Estimation Of Site Effects Research Articles

Applications of a GIS-based geotechnical tool to assess spatial earthquake hazards in an urban area

A geotechnical information system (GTIS) was constructed within a spatial geographic information system (GIS) framework to reliably predict geotechnical information and accurately estimate site effects at Gyeongju, an urban area in South Korea. The system was built based on both collected and performed site investigation data in addition to acquired geo-knowledge data. Seismic zoning maps were constructed using the site period (TG) and mean shear wave velocity to a depth of 30 m (VS30), and these maps were presented as a regional strategy to mitigate earthquake-induced risks in the study area. In particular, the TG distribution map indicated the susceptibility to ground motion resonance in periods ranging from 0.2 to 0.5 s and the corresponding seismic vulnerability of buildings with two to five stories. Seismic zonation of site classification according to VS30 values was also performed to determine the site amplification coefficients for seismic design and seismic performance evaluation at any site and administrative subunit in the study area. In addition, we investigated the site effects according to subsurface and surface ground irregularities at Gyeongju by seismic response analyses in time domains based on both two- and three-dimensional spatial finite element models, which were generated using spatial interface coordinates between geotechnical subsurface layers predicted by the GTIS. This practical study verified that spatial GIS-based geotechnical information can be a very useful resource in determining how to best mitigate seismic hazards, particularly in urban areas.

The L’Aquila earthquake—A view of site effects and building behavior from temporary networks

The idea of this Special Issue comes from a “joint venture” of research groups from several Italian and European Institutions (Istituto Nazionale di Geofisica e Vulcanologia and University of Basilicata, Italy; GeoForschungsZentrum-GFZ, Germany; CETE Mediterranee—LRPC Nice and OCA-UMR Geoazur, France) which carried out a large seismic survey for site effect estimation in the wide area hit by the 2009 L’Aquila, Italy, earthquake (Fig. 1). The Mw 6.3 mainshock of April 6th, 2009, represented the most damaging event in Italy since the 1980 Irpinia earthquake (Mw 6.9). Several large aftershocks (Mw > 5) and thousands of smaller events occurred in the following months and the deployment of different rapid response seismic networks from Italy, France and Germany allowed the scientific community to collect a very large and high quality data set, probably the largest ever acquired during a normal fault seismic sequence (Marzorati et al. 2011). Among many scientific issues, the L’Aquila earthquake seismic sequence represented a laboratory to better understand the occurrence of site effects in intra-mountain regions and their influence on damage distribution for traditional dwellings and recent residential reinforced concrete buildings (D’Ayala and Dolce 2011). The macroseismic survey (Galli and Camassi 2009) indicates that the strongest damages were observed in the city of L’Aquila and in the villages to the SE, in agreement with the rupture propagation pattern toward up-dip and SE (Cirella et al. 2009) and according to the orientation of the Aterno River Valley. In particular, sixteen localities including L’Aquila downtown, on the hanging-wall side of the

Variance Reduction and Signal-to-Noise Ratio: Reducing Uncertainty in Spectral Ratios

This paper uses an unusually large dataset to study scatter in site-effect estimation, focusing on how the events that increase uncertainty can be removed from the dataset. Four hundred seventy-three weak motion earthquake records from the surface and bedrock of a 178-m-deep borehole in Aegion, Gulf of Corinth, Greece, are used to evaluate spectral ratios. A simple statistical tool, variance reduction (VR), is first used to identify two groups of events that lie closest and farthest from the average, which is considered here as the initial best estimate of the site response. The scatter in the original dataset is found to be due to the group of events with smallest VR. These events can be removed from the dataset in order to compute a more reliable site response. However, VR is not normally used to choose records for site-effect studies, and it cannot be applied to the usual small datasets available. The signal-to-noise ratio (SNR) is normally used to this end, for which reason we investigate whether SNR can be used to achieve similar results as VR. Signal-to-noise ratio is estimated using different definitions. Data selection based on SNR is then compared to that using VR in order to define an SNR-based criterion that discriminates against events that, according to VR, increase scatter. We find that defining the SNR of a surface record as the mean value over a frequency range around the resonant peak (here, 0.5–1.5 Hz) and using a cutoff value of 5 may be used in this case to exclude most events for which VR is small. This process is also applied to the downhole station, where we obtain similar results for a cutoff value of 3.

