Site Effects of Onna during the 2009 L’Aquila (Central Italy) Seismic Sequence: Constraints on Bedrock Depth and 1D Local Velocity Structure from Aftershock Seismograms

2020 ◽  
Vol 110 (2) ◽  
pp. 399-409
Author(s):  
Giuliana Mele ◽  
Antonio Rovelli ◽  
Antonio Fodarella ◽  
Marco Mancini
Keyword(s):  
Velocity Structure ◽  
Transfer Functions ◽  
Ground Motions ◽  
Central Italy ◽  
Spectral Ratio ◽  
Spectral Ratios ◽  
Linear Approach ◽  
Bedrock Depth ◽  
The Village ◽  
Sequence Constraints

ABSTRACT After the 2009 L’Aquila Mw 6.1 earthquake, particular attention was paid to the large difference of Mercalli–Cancani–Sieberg (MCS) macroseismic intensity between the nearby villages of Onna (9.5 MCS) and Monticchio (6 MCS). Several authors estimated that in Onna, settled in the Aterno river valley, ground motions were amplified at 2–3 Hz by up to a factor of 5 with respect to Monticchio, settled on more competent rocks. Although there was a general agreement that the spectral peak was caused by the resonance of the uppermost 40 m layer, a satisfactory fit of the amplitudes was not provided. Here, we apply spectral ratio techniques to 1437 aftershock seismograms (magnitude between 1.8 and 3.9) to compare ground motions within Onna and between Onna and Monticchio. Spectral amplitudes at stations located outside and inside the “red zone” of Onna show that the seismic response was uniform, confirming that vulnerability was crucial for the heavier damage of the ancient part of the village. We have also estimated the empirical transfer function of Onna through the spectral ratios between Onna and Monticchio. Although in a 1D simplification, a model with a further velocity contrast of ∼2 at 200 m of depth produces a more accurate fit of observations. Using the new velocity profile, we modeled the mainshock ground motion at Onna in an equivalent-linear approach. Accelerations are amplified by a factor of 2 and spectral ordinates increase from 0.7g at 0.2 s to 1g at 0.5 s, a shaking level that can be destructive for nonductile ancient buildings. This study shows that accurate estimates of empirical transfer functions, even in a simplified 1D approach, provide useful constraints to the deeper velocity structure where measurements are shallow or lacking.

Modelling the three-dimensional site response in the village of Amatrice, Central Italy

2020 ◽  
Author(s):  
Roberto Razzano ◽  
Iolanda Gaudiosi ◽  
Massimiliano Moscatelli ◽  
Callisto Luigi ◽  
Giuseppe Lanzo ◽  
...  
Keyword(s):  
Finite Difference ◽  
Transfer Functions ◽  
Site Response ◽  
Central Italy ◽  
Motion Field ◽  
Frequency Range ◽  
Reconstruction Process ◽  
Array Measurements ◽  
Acceleration Time Histories ◽  
The Village

<p>We analyzed the seismic site response of the Amatrice village, that experienced extensive and very high level of damage after the 24th of August 2016 earthquake, further aggravated by the following shocks of October 2016. In particular, site response was investigated by simulating seismic wave propagation through an advanced 3D subsurface model of the site. Availability of experimental site transfer functions allowed validating simulation results and evaluating advantages and drawbacks of this approach.</p><p>The 3D subsoil model was developed based on the available dataset of borehole stratigraphic logs, shear wave velocity profiles obtained from Down-Hole tests and 2D ARRAY measurements as well physical and mechanical properties measured by means of laboratory tests (EmerTer Project, 2018; CNR IGAG Report, 2018).</p><p>The model was forced by 3-component (3C) input constituted of acceleration time histories that were selected from the European Strong-Motion database (www.esm.mi.ingv.it ; Luzi et al., 2016) by considering a return period = 975y<strong>. </strong></p><p>The explicit finite-difference code FLAC3D (ITASCA Consulting group Inc., 2017) was used for numerical simulations; this code operates in the time domain, incorporates a compliant base, free-field lateral boundaries and uses a fully nonlinear approach to model the dynamic soil properties. The identification of the seismic bedrock depth was carried out by an iterative procedure that minimizes the difference between recorded motions after deconvolution at depth. A hysteretic-damping model and Rayleigh damping formulation were used to account for viscous damping in dynamic condition. Rule by Kuhlemeyer and Lysmer (1973) was adopted for element size definition to achieve a satisfactory level of accuracy up to 10 Hz. The finite difference mesh consists of about 1.1 million tetrahedral-shaped elements.</p><p>Three control points in correspondence with three temporary seismic stations, i.e., MZ12, MZ28 and MZ31, were considered in order to compare the simulated 3D transfer functions with the experimental ones. In particular, MZ12 was located in the historical center of Amatrice village, MZ28 in the southeastern part of the village, while MZ31 in the western sector. Available Standard Spectral Ratios (Borcherdt, 1970, Milana et al., 2019) were used to determine the experimental frequencies and amplifications. The results showed that the average amplification is about 4 for MZ12 in the frequency range 5-7Hz and about 2 for MZ28 station at 3Hz, while amplification function is essentially flat at MZ31. In the historical part of the village, only Horizontal-to-Vertical Spectral Ratio (Nakamura, 1989) measurements were available. Reasonable agreements were found in the considered frequency range 1-10Hz.</p><p>This approach, which simulated the 3C ground motion field, demonstrated to be useful to evaluate the most important 3D model features relevant for site amplification.</p><p>The present work was performed in the frame of the SISMI Project, funded by Regione Lazio and devoted to developing new technologies for improving the security and the reconstruction process of the historical centers in central Italy. All the activities were carried out under in the framework of the “DTC Lazio” (https://dtclazio.it/).</p>

