. 875529302110608
Author(s):  
Chuanbin Zhu ◽  
Fabrice Cotton ◽  
Hiroshi Kawase ◽  
Annabel Haendel ◽  
Marco Pilz ◽  
...  
Keyword(s):  
Site Effects ◽  
Seismic Waves ◽  
Site Response ◽  
Spectral Ratio ◽  
Response Analysis ◽  
Ground Response Analysis ◽  
Site Specific ◽  
Quarter Wavelength ◽  
High Level ◽  
Surface Geology

Earthquake site responses or site effects are the modifications of surface geology to seismic waves. How well can we predict the site effects (average over many earthquakes) at individual sites so far? To address this question, we tested and compared the effectiveness of different estimation techniques in predicting the outcrop Fourier site responses separated using the general inversion technique (GIT) from recordings. Techniques being evaluated are (a) the empirical correction to the horizontal-to-vertical spectral ratio of earthquakes (c-HVSR), (b) one-dimensional ground response analysis (GRA), and (c) the square-root-impedance (SRI) method (also called the quarter-wavelength approach). Our results show that c-HVSR can capture significantly more site-specific features in site responses than both GRA and SRI in the aggregate, especially at relatively high frequencies. c-HVSR achieves a “good match” in spectral shape at ∼80%–90% of 145 testing sites, whereas GRA and SRI fail at most sites. GRA and SRI results have a high level of parametric and/or modeling errors which can be constrained, to some extent, by collecting on-site recordings.

Analysis of spectral ratios for estimating ground motion in deep basins

1996 ◽  
Vol 86 (3) ◽  
pp. 646-654 ◽  
Author(s):  
M. Dravinski ◽  
G. Ding ◽  
K.-L. Wen
Keyword(s):  
Resonant Frequency ◽  
Ground Motion ◽  
Three Dimensional ◽  
Sedimentary Basins ◽  
Spectral Ratio ◽  
Spectral Ratios ◽  
Resonant Frequencies ◽  
Ground Motion Amplification ◽  
Three Dimensional Models ◽  
Incident Waves

Abstract Use of Nakamura's spectral ratio (horizontal versus vertical components) is investigated theoretically for deep sedimentary basins by considering semi-circular and semi-spherical valleys. The ratio is evaluated from the steady-state surface response for different incident waves. Based on this ratio, both the resonant frequencies and ground motion amplification are determined. The results based on Nakamura's ratio are compared with those based on the sediment-to-bedrock spectral ratios (Kagami's ratio). The results show that for both two- and three-dimensional models, Nakamura's technique predicts well the fundamental resonant frequency, but it could not determine higher resonant frequencies of the basins. The error in Nakamura's estimate of the fundamental resonant frequency increases for stations near the valley center. For alluvial valleys considered in this article, Nakamura's ratio failed to predict accurately surface ground-motion amplification.

scholarly journals Sediment-induced amplification in the Northeastern United States: A case study in Providence, Rhode Island

1995 ◽  
Vol 85 (5) ◽  
pp. 1388-1397
Author(s):  
Karen M. Fischer ◽  
Lynn A. Salvati ◽  
Susan E. Hough ◽  
Edward Gonzalez ◽  
Chad E. Nelsen ◽  
...  
Keyword(s):  
Ground Motion ◽  
Rhode Island ◽  
Site Response ◽  
P Wave ◽  
Earthquake Ground Motion ◽  
Soft Sediment ◽  
Spectral Ratios ◽  
Study Region ◽  
Entire Study ◽  
Sediment Layers

