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Evaluation of effective parameters on soil layers seismic amplification ratios (A case
study of Bam earthquake)
Évaluation des paramètres effectifs sur les ratios d'amplification sismique des couches de sol
(Une étude de cas de tremblement de terre de Bam)
Tabatabaie S.H.
Faculty member of Building and Housing Research Center, Tehran, Iran
M. Hassanlourad
Faculty of Engineering, Imam Khomeini International University, Qazvin, Iran.
Yazdanparast M.
Faculty of Engineering and Technology, Imam Khomeini International University, Qazvin, Iran.
Mohammadi A.
Senior expert of Building and Housing Research Center, Tehran, Iran.
ABSTRACT: The average of shear wave velocity in 30 m of top soil in boreholes is used in standard seismic codes like Euro Code
and Iranian Earthquake Code (2800) for finding the soil layer type underlined by bedrock. At least 50% of observed seismic
amplification ratio and surface destructions after bam earthquake did not verify the seismic codes expectations especially in locations
of E type soil layers, the soil layer in which its shear wave velocity is about lower than 360 m/s having the thickness of 5 to 20 m
underlined by a hard rock. In this research it is tried to find how stratifications and sub-layers can affect the seismic amplification
ratio. The depth of bedrock, its determination criteria and the thickness of sub-layers were investigated by comprehensive analysis in
several stages. In this procedure the EERA soft ware was used based on one dimensionally shear wave propagation theory in soil
layers. In addition the effect of sub-layers thickness in detailed sensitive analyses was studied. Analyses with respect to amplification
ratios in sub-layers that are classified as Euro Code and Iranian Earthquake Code (2800) based on their shear wave velocity, showed
the considerable relation between the thickness of E, B and D type soil layers and seismic amplification ratios.
RÉSUMÉ : La moyenne de la vitesse des ondes de cisaillement à 30 m de couches de sol au-dessus des trous de forage est utilisée
dans la norme des codes sismiques comme Code d’Euros et Code de Tremblement de terre iranienne (2800) pour trouve rle type de
couche de sol soulignée parle substratum rocheux. Au moins 50 % du ratio d’amplification sismique observée et destructions de
surface après le tremblement de terre de Bam n’a pas vérifié les attentes des codes sismiques surtout dans les lieux de couches de sol
de type E, la couche du sol où sa vitesse d’onde de cisaillement est d’environ inférieure