The Microzonation of Perth, Western Australia, Using Microtremor Spectral Ratios

1995 ◽  
Vol 11 (2) ◽  
pp. 173-191 ◽  
Author(s):  
Brian A. Gaull ◽  
Hiroshi Kagami ◽  
Hitoshi Taniguchi
Keyword(s):  
Western Australia ◽  
Metropolitan Area ◽  
Reference Site ◽  
Hard Rock ◽  
Ground Motions ◽  
Spectral Ratio ◽  
Earthquake Risk ◽  
Spectral Ratios ◽  
Frequency Bands ◽  
Seismic Zones

This paper indicates new microzonation maps of Perth, Western Australia, utilising microtremor spectral ratios. This metropolitan area has been developing in recent times on Perth Basin which is one of the most active seismic zones in the country. The authors carried out simultaneous measurings of microtremors over most of metropolitan Perth, using a 3 km grid as a basis and hard rock reference site throughout. They calculated spectral ratios of microtremors at deposit site to rock reference site and plotted and contoured on maps for 6 frequency bands from 0.2 to 5.0 Hz. Spectral ratio contours appeared to correlate well with various geological subsurface contours. They also showed that previously estimated earthquake risk estimates underestimated ground motions by up to a factor of two.

Investigation of Relationship between the Response and Fourier Spectral Ratios Based on Statistical Analyses of Strong-Motion Records

2020 ◽  
pp. 2150008
Author(s):  
Haizhong Zhang ◽  
Yan-Gang Zhao
Keyword(s):  
Seismic Design ◽  
Site Effects ◽  
Epicentral Distance ◽  
Ground Motions ◽  
Scaling Law ◽  
Ground Surface ◽  
Strong Motion ◽  
Spectral Ratio ◽  
Spectral Ratios ◽  
Earthquake Scenario

In both seismic design and probabilistic seismic-hazard analyses, site effects are typically characterized as the ratio of the response spectral ordinate on the ground surface to that on the bedrock based on the scaling law borrowed from the Fourier spectral ordinate. Recent studies have shown that different from the Fourier spectral ratio (FSR), the response spectral ratio (RSR) does not purely reflect the site effects but also depends on the earthquake scenario even for linear analysis. However, previous studies are limited to theoretical analysis. This study statistically compares the two spectral ratios by analyzing many actual seismic ground motions recorded at nearby soil and rock sites. It is observed that the average RSR and FSR have similar overall shapes, and their maximum values occur at approximately the same period; however, the values around the peak are clearly different with FSRs consistently exceeding the RSRs. The RSR–FSR relationship depends on the earthquake scenario and the oscillator damping; their difference at periods longer than the site’s fundamental period decreases as the magnitude and epicentral distance increase, and the RSRs generally approach the FSRs as the oscillator damping decreases.

scholarly journals Nonlinearity, liquefaction, and velocity variation of soft soil layers in Port Island, Kobe, during the Hyogo-ken Nanbu earthquake

