Site Effect Study in Urban Area: Experimental Results in Grenoble (France)

—?Three methods are used to determine the site effect in the town of Grenoble, located in the Western Alps. First we use the classical spectral ratio method in 14 sites to calculate the transfer function of the basin. We find an amplification of 10 in the frequency range of 0.25 to 10?Hz. Second, we compare these results with the H over V spectral ratio method, and propose a map of resonance frequency of the basin. We find a lower resonance frequency in the center of the basin than on the edge, that is consistent with the structure deduced from a gravity Bouguer anomaly map. Finally we use the empirical Green's function method to simulate a M _w 5.5 earthquake at a distance of 20?km from the town. The simulated acceleration reaches the level of 2?m/s² in the center of the basin compared to 0.2?m/s² on the edges. The simulated ground motion we compute is smaller than the French seismic codes on the edge of the valley but significantly larger in the center.     

Response spectral amplification ratios from 1- and 2-dimensional nonlinear soil site models

In order to determine the effect of geometry on the ground response of 2-dimensional (2-D) basins filled with soils that can develop nonlinear response, we use three basin models with width/depth ratios 3, 6 and 10. The three basins are subjected to a suite of rock site records with various magnitudes and source distances. We compute response spectral amplification ratios at four locations on the surface of the 2-D basins, and determine the average variation of the amplification ratios with respect to excitation spectra, for peak ground acceleration (PGA) and 3 spectral periods of 0.2, 0.5, 1 s. Similarly, we compute the average response spectral amplification ratios for two 1-dimensional (1-D) nonlinear models, one having the soil profile at the basin centre and the other having a soil profile at half the depth of the basin. From the relationship between the average amplification ratios and excitation spectra, we determine the cross-over point in terms of excitation spectral values that separate the amplification range from the deamplification range. Our results show that the cross-over point varies significantly from one location to another on the ground surface and from one basin to another, in a range of 0.3–1.1g for PGA. The effects of basin geometry are very strong at weak and moderate excitation, but decrease with increasing excitation spectra in a significant portion around the basin centre. Our results provide some justification for using 1-D models for 2-D basins with a width/depth ratio ?6 if the soil site is subjected to strong ground shaking.     

Site effect and damage distribution in Pujili (Ecuador) after the 28 March 1996 earthquake

The 28 March 1996 earthquake (Mw=5.7) produced extensive damage in Pujili, a small town located in the central part of the Inter-Andean valley of Ecuador. Variations in the damage rate of adobe constructions throughout the city let us suppose a ground motion amplification related to site effects. A site effect study using the H/V spectral ratio confirmed the good agreement with geological formations and showed that a second peak in some soil response appears at sites that are located over a zone of alluvial deposits, in the most damaged area of the city. This second peak is in the frequency range 5–7 Hz, close to the natural response of adobe building frequency. Thus, the second peak seems to increase the damage rate of adobe buildings and may be related to superficial alluvial deposits of a river, acting independently of the rest of the soil column.     

Effects of surface topography on seismic ground response in the Egion (Greece) 15 June 1995 earthquake

The Greek coastal town of Egion on 15 June 1995 was shaken by a strong, small epicentral distance, earthquake that caused heavy damages to buildings and loss of life. The damages were concentrated in the central elevated part of the town whereas the flat coastal region remained almost intact. This non-uniform distribution of damage is studied in this article in terms of surface topography effects by conducting seismic response analyses of a simplified 2-D profile of the town. A dynamic finite element code implementing the equivalent-linear soil behavior (FLUSHPLUS) was used for the analyses and it was found that the step-like topography amplified greatly the intensity of motion without affecting its frequency content. The analyses showed that the motion recorded by an accelerograph installed at the center of the town is in agreement with the computed values; they also indicated a particularly intense amplification close to the crest of the steep slope, where a multi-story RC residential building partially collapsed. In contrast, the level of motion was found to be low at the flat coastal zone of the town where the earthquake damages were insignificant. It is concluded that the characteristic surface topography of the town played an important role in modifying the intensity of base motion.     