A new approach to liquefaction hazard zonation: Application to Laoag City, Northern Philippines

To help mitigate liquefaction hazards in the Philippines, an inexpensive yet effective approach to liquefaction hazard zonation was developed in this study. The proposed approach is also useful in other areas especially where funds for more rigorous procedures may not be available. The approach utilizes the geomorphology-based criteria to identify liquefaction-prone deposits based on geology and grain characteristics, and generate a preliminary liquefaction susceptibility map. Then, microtremor recordings, popularly used in site effect estimation, are gathered to derive qualitative information on the density and thickness of these deposits and generate a site classification map. This latter map is also essentially a ground shaking hazard map in that it shows those areas where thick, soft deposits likely to amplify and prolong the duration of ground motion can be found. Therefore, it also identifies areas where seismic demand can be high that the possibility of liquefaction being triggered is likewise high. Combining the two maps, an integrated liquefaction hazard zonation map is produced which provides not only an improved characterization of the soils’ capacity to resist liquefaction but also integrates qualitative information on the seismic demand on these deposits as well. With information about the relative thickness of the deposits, the severity of potential damage can likewise be inferred from the map since thicker deposits relate to more serious damage. The proposed approach was applied to Laoag City, Northern Philippines, where it was shown to reliably identify areas that are vulnerable to the hazard.

A detailed study of the site effects in the volcanic area of Campi Flegrei using empirical approaches

SUMMARY Campi Flegrei is a highly populated active caldera in the south of Italy. Several hundred thousand people live within this area, which is characterized by seismicity and ground deformation episodes, known as ‘bradyseism’. For this reason, this area falls into a high-risk category and thus the Italian Civil Defence requires a detailed site-effect estimation. To determine the local amplification of the seismic waves for a high number of sites, we have analysed the seismic recordings of three seismic networks that have been deployed in the Campi Flegrei area over different time periods. The first network was deployed during the bradyseismic crisis of 1982–1984. We selected 22 of the highest magnitude earthquakes that were recorded during this crisis. An additional 22 seismic events were selected from those recorded by the mobile seismic network that has been in operation in the Campi Flegrei area since 2006. The third data set comprises noise recorded by 34 seismic stations that were deployed during the active SERAPIS experiment in 2001 September. The generalized inversion technique and the H/V spectral ratio method were applied to the S waves and coda waves of the earthquakes recorded by the first two seismic networks, to determine the site-transfer functions of the recording stations. The seismic noise recorded by the third network was analysed using the Nakamura’s technique. The results show that the high topographical and geological heterogeneity of the sites located inside the caldera has an important influence on the seismic-wave amplification. Consequently, the site-transfer functions can be different even at sites close to each other. The transfer functions of the sites located outside the caldera are much more regular, apparently due to the more regular topography and geology.

Site response in Tecoman, Colima, Mexico—I: Comparison of results from different instruments and analysis techniques

This paper presents a study of site effects in the urban area of Tecoman in Colima, Mexico. A variety of instruments (both accelerometers and seismometers) were used to record earthquakes and ambient vibration throughout the city. Earthquake records were analysed using several techniques to estimate site effects: spectral ratios relative to a reference station, spectral ratios of the horizontal components relative to the vertical recorded at the same site, and a parametric inversion of Fourier spectra. Ambient noise records were used to estimate a local transfer function using horizontal to vertical spectral ratios. The results show that local amplification at Tecoman is significant. Dominant frequency varies between 0.5 and 0.7 Hz, suggesting a large thickness of the soft sedimentary deposits. We did not observe systematic variations throughout the city. Our more reliable estimates indicate that maximum amplification is comprised between a factor 6 and 8. Comparisons among different sensors and recorders show that all combinations between velocimeters, accelerometers, and recorders provide reliable results provided that the electronic noise is smaller than the noise being recorded. This is notably not the case for accelerometers at quiet sites and for frequencies smaller than 2 Hz. This explains why previous studies disagree as to the usefulness of accelerometers to record ambient noise for site effect studies. This factor is, however, a function of noise amplitude at each site.