1997 ◽  
Vol 87 (5) ◽  
pp. 1244-1258 ◽  
Author(s):  
Jorge Aguirre ◽  
Kojiro Irikura
Keyword(s):  
Wave Velocity ◽  
Velocity Structure ◽  
Ground Motions ◽  
Peak Acceleration ◽  
Spectral Ratios ◽  
S Wave ◽  
Strong Ground Motions ◽  
Before And After ◽  
Different Depths ◽  
Strong Ground

Abstract Clear nonlinear behavior is analyzed from the acceleration records of the 1995 Hyogo-ken Nanbu earthquake at Port Island, Kobe. From four triaxial instruments placed at four different depths, the surficial effects during strong ground motions were compared with those during weak motions before and after the mainshock. We used a spectral ratio technique and a nonlinear inversion for velocity structure to analyze the data. From the spectral analysis, we observed a large variation of the spectral ratios between the surface and different depths during the strong ground motions and during the liquefied state. The spectral ratios after the mainshock (i.e., after the liquefied state) are different from those before the mainshock. The peak frequencies in the spectral ratios after the mainshock are shifted to lower frequencies with respect to those in the spectral ratios before the mainshock. We inverted the S-wave velocities using a genetic algorithm technique to determine the velocity structure before, during, and after the mainshock. The S-wave velocity structure before and after the mainshock was found to be different. Specifically, the S-wave velocity of the second layer (5 m to 16 m depth) after the mainshock was 20% lower than before. Our analysis shows that the liquefied state remains at least 3 hr after the mainshock but no more than 24 hr. The rigidity of the soil decreased close to zero when liquefaction happened and later increases gradually following a trend that resembles a consolidation curve. The strong influence of nonlinearity during the mainshock yielded a big reduction of the horizontal surface ground motions, so that the observed horizontal peak acceleration was only about 25% of the peak acceleration expected from the linear theory. However, the nonlinear effects in the vertical peak acceleration were not significant.

Site effects of the Denis-Perron dam (SM-3): A case study in Eastern North America

2021 ◽  
pp. 875529302110194
Author(s):  
Daniel Verret ◽  
Denis LeBœuf ◽  
Éric Péloquin
Keyword(s):  
North America ◽  
Ground Motion ◽  
Site Effects ◽  
Transfer Functions ◽  
Ground Motions ◽  
Published Data ◽  
Eastern North America ◽  
Ground Acceleration ◽  
Large Dams ◽  
Moderate Seismicity

Eastern North America (ENA) is part of a region with low-to-moderate seismicity; nonetheless, some significant seismic events have occurred in the last few decades. Recent events have reemphasized the need to review ENA seismicity and ground motion models, along with continually reevaluating and updating procedures related to the seismic safety assessment of hydroelectric infrastructures, particularly large dams in Québec. Furthermore, recent researchers have shown that site-specific characteristics, topography, and valley shapes may significantly aggravate the severity of ground motions. To the best of our knowledge, very few instrumental data from actual earthquakes have been published for examining the site effects of hydroelectric dam structures located in eastern Canada. This article presents an analysis of three small earthquakes that occurred in 1999 and 2002 at the Denis-Perron (SM-3) dam. This dam, the highest in Québec, is a rockfill embankment structure with a height of 171 m and a length of 378 m; it is located in a narrow valley. The ground motion datasets of these earthquakes include the bedrock and dam crest three-component accelerometer recordings. Ground motions are analyzed both in the time and frequency domains. The spectral ratios and transfer functions obtained from these small earthquakes provide new insights into the directionality of resonant frequencies, vibration modes, and site effects for the Denis-Perron dam. The crest amplifications observed for this dam are also compared with previously published data for large dams. New statistical relationships are proposed to establish dam crest amplification on the basis of the peak ground acceleration (PGA) at the foundation.

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.