Abstract We employed ambient-noise measurements to assess the potential for seismic site response in sediment-filled valleys that intersect beneath downtown Providence, Rhode Island. At eight valley stations and at two sites on an adjacent bedrock highland, we recorded ground motion from two types of sources: pile drivers at a local construction site and ambient microtremors. At all valley sites where sediment thicknesses exceed 10 m, spectral ratios contain amplitude peaks at frequencies of 1.5 to 3.0 Hz. In contrast, spectral ratios from the two sites on the bedrock highland where sediment cover is less than 4-m thick are relatively flat within this frequency range. A variety of borehole logs identified two fundamental sediment types (soft sediment and a consolidated glacial till) and were used to map layer thicknesses over the entire study region. Refraction data constrained P-wave velocity in the bedrock to be 3680 ± 160 m/sec and indicated two soft-sediment layers with P-wave velocities of 300 ± 50 and 1580 ± 120 m/sec. Using a one-dimensional reflection matrix technique, we matched the spectral-ratio peak observed at each valley site with the frequency of fundamental resonance predicted for local layer thicknesses and velocities. A positive correlation between the best-fitting soft-sediment velocities and bedrock depth may reflect greater compaction in the deepest sediments or a locally two-dimensional sediment resonance at the deepest sediment sites. We conclude that unconsolidated sediment layers under downtown Providence have the potential to amplify earthquake ground motion at frequencies damaging to engineered structures.

Observed Variation of Earthquake Motion across a Basin—Taipei City

1998 ◽  
Vol 14 (1) ◽  
pp. 115-133 ◽  
Author(s):  
Chin-Hsiung Loh ◽  
Jeng-Yaw Hwang ◽  
Tzay-Chyn Shin
Keyword(s):  
Ground Motion ◽  
Seismic Waves ◽  
Principal Direction ◽  
Response Spectrum ◽  
Strong Motion ◽  
Seismic Source ◽  
Spectral Ratio ◽  
Site Amplification ◽  
Taipei Basin ◽  
Basin Effects

Local site amplification of sedimentary deposit during earthquakes is an important issue in strong ground motion analysis. The phenomenon is more obvious for sediment basin. From the strong-motion instrumentation network of Taipei basin, the ground motion characteristics of the basin effects are studied from two seismic events: the June 5, 1994 earthquake with ML = 6.57 and the June 25, 1995 earthquake with ML = 6.50. The objective is to investigate the effects of the basin structure on the patterns of the recorded ground motions. The analyses include: (1) response spectrum and spectral ratio analyses; (2) correlation of seismic source, PGA distribution and strong-motion duration with site amplification, (3) principal direction analysis of seismic waves in the basin. The observed variations of ground motion across the basin are different from each other because of the basin effect. It means that for the Taipei basin, the basin effects for shallow sources are going to be much more significant than for the deep sources.

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

<p>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).</p><p>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.</p><p>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.</p><p>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.</p><p>REFERENCES</p><p>Borcherdt, R.D., 1970. Effects of local geology on ground motion near San Francisco Bay. Bull. Seismol. Soc. Am. 60, 29–61.</p><p>Perron, V., Gélis, 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, 1442–1454.</p>

Distinguishing cryoseisms from earthquakes in Alberta, Canada

2018 ◽  
Vol 55 (10) ◽  
pp. 1183-1195 ◽  
Author(s):  
Hadi Ghofrani ◽  
Gail M. Atkinson
Keyword(s):  
Ground Motion ◽  
Seismic Waves ◽  
Seismic Events ◽  
Seismic Signals ◽  
Local Magnitude ◽  
Earthquake Catalogues ◽  
Small Magnitude ◽  
Rock Bursts ◽  
Cultural Noise

Characterizing the ground-motion signature of various event types is important for event discrimination and for the development of reliable earthquake catalogues. Small-magnitude seismic events are often enigmatic and determining their cause can be challenging. This is because there are various sources of energy (e.g., rock bursts, construction blasts, cultural noise, etc.) that generate seismic waves similar to those from small-magnitude earthquakes. A sequence of eight unusual small-magnitude (local magnitude, ML, of −0.17 to 1.39) seismic signals were detected in the area surrounding the Brazeau Reservoir and river (Alberta), beginning in mid-2014. Detailed analyses of the waveforms and spectra for these events, as well as their timing, strongly suggest that at least seven of these events are cryoseismic signals associated with freeze-up on the reservoir and Brazeau River.

scholarly journals Estimation of ground motion in Xalapa, Veracruz, Mexico during the 1920 (M~6.4) crustal earthquake, and some significant intraslab earthquakes of the last century