GFZ Wireless Seismic Array (GFZ-WISE), a Wireless Mesh Network of Seismic Sensors: New Perspectives for Seismic Noise Array Investigations and Site Monitoring

Over the last few years, the analysis of seismic noise recorded by two dimensional arrays has been confirmed to be capable of deriving the subsoil shear-wave velocity structure down to several hundred meters depth. In fact, using just a few minutes of seismic noise recordings and combining this with the well known horizontal-to-vertical method, it has also been shown that it is possible to investigate the average one dimensional velocity structure below an array of stations in urban areas with a sufficient resolution to depths that would be prohibitive with active source array surveys, while in addition reducing the number of boreholes required to be drilled for site-effect analysis. However, the high cost of standard seismological instrumentation limits the number of sensors generally available for two-dimensional array measurements (i.e., of the order of 10), limiting the resolution in the estimated shear-wave velocity profiles. Therefore, new themes in site-effect estimation research by two-dimensional arrays involve the development and application of low-cost instrumentation, which potentially allows the performance of dense-array measurements, and the development of dedicated signal-analysis procedures for rapid and robust estimation of shear-wave velocity profiles. In this work, we present novel low-cost wireless instrumentation for dense two-dimensional ambient seismic noise array measurements that allows the real–time analysis of the surface-wavefield and the rapid estimation of the local shear-wave velocity structure for site response studies. We first introduce the general philosophy of the new system, as well as the hardware and software that forms the novel instrument, which we have tested in laboratory and field studies.

TheMw6.3, 2009 L���Aquila earthquake: source, path and site effects from spectral analysis of strong motion data

SUMMARY The strong motion data of 2009 April 6 L’Aquila (Central Italy) earthquake (Mw= 6.3) and of 12 aftershocks (4.1 ≤Mw≤ 5.6) recorded by 56 stations of the Italian strong motion network are spectrally analysed to estimate the source parameters, the seismic attenuation, and the site amplification effects. The obtained source spectra for S wave have stress drop values ranging from 2.4 to 16.8 MPa, being the stress drop of the main shock equal to 9.2 MPa. The spectral curves describing the attenuation with distance show the presence of shoulders and bumps, mainly around 50 and 150 km, as consequence of significant reflected and refracted arrivals from crustal interfaces. The attenuation in the first 50 km is well described by a quality factor equal to Q(f) = 59f0.56 obtained by fixing the geometrical spreading exponent to 1. Finally, the horizontal-to-vertical spectral ratio provides unreliable estimates of local site effects for those stations showing large amplifications over the vertical component of motion.

Comparison of Site-Effects Estimation Methods Using the Lefkas, Greece, 2003 Earthquake Aftershocks

Abstract This study aims at the comparison of different site-effect estimation techniques. We used aftershock records of the 14 August 2003 ( M w 6.2) earthquake that occurred in the western part of Greece next to Lefkas Island. A data set of 15 events recorded at five stations was selected for the period of 20 August 2003–30 September 2003. The moment magnitudes range between 3.3 and 4.2, and the hypocentral distances vary from 5 to 34 km. The site effects are determined using the standard spectral ratio (SSR) method, the horizontal-to-vertical spectral ratio for earthquake data method (the so-called receiver function [RF] method), and an inversion method for simultaneous source, path, and site-effects estimation. These methods are applied to both P and S waves, using exactly the same time windows in order to compute the spectra. Finally, using geotechnical information available for the city of Lefkas, the experimental site transfer functions at each site are compared to theoretical one-dimensional 1D transfer functions computed using Kennett’s reflectivity coefficient method. We show that the SSR method and the inversion method give similar site transfer functions. However, the experimental SSR fits even better with the spectral ratios of the inversion results, which indicates that our reference site is affected by a small site effect. Moreover, the RF method results fit well with the horizontal-to-vertical ratios coming from the inversion results, indicating in this case a nonnegligible amplification on the vertical component. This is the reason why the RF gives only the frequency of amplification but not the amplitude. These conclusions hold for both P and S waves. Finally, the site effects computed using the whole record or a window including only S waves are similar, while the P waves results, although showing some similar features with those for S waves, give different transfer functions for the sites investigated.