Characteristics of long-period microtremors and their applicability in exploration of deep sedimentary layers

1994 ◽  
Vol 84 (6) ◽  
pp. 1831-1841 ◽  
Author(s):  
Hiroaki Yamanaka ◽  
Masayuki Takemura ◽  
Hiroshi Ishida ◽  
Masanori Niwa
Keyword(s):  
Rayleigh Waves ◽  
Ground Motions ◽  
Spectral Ratio ◽  
Northwestern Part ◽  
Trial And Error ◽  
Sediment Thickness ◽  
Short Term ◽  
Subsurface Structures ◽  
Long Period ◽  
Good Agreement

Abstract Applicability of long-period microtremors in inferring subsurface structure is examined using measurements of microtremors in the northwestern part of the Kanto Plain in Japan. Short-term continuous measurements of long-period microtremors at both sediment and basement sites were taken. A spectral peak at a period of 4 to 5 sec is stable with time, while peaks at periods less than 2 sec are time variant, suggesting a variation of microtremor sources. However, it was found that the spectral ratio between vertical and horizontal microtremors (ellipticity) at each site is stable with time. Good agreement was found between ellipticities of microtremors at the sediment site and those computed for Rayleigh waves in which the structure of the sediments beneath the site was taken into account. We also found that the ellipticities of Rayleigh waves in earthquake ground motions were consistent with those of the microtremors. These comparisons provide strong evidence that long-period microtremors in the area studied consist mainly of Rayleigh waves. The ellipticity of microtremors was investigated by observing microtremors at temporary observation sites in the Kanto Plain where the sediment thickness varied from 0 to 1 km. The subsurface structures were deduced by trial-and-error fitting of observed ellipticities with theoretical ellipticities that were calculated assuming Rayleigh waves. These results show that ellipticity of long-period microtremors is effective for deducing structure from microtremor data at a single site.

Two-parameter scaling of source spectra: Love waves from deep-focus Bonin Islands earthquakes

1977 ◽  
Vol 67 (2) ◽  
pp. 285-300
Author(s):  
R. James Brown
Keyword(s):  
Body Wave ◽  
Scaling Law ◽  
Spectral Ratio ◽  
Love Waves ◽  
Corner Frequency ◽  
Spectral Ratios ◽  
Bonin Islands ◽  
Deep Focus ◽  
Two Parameter ◽  
Fault Length

Abstract Starting with the one-parameter scaling law of Aki, a two-parameter expression is developed to model the source factor of the far-field spectrum from a dislocation fault source for both ω−2 and ω−3 high-frequency asymptotic types. Aki's assumption of similarity is relaxed in two respects: it is neither here assumed that wD0 ∞ L2 (L = fault length, w = fault width, D0 = average dislocation) nor that kT = v kL (kT−1 = correlation time, kL−1 = correlation length, v = velocity of rupture propagation), the latter being equivalent to allowing for Brune's fractional stress drop. From this two-parameter model a four-parameter model of spectral ratio is obtained and fitted to observed spectral ratios by computer optimization of the four parameters. Observed spectral ratios have been determined from the Love waves recorded at NORSAR from six deep-focus Bonin Islands earthquakes using a common-path method. From the optimal values of the four parameters, values are determined for corner frequency (f ≈ 0.2 Hz for m 6.0; f ≈ 0.3 Hz for m = 5.3; m = PDE body-wave magnitude), relative fault length, relative seismic moment (and magnitudes), and p, the slope of the corner-frequency locus. Values found for p are all greater than 3 and such p, in combination with an ω−3 scaling law, can yield a reasonable m:M relation, i.e., with no ceiling imposed on m. A slightly better fit is obtained by starting with an ω−3 model than with ω−2.

Focal mechanism and source depth of earthquakes from body- and surface-wave data

1971 ◽  
Vol 61 (5) ◽  
pp. 1369-1379 ◽  
Author(s):  
Nezihi Canitez ◽  
M. Nafi Toksöz
Keyword(s):  
Surface Wave ◽  
Body Wave ◽  
Source Parameters ◽  
Focal Depth ◽  
Spectral Ratio ◽  
The Body ◽  
Spectral Ratios ◽  
Motion Data ◽  
Wave Data ◽  
Surface Wave Data

abstract The determination of focal depth and other source parameters by the use of first-motion data and surface-wave spectra is investigated. It is shown that the spectral ratio of Love to Rayleigh waves (L/R) is sensitive to all source parameters. The azimuthal variation of the L/R spectral ratios can be used to check the fault-plane solution as well as for focal depth determinations. Medium response, attenuation, and source finiteness seriously affect the absolute spectra and introduce uncertainty into the focal depth determinations. These effects are nearly canceled out when L/R amplitude ratios are used. Thus, the preferred procedure for source mechanism studies of shallow earthquakes is to use jointly the body-wave data, absolute spectra of surface waves, and the Love/Rayleigh spectral ratios. With this procedure, focal depths can be determined to an accuracy of a few kilometers.

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