Application of horizontal-to-vertical (H/V) Fourier spectral ratio for analysis of site effect on rock (NEHRP-class B) sites in Taiwan

The frequency-dependent amplification for rock (NEHRP-class B) sites was studied using earthquake ground-motion database collected in Taiwan during implementation of the Taiwan Strong Motion Instrumentation Program. The database used includes several hundred records from earthquakes of M_L 4.0–7.3 occurred between 1993 and 2004. The characteristics of amplification were evaluated using the well-known technique of horizontal-to-vertical Fourier spectral ratio (H/V) of the S-wave phase [Lermo J, Chavez-Garcia FJ. Site effect evaluation using spectral ratios with only one station. Bull Seism Soc Am 1993;83:1574–94]. The study allows us to analyze peculiarities of rock sites amplification in Northern and Eastern Taiwan. It was suggested to divide the NEHRP-class B site amplification into four types based on frequency of maximum amplification and the shape of amplification function. The applicability of the technique was also checked for a few stiff and soft soil sites (NEHRP-classes D and E).     

Study of site effects in the area of Nocera Umbra (Central Italy) during the 1997 Umbria-Marche seismic sequence

During the M_L = 5.6 and 5.8 earthquakes occurredin central Italy on 26 September 1997 the historiccentre of Nocera Umbra, lying on top of a 120 m highhill, was diffusely damaged (VII-VIII degrees of MCSintensity). Some recently built houses in the modernpart of the town suffered an even higher level ofdamage. A temporary seismic array was deployed toinvestigate a possible correlation between localamplifications of ground motion in this area and theobserved pattern of damage. After a geologic andmacroseismic survey, eight sites were selected asrepresentative of different local conditions, such astopographic irregularities, sharp hard-to-softlithology transitions, alluvium-filled valleys, andboth undisturbed and deformed rocks.Horizontal-to-vertical spectral ratios for bothmicrotremor and earthquake recordings, as well asspectral ratios referred to undisturbed rock sites,were used to quantify local variations of groundmotion. In spite of the diffuse damage in the historiccentre of Nocera Umbra, a small amplification isobserved at the stations on the hill's top. Thissuggests that the higher vulnerability of the ancientbuildings mainly accounts for the diffuse damage inthat part of the town. In the frequency band ofengineering interest (1 to 10 Hz) the largestamplifications of ground motion are found at softsites: in the Topino river valley, where many episodesof severe structural damage occurred, spectralamplification is significant over a broad frequencyband ranging from 2 Hz to more than 20 Hz. Inparticular, in the central part of the valley highamplification (> 4) is found from 3 to 10 Hz,reaching a maximum of 20 around 4 Hz. At the edge ofthe valley, close to the soil-to-rock transition,amplification is as large as 10 in a frequency bandranging from 4 to more than 20 Hz. A significantamplification (by a factor of 10 around 10 Hz) isobserved also at one of the rock sites, possibly dueto the presence of a cataclastic zone related to theactivity of a regional fault that altered themechanical properties of the rock.     

Contributions of Basin-edge-induced Surface Waves to Site Effect in the Dinar Basin,Southwestern Turkey

Ground motion amplifications in the Dinar basin, and contributions of the surface waves generated from basin edges are investigated in frequency and time domains. Amplification functions are computed from the aftershock data of the October 1, 1995 Dinar earthquake (M_W=6.4) using the Standard Spectral Ratio method which requires a pair of instruments; one located at the site under investigation (generally on alluvium) and the other on a reference site, preferably a nearby rock site. First, a time window covering the whole signal is used to compute the amplification function, and, successively, the noise, P wave, S wave and the surface wave time windows are used in computation to observe their contributions to the amplification function. It is seen that the maximum amplifications observed at about 2.0 Hz on the amplification functions of the stations located in the basin are largely due to basin edge induced surface waves. These waves have significantly increased the duration of signals recorded within the basin. Particularly, on the vertical component records, the amplitudes of surface waves are larger than the S-wave amplitudes. The periods of waves amplified maximally due to the basin structure coincide with the natural periods of 4–6-story buildings which were heavily damaged in Dinar. This indicates that the site effects may have been important regarding the damage which occurred during the Dinar earthquake of October 1995.     

Simulating the Taipei basin response by numerical modeling of wave propagation

Taipei, the capital of Taiwan, suffered from destructive earthquakes four times during the 20th century (M _L = 7.3 on April 15, 1909; M _L = 6.8 on November 15, 1986; the Chi–Chi M _L = 7.3 earthquake on September 21, 1999; and M _L = 6.8 on March 31, 2002). Analysis of recorded data shows a strong dependence of spectral amplification in the Taipei Basin on earthquake depth and azimuth. At low frequencies (f < 3 Hz) significant larger amplifications are observed for shallow earthquakes as compared to intermediate depth events. The former ones also display strong azimuthal dependence. As structures with large response periods such as bridges and tall buildings are sensitive to these low frequencies the understanding of the associated wave effects within the basin and their role for site effect amplification is critical. The tool we employ is 3D finite-difference modeling of wave propagation of incident wave fronts. The available detailed model of the basin allows studying the wave effects. Modeling clearly reveals that basin edge effects as observed in data are related to surface wave generation at the basin edges with a high degree of azimuthal dependency. The reproduced site amplification effects are in qualitative agreement with the observations from strong motion data.     