Mapping of Average Shear Wave Velocity for Bangalore Region: A Case Study

Mapping the shear wave velocity profile is an important part in seismic hazard and microzonation studies. The shear wave velocity of soil in the city of Bangalore was mapped using the Multichannel Analysis of Surface Wave (MASW) technique. An empirical relationship was found between the Standard Penetration Test (SPT) corrected N value [Formula: see text] and measured shear wave velocity [Formula: see text]. The survey points were selected in such a way that the results represent the entire Bangalore region, covering an area of [Formula: see text]. Fifty-eight 1-D and 20 2-D MASW surveys were performed and their velocity profiles determined. The average shear wave velocity of Bangalore soils was evaluated for depths of [Formula: see text], [Formula: see text], [Formula: see text], [Formula: see text], [Formula: see text] and [Formula: see text]. The sub-soil classification was made for seismic local site effect evaluation based on average shear wave velocity of [Formula: see text] depth [Formula: see text] of sites using the National Earthquake Hazards Reduction Program (NEHRP) and International Building Code (IBC) classification. Mapping clearly indicates that the depth of soil obtained from MASW closely matches with the soil layers identified in SPT bore holes. Estimation of local site effects for an earthquake requires knowledge of the dynamic properties of soil, which is usually expressed in terms of shear wave velocity. Hence, to make use of abundant SPT data available on many geotechnical projects in Bangalore, an attempt was made to develop a relationship between Vs (m∕s) and [Formula: see text]. The measured shear wave velocity at 38 locations close to SPT boreholes was used to generate the correlation between the corrected N values and shear wave velocity. A power fit model correlation was developed with a regression coefficient [Formula: see text] of 0.84. This relationship between shear wave velocity and corrected SPT N values correlates well with the Japan Road Association equations.

2007年能登半島地震の余震観測と微動探査による輪島市門前町の地盤震動特性の評価

2007年能登半島地震では，能登半島北部地域を中心に大きな被害が発生した。とくに，震源域の輪島市門前町中心部の走出や道下地域で木造家屋の倒壊が数多くみられた。この地域は，南北を山地に挟まれた八ケ川沿いに広がっており，地盤条件は良好ではない。Yamanaka et al.（2008）は，この地域で余震観測を実施し，被害の多かった地点では表層地盤による地震波の増幅を示唆する明瞭な卓越周期が存在することを明らかにした。こうした地盤増幅特性を理解するために，この余震観測が行われた７地点において半径1mから1kmまでの複数のアレイで微動の上下成分の観測を実施し，周期0.1秒から３秒の間でレイリー波の位相速度を求めた。各地点での位相速度の逆解析により，深部および表層地盤の１次元Ｓ波速度構造を明らかにした。これらの地盤モデルを用いて，門前町地域でのＳ波の地盤増幅特性について検討した。表層地盤に対するＳ波の１次元理論増幅特性の卓越周期は，余震観測による地盤増幅特性の卓越周期とほぼ一致しており，地盤モデルが妥当なものであることがわかった。また，観測増幅特性のピーク振幅は，得られた地盤モデルの深さ30ｍまでの平均Ｓ波速度と線形関係にあることがわかった。しかし，ピーク振幅の絶対値は，１次元地盤モデルでは説明できず，表層地盤の２・３次元構造の影響の可能性が示唆された。

Improvement of H/V technique by rotation of the coordinate system

In this work we investigate the application of the rotation of the coordinate system before applying denoising techniques aimed at improving the H/V spectral ratio for seismic site effects estimation. By this rotation, one can detect the direction of maximum and minimum matching between any couple of orthogonal H/V ratios. The direction of minimum matching should represent the direction of maximum anisotropy of the surface layer or a directional noise. To further improve the method, singular spectrum analysis and WASEE techniques are then applied.