2018 ◽  
Vol 57 (2) ◽  
Author(s):  
Francisco Córdoba-Montiel Córdoba-Montiel ◽  
Srhi Krishna Singh ◽  
Arturo Iglesias ◽  
Xyoli Pérez-Campos ◽  
K. Sieron
Keyword(s):  
Ground Motion ◽  
Stress Drop ◽  
Seismic Waves ◽  
Volcanic Belt ◽  
Spectral Ratio ◽  
Site Effect ◽  
Peak Ground Velocity ◽  
Ground Acceleration ◽  
Scenario Earthquakes ◽  
The City

Ground motions in Xalapa, Veracruz, Mexico, during the earthquake of January 4, 1920 (M~6.4), and three significant intraslab earthquakes (Mw7.0) of the last century were estimated. These events are reasonable scenario earthquakes for Xalapa. Towards this goal, portable broadband seismographs at nine sites in the city and an additional one at a reference hard site outside the city were deployed. Peak ground acceleration (Amax) and peak ground velocity (Amax) in Xalapa were estimated based on Brune w -2 source model and the site effect, obtained from earthquake recordings by using the standard spectral ratio (SSR) technique, and the application of a stochastic method. During the 1920 Xalapa earthquake the estimated Amax values corresponding to a stress drop, Ds, of 50 bar are between 100 and 250 cm/s2, except at two sites where the site effect is very large and Amax values reach 300 and 600 cm/s2. Estimated Vmax values are between 10 and 20 cm/s, except at the site with the largest site effect where it is ~ 40 cm/s. Ds of 30 and 100 bar produce about half and twice of these peak values, respectively. The main uncertainty in the present estimations is due the Ds value, because although a range of 30 to 100 bar for crustal earthquakes in the Trans-Mexican Volcanic Belt (in which Xalapa is located) seems reasonable, it is not constrained by the data. The mean stress drop for intraslab events, ~ 300 bar, is better constrained from previous studies. A median Amax of ~ 30 cm/s2 and a median Vmax of 4 cm/s in Xalapa during the 1973 (Orizaba) and 1999 (Tehuacán) earthquakes was estimated; the corresponding values during the 1980 (Huajuapan) earthquake are ~ 10 cm/s2 and 2 cm/s. The uncertainty in the estimation is probably within a factor of 2 to 3.The ground motion prediction equations developed from data in the forearc region with less attenuation (than the backarc region) and recorded at hard sites appear to work reasonably well for Xalapa sites, which lie in the back arc. This observation suggests that the seismic waves from intraslab earthquakes, traveling through the mantle wedge before arriving Xalapa, suffer relatively large attenuation. However, these waves get amplified due to local site effects. It seems that in Xalapa these two effects, roughly, balance each other.

Numerical evaluation of the effect of cross-coupling of different components of ground motion in site response analyses

1999 ◽  
Vol 89 (4) ◽  
pp. 877-887
Author(s):  
Roberto Paolucci
Keyword(s):  
Transfer Function ◽  
Ground Motion ◽  
Site Response ◽  
Strong Motion ◽  
Cross Coupling ◽  
Response Functions ◽  
Spectral Ratios ◽  
Acceleration Time Histories ◽  
Time Histories ◽  
Input Motion

Abstract The effect of cross-coupling between the three components of ground motion in the evaluation of site-response functions, such as standard spectral ratios (SSRs) and horizontal-to-vertical spectral ratios (HVSRs), is analyzed in this article. Numerical analyses of the seismic response of fully 3D geological structures, namely, a real topographic irregularity and an ideal stratigraphic inclusion, have been carried out to obtain a 3D transfer function in the form of a 3 × 3 matrix. Each element of this matrix contains the frequency response in the ith direction due to an input motion in the jth direction. A synthetic set of acceleration time histories at the surface of the geological irregularity has been created by convolution with the 3D transfer function, using as input motion different real multicomponent strong-motion accelerograms recorded at stiff-soil or rock sites. The SSRs and HVSRs are calculated and compared with the theoretical 3D transfer function in order to highlight the effect of cross-coupling terms. These are found to generate a rather large dispersion in the site-response functions, as well as response peaks that could be misleading in the interpretation of both numerical and observed spectral ratios.

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