Long period microtremors, microseisms and earthquake damage: Northridge, CA, earthquake of 17 January 1994

Three studies of site amplification factors, based on the recorded aftershocks, and one study based on strong motion data, are compared one with another and with the observed distribution of damage from the Northridge, CA, earthquake of 17 January 1994 (M_L=6.4). In the epicentral area, when the peak ground velocities are larger than v_m≈15 cm/s, nonlinear response of soil begins to distort the amplification factors determined from small amplitude (linear) wave motion. Moving into the area of near-field and strong ground motion (v_m>30 cm/s), the site response becomes progressively more affected by the nonlinear soil response. Based on the published results, it is concluded that site amplification factors determined from small amplitude waves (aftershocks, small earthquakes, coda waves) and their transfer-function representation may be useful for small and distant earthquake motions, where soils and structures respond to earthquake waves in a linear manner. However in San Fernando Valley, during the Northridge earthquake, the observed distribution of damage did not correlate with site amplification determined from spectra of recorded weak motions. Mapping geographical distribution of site amplification using other than very strong motion data, therefore appears to be of little use for seismic hazard analyses.     

Seismicity induced by gas production: II. Lithology correlated events,induced stresses and deformation

A 3D relocation technique permits precise locations of induced earthquakes. Geostatistical processing using the data of 87 boreholes provides the basis of a precise 3D structure, with a dome geometry. Conventional laboratory mechanical tests performed on deep rock samples (1000 m to 5000 m) define the rock properties at depths similar to those of the seismic events (1<M _L<4.2) that range from 1 to 7 km.In the studied period, most (85%), of the events were located above the gas reservoir, with very few located in the reservoir itself. Because the production parameters (50 MPa depletion of the gas pressure reservoir) are homogeneous throughout the gas field, the lateral inhomogeneity of the seismic rupture locations are a consequence of variations in the rheological response of the dome to the deformation induced by gas production.Here a ratio of two is found between the elastic modulus of the seismic rock matrix and the elastic modulus of the aseismic rock matrix. The contrast in strength is at least as great, if not greater. Repeated measured surface deformations involve the whole structure. Spatial and temporal deformations indicate that aseismic deformation is quantitatively the main process of this structural deformation. The heterogeneous stress pattern inferred fromP-axes of induced earthquakes disagrees with the tectonic regional stress field. The radial distribution ofP-axes towards the gas reservoir probably reflects the production induced deformation. The inferred deformation of the dome occurs in response to weak induced stresses.     

Side-effect of using response spectral amplification ratios for soft soil sites—Earthquake source-type dependent amplification ratios

Our previous studies show that site effects (amplification of rock motions), source and path effects are coupled when response spectra are used to characterize the amplification ratios for a soil site modelled as nonlinear or elastic. The coupling is referred to as a “side effect” of using response spectral amplification ratios. In the present study we use a suite of rock site records, well distributed with respect to magnitude and source distance, from crustal, subduction interface and slab earthquakes to evaluate the response spectral amplification ratio for soft soil sites. We compare these side-effects for ground motions generated by three types of earthquakes, and we find that, at periods much shorter or much longer than the natural period of a soil site modelled as elastic, the average amplification ratios with respect to rock site ground motions from three types of earthquakes are moderately different and are very similar for other spectral periods. These differences are not statistically significant because of the moderately large scatter of the amplification ratios. However, the extent of magnitude- and source-distance-dependence of amplification ratios differs significantly. After the effects of magnitude and source distance on the amplification ratios are accounted for, the differences in amplification ratios between crustal and subduction earthquake records are very large in some particular combinations of source distance and magnitude range. These findings may have potential impact in establishing design spectra for soft soil sites using strong motion attenuation models or numerical modelling.     

Site characterizations for the Tehran network (TDMMO) in Tehran Region Using Micro-Earthquake,Microtremor and Quarry Blast Data

We did a Study of Horizontal-to-Vertical Component Spectral Ratio in the Tehran seismic zone. Micro-earthquakes, microtremors and quarry blasts data were used as an estimation of the site response in the Tehran zone. Site effects were studied based on horizontal to vertical ratios by the Nakamura׳s technique. Also, we used the spectra of signals for three components with the lowest noise levels for spectral slope studies. The analysis used seismic events from a network of 13 seismic stations by the permanent local seismological network of the Tehran Disaster Mitigation and Management Organization (TDMMO) from 2004 to 2007. The number of events used were different for each station. Quarry blast events were with 1.2≤M_L≤2.2 and micro-earthquakes were with 1.1≤M_L≤4.1.By comparing results for earthquake, microtremor and quarry blast, we could see that there is a significant difference between them. The data showed clear observations, especially in high-frequencies. The H/V spectral ratios indicate dominant frequency for rock/soft site with a higher ratio level for quarry blast ratios, which are comparable to the earthquake results due to their difference sources. The results derived by spectral H/V ratios and spectral analysis may be used to distinguish between local earthquakes and quarry blasts.     

Evidence of nonlinear site response in horizontal-to-vertical spectral ratio from near-field earthquakes

The issue is addressed as to whether the horizontal-to-vertical spectral ratio (HVSR) method is sensitive to the amplitude of ground motion from near-field earthquakes. Twenty-one three-component accelerograms from two closely located similar soil sites in the town of Lefkas are used. The recordings represent 17 earthquakes covering a wide range of magnitudes, epicentral distances and azimuths. Peak horizontal accelerations (PGA) and velocities (PGV) lie in the ranges 20–540 cm/s² and 1.4–55.2 cm/s. For each HVS ratio, the site's fundamental-resonance frequency, f_res, is determined visually. Linear correlation analysis shows that f_res is strongly (negatively) correlated to PGA and PGV (r between −0.7 and −0.8); no correlation is found with resonance amplitude or epicentral distance. We show that the observed correlation is attributable to soil nonlinearity and indicate how weak-motion estimates of f_res can be corrected for use in assessing site response during strong shaking.     

Side effect of using response spectral amplification ratios for soil sites—variability and earthquake-magnitude and source-distance dependent amplification ratios for soil sites

We investigate a special type of variability in response spectral amplification ratios computed from numerical “engineering” models for a soft soil site. The engineering models are defined by shallow soil layers over “engineering” bedrock with a shear-wave velocity over 600–700 m/s and the model is subjected to vertical propagating shear waves. The variability, perhaps unique in earthquake engineering, is a result of the “perfectly accurate” computational procedure. For example, an engineering soil site model, subjected to two rock site records or the two horizontal components of a rock site record, produces different response spectral amplification ratios. We use a large number of strong-motion records from “engineering” rock sites, with a reasonably balanced distribution with respect to magnitude and source distance, generated by subduction earthquakes in Japan, to investigate the nature of the variability. In order to avoid any approximation in removing the effect of soil nonlinear response, we use a simple model, a single horizontal soil layer over a bedrock, modelled as elastic. We then demonstrate that a similar type of variability observed in the one- or two-dimensional nonlinear soil models is caused by the nature of response spectral amplification ratios, not a direct result of soil nonlinear response. Examination of variability reveals that the average of response spectral amplification ratios systematically depends on both earthquake magnitude and source distance. We find that, at periods much longer than the site natural periods of the soil sites, the scatter of the amplification ratios decreases with increasing magnitude and source distance. These findings may have a potential impact in establishing design spectra for soft soil sites using strong-motion attenuation models or dynamic numerical modelling.     

Site effects of the 2002 Molise earthquake, Italy: analysis of strong motion, ambient noise, and synthetic data from 2D modelling in San Giuliano di Puglia

On 31 October and 1 November 2002, the Basso Molise area (Southern Italy) was struck by two earthquakes of moderate magnitude (M _L = 5.4 and 5.3). The epicentral area showed a high level of damage, attributable both to the high vulnerability of existing buildings and to site effects caused by the geological and geomorphological settings. Specifically, the intensity inside the town of San Giuliano di Puglia was two degrees higher than in neighbouring towns. Also, within San Giuliano di Puglia, the damage varied notably. The site response in the city was initially evaluated from horizontal-to-vertical spectral ratios (HVSR) from a limited number of strong motion recordings of the most severe aftershocks. Several microtremor measurements were also available. Both data sets indicated the simultaneous presence of two amplification peaks: one around 6 Hz, attributed in previous studies to the strong, shallow impedance contrast among landfill/clay and calcarenites, and one at 2 Hz related to the first S-wave arrivals and predominantly seen only on one receiver component. Further studies performed on weak-motion recordings also showed strong amplification on the vertical receiver component, thus indicating an underestimation of the amplification by the HVSR technique. Additionally, a 2D-model of the geology of the sub-surface was developed, reproducing the flower-shaped structure generated during the late orogenic transpressive regime. The numerical (finite-difference hybrid) simulation reproduced the two peaks of the observed data at slightly higher frequencies. The model also confirmed that the borders of the flower structure define a boundary between amplification levels, with higher amplification inside.     

Effects of Impedance Contrast and Soil Thickness on Basin-Transduced Rayleigh Waves and Associated Differential Ground Motion

This paper presents the effects of impedance contrast (IC) across the basin edge, velocity contrast between the basin and underlying bedrock, Poisson’s ratio and soil thickness on the characteristics of basin-transduced Rayleigh (BTR) waves and associated differential ground motion (DGM). Analysis of simulated results for a two-dimensional (2D) basin revealed complex mode transformation of Rayleigh waves after entering the basin. Excellent correlation of frequencies corresponding to different spectral ratio peaks in ellipticity curves of BTR waves and spectral amplification peaks was obtained. However, such correlation was not observed between values of peaks in ellipticity curves and spectral amplification at the corresponding frequencies. An increase of spectral amplification with IC was obtained. The largest spectral amplification was more than twice the IC in the horizontal component and more than the IC in the vertical component in the case of large and same impedance contrast for P- and S-waves. It was concluded that the frequency corresponding to the largest spectral amplification was greater than the fundamental frequency of soil by around 14% and 44% in the vertical and horizontal components, respectively. Spectral amplification of the vertical component was negligible when soil thickness was less than around 15–20 times the S-wave wavelength in the basin. The largest values of peak ground displacement (PGD) and peak differential ground motion (PDGM) were obtained very near the basin edge, and their values with offset from the edge were strongly dependent on the IC across the basin edge, Poisson’s ratio, velocity contrast between the basin and underlying bedrock (dispersion), damping and soil thickness. The obtained value of PDGM for a span of 50 m in the horizontal and vertical components due to the BTR wave was of the order of 0.75 × 10^?3 and 1.32 × 10^?3 for unit amplitude (1.0 cm) in the horizontal component of the Rayleigh wave at rock very near the basin edge.     

Response of sedimentary basin to obliquely incident SH waves

This research studies the impact of the incident angle of SH waves on the seismic response of two-dimensional sedimentary basins by using a nonlinear method. At first Ricker wavelet is input for a detailed analysis, followed by a statistical analysis based on a total of 100 real earthquake motions recorded at rock sites. The results show that the incident angle has a significant implication on the basin ground motion. First, the incident angle affects the short-period components more than the long-period ones of the spectral response acceleration, but the dominant period of the spectral response acceleration is insensitive to incident angle and location. Second, the MDIA of a basin is not necessarily 0° (vertical incidence) but in the range of approximately 0°–30°, and hence due attention should be paid to the influence of incident angle in seismic response analysis. Third, basin central areas are seismically preferable to edge regions for short-period buildings located on the basin, while, for long-period buildings, the edge areas become preferable. However, with the increase in incident angle, the difference between edge and central areas diminishes gradually. Finally, given that the dimensions of a basin are perceivable to incidence waves, the slope angle has a considerable impact on the PGA distribution pattern by controlling whether or not peak appears in the edge area. The MDP is most likely to be in the edge area of a basin with small slope angle when subjected to excitation with small incident angle (including vertical incidence).     

Separation of Source, Propagation and Site Effects from S Waves¶of Local Earthquakes in Bursa Region, Northwestern Turkey

—?We have used micro-earthquake recordings (M= 1.8–4.1) of local events in the distance range of 5–60?km in order to quantify the attenuation and site effects in the vicinity of the Bursa city, Marmara region, Turkey. The data set consists of 120 three-component recorded accelograms from 69 earthquakes, recorded at six stations. Each station is deployed on different geologic units, such as massive limestone, slope deposit and Quaternary young sediments, in the framework of the Marmara Poly-Project.¶In this study a nonparametric inversion method was applied to acceleration records from the Bursa region to estimate source, site and path effects using a two-step inversion. At the first step, we determined attenuation functions by analyzing the distance dependence of the spectral amplitudes and retrieved values of Q _s (f) = 46.59f ^0.67. At the second step, the corrected S-waves spectral records for the attenuation function, including the geometrical spreading effect, were inverted to separate source and site response for 21 different frequencies selected between 0.5 and ～25?Hz. The near-surface attenuation, κ value, was also estimated by using the model proposed by Anderson and Hough (1984) at each site. We observed that κ₀ is smaller for stations located on rock site (I?dιr, SIGD, κ₀～0.004) compared to the one that is located on Neogene sediment (Çukurca, SCKR, κ₀～0.018).¶Site amplifications from inversion showed that the station located within the Bursa basin, Çukurca (SCKR), is the most important site with about 4.0 amplification value at 1.8?Hz. Demirta? (SDEM) amplifies the spectral amplitudes about 3.0 times at 2.0?Hz, SHMK about 3.5 times between 2.5 and 3.5?Hz and SHMT nearly reaching 3.5 times between 1.5 and 4.0?Hz. However, stations located on the Uluda? Mountain Massif (SKAY and SIGD), which correspond to a deep limestone geological unit, have the smallest amplification, that values between 0.6 and 1.4.     

Study of Basin-edge Effects on the Ground Motion Characteristics Using 2.5-D Modelling

This paper presents the role of basin-edge geometry in the generation of surface waves using 2.5-D modelling. The simulated responses of various basin-edge models revealed surface wave generation near the basin edge and their propagation normal to the edge. Seismic responses of basin-edge models using different fundamental frequency of soil along with spectral analysis of differential ground motion confirmed that surface waves start generating near the basin edge when body-wave frequency exceeds the fundamental frequency of soil. Spectral analysis of differential ground motion also confirmed the generation of high frequency surface wave. An increase of surface-wave amplitude with soil thickness was obtained. Large ground displacement observed near the basin edge may be due to the interference of surface/diffracted waves with the direct waves and their multiples. The effect of edge roughness on the surface-wave characteristics was found to be negligible as compared with the edge geometry. Simulated results revealed a decrease of surface-wave amplitude with edge slope, particularly in the case of surface waves caused by S waves. Surface wave generation near the basin edge was obtained for all four considered angles of incidence. At the same time, it was also inferred that the characteristics of these surface waves depend on the angle of incidence to some extent. The findings of this paper reveal that basin-edge effects deserve a particular attention for the purpose of earthquake-resistant design and seismic microzonation.Acknowledgement Financial assistance by the Department of Science and Technology (DST) New Delhi and the Indian National Science Academy (INSA), New Delhi is gratefully acknowledged.     

Evidence for fault-related directionality and localized site effects from strong motion recordings of the 2003 Boumerdes (Algeria) earthquake: Consequences on damage distribution and the Algerian seismic code

The Algiers–Boumerdes region has been struck by a destructive magnitude 6.8 (M_w) earthquake on May 21, 2003. The study presented in this paper is based on main shock strong motions from 13 stations of the Algerian accelerograph network. A maximum 0.58g peak ground acceleration (PGA) has been recorded at 20 km from the epicenter, only about 150 m away from a PGA of 0.34g, with both a central frequency around 5 Hz, explained by a strong very localized site effect, confirmed by receiver function technique results showing peaks at 5 Hz with amplitudes changing by a factor of 2. Soil amplifications are also evidenced at stations located in the quaternary Mitidja basin, explaining the higher PGA values recorded at these stations than at stations located on firm soil at similar distances from the epicenter. A fault-related directionality effect observed on the strong motion records and confirmed by the study of the seismic movement anisotropy, in agreement with the N65 fault plan direction, explains the SW–NE orientation of the main damage zone. In the near field, strong motions present a high-frequency content starting at 3 Hz with a central frequency around 8 Hz, while in the far field their central frequency is around 3 Hz, explaining the high level of damage in the 3- to 4-story buildings in the epicentral zone. The design spectra overestimate the recorded mean response spectra, and its high corner frequency is less than the recorded one, leading to a re-examination of the seismic design code that should definitively integrate site-related coefficient, to account for the up to now neglected site amplification, as well as a re-modeling of the actual design spectra. Finally, both the proposed Algerian attenuation law and the worldwide laws usually used in Algeria underestimate the recorded accelerations of the 6.8 (M_w) Boumerdes earthquake, clearly showing that it is not possible to extrapolate the proposed Algerian law to major earthquakes.