Development and application of a GIS-based tool for earthquake-induced hazard prediction

An integrated GIS-based tool (GTIS) was constructed to estimate site effects related to the earthquake hazards in the Gyeongju area of Korea. To build the GTIS for the study area, intensive site investigations and geotechnical data collections were performed and a walk-over site survey was additionally carried out to acquire surface geo-knowledge data in accordance with the procedure developed to build the GTIS. For practical applications of the GTIS used to estimate the site effects associated with the amplification of ground motion, seismic microzoning maps of the characteristic site period and the mean shear wave velocity to a depth of 30 m were created and presented as a regional synthetic strategy addressing earthquake-induced hazards. Additionally, based on one-dimensional site response analyses, various seismic microzoning maps for short- and mid-period amplification potentials were created for the study area. Case studies of seismic microzonations in the Gyeongju area verified the usefulness of the GTIS for predicting seismic hazards in the region.

Improvement of HVSR technique by wavelet analysis

In this work we investigate the application of the wavelet analysis to improve the well-known horizontal to vertical spectral ratio (HVSR) technique for site effect estimation. Wavelet analysis has been widely used in the last decade for its capacity to localise the signal in both the time and frequency domains. The wavelet packet transform and the evaluation of a cost function for every node permit one to remove from the signal the least important components and lead to significant improvements in the identification of the fundamental frequency. In this paper a new algorithm is presented based on wavelet analysis, that allows, in some cases, to improve the identification of the peak in the QTS ratio. A number of examples of application are also presented.

Seismic site effect estimation in the city of Rabat (Morocco)

This study is based on a site survey carried out in the capital city of Morocco, Rabat. For this aim, we used a technique based on the recording and processing of seismic ambient noise to characterize local geological conditions in terms of the dynamic response of soil during earthquakes. This technique consists of an assessment of the spectral ratio of the horizontal to vertical components of microtremors recorded at the surface during a few minutes, and allows determination of the fundamental period and the maximum amplification factor at the site of measurement. We applied this technique in the city of Rabat to obtain distribution maps of dominant periods and amplification factors assessed in more than 250 sites. In order to discuss the results of this study, we used the information available in this zone about the lithology and topography to correlate the obtained values of dominant periods and amplification factors with the ground conditions. Finally, we established a microzoning map of the city of Rabat based on the contours of dominant periods. The microzoning map established in this study can be used by engineers and decision makers for urban and land use planning and also as a guide in reduction of the seismic vulnerability of buildings.

Estimation of local site effects in Ojiya city using aftershock records of the 2004 Mid Niigata Prefecture earthquake and microtremors

AbstractAftershock observations of the 2004 Mid Niigata Prefecture earthquake were conducted in the central part of Ojiya city, the Niigata prefecture, central Japan, to investigate local site effects. We installed eight accelerographs in the vicinity of the K-NET and JMA stations in the area. The stations of the aftershock observations are situated under different geological conditions including one installed in a mountainous area on Tertiary layers to serve as a reference site. We examined the ground-motion characteristics of the records for a Mj 6.1 aftershock focusing on local site effects. The amplification, at a period of less than 1 sec, is the largest in the vicinity of the K-NET station. The amplification at periods longer than 2 sec is larger in the western part of the city than those in the east. We estimated theS-wave velocity structure in the sediments above the basement with anS-wave velocity of 3.4 km/sec from the inversion of phase velocities measured by the array observations of vertical microtremors. We discuss the amplification factors using theS-wave velocity profile and show that shallow soils over the layer with anS-wave velocity of 0.49 km/sec are responsible for the amplification at periods shorter than 0.4 sec. Deeper sedimentary layers are needed to explain amplification at periods of 1 sec.

Ambient noise horizontal-to-vertical spectral ratio in site effects estimation and correlation with seismic damage distribution in urban environment: the case of the city of Thessaloniki (Northern Greece)

The validity of the estimation of seismic site response characteristics from ambient noise measurements was investigated in the downtown district of the city of Thessaloniki (Northern Greece), which was strongly affected by the 20/6/1978 ( M=6.5) damaging earthquake. For this purpose 250 ‘single site’ ambient noise measurements were performed in a dense grid of points covering the center of the city. The ambient noise H/ V spectral ratio for each site was calculated and the fundamental frequency (fo) and corresponding H/ V amplitude level (Ao) were estimated. Contour maps of both, fo and Ao, were compared with results from geological and geotechnical studies as well as with macroseismic data of the 1978 earthquake and were found to be well correlated. These comparisons provide strong evidence that ambient noise measurements properly processed with the ( H/ V) spectral ratio technique can be used as an inexpensive and fast tool for microzonation studies in urban environments.

Evaluation of seismic site factors in the Mississippi Embayment. I. Estimation of dynamic properties

Deep deposits of the Mississippi Embayment, overlying the New Madrid Seismic Zone, present unique challenges for the estimation of local site effects on propagated ground motion. Limited information is available on the dynamic properties of these deposits. This paper develops generalized depth dependent modulus degradation and damping curves specific to the embayment deposits. Depth dependent small strain damping is estimated using weak motion recordings during the Enola earthquake, 2001. Modulus degradation and damping curves are further constrained using limited laboratory test data of embayment soils at low confining pressure. At high confining pressures embayment specific data is unavailable and test data from outside the embayment is used in developing the dynamic properties. The representative modulus degradation and damping curves are used in non-linear and equivalent linear one-dimensional site response analyses. A companion paper describes a large-scale probabilistic seismic hazard analysis study in the Mississippi Embayment that integrates non-linear site effects.

P wave attenuation structure by spectral inversion of seismic waveforms

We introduce the 3-D attenuation tomographic approach that combines the frequency-domain inversions scheme consisting of a set of sub-inversions and the site effect estimation scheme based on the residual spectra. We apply it to data observed at the Japanese Islands to estimate three-dimensional P wave attenuation structure beneath the Japanese islands. The results show that the Qp-1 distribution is highly variable over the entire Japanese Islands, with the spatial wavelength of 2-3 degrees. This heterogeneous material property of the underground structure may be related to a tectonic state beneath the Japanese Islands.

Observation of aftershocks of the 2003 Tokachi-Oki earthquake for estimation of local site effects

Observation of aftershocks of the 2003 Tokachi-Oki earthquake was conducted in the southern part of the Tokachi basin in Hokkaido, Japan for estimation of local site effects. We installed accelerographs at 12 sites in Chokubetsu, Toyokoro, and Taiki areas, where large strong motion records were obtained during the main shock at stations of the K-NET and KiK-net. The stations of the aftershock observation are situated with different geological conditions and some of the sites were installed on Pleistocene layers as reference sites. The site amplifications are investigated using spectral ratio of S-waves from the aftershocks. The S-wave amplification factor is dominant at a period of about 1 second at the site near the KiK-net site in Toyokoro. This amplification fits well with calculated 1D amplification of S-wave in alluvial layers with a thickness of 50 meters. In addition to the site effects, we detected nonlinear amplification of the soft soils only during the main shock. The site effects at the strong motion site of the K-NET at Chokubetsu have a dominate peak at a period of 0.4 seconds. This amplification is due to soft soils having a thickness of about 13 meters. Contrary to the results at the two areas, site effects are not significantly different at the stations in the Taiki area, because of similarity on surface geological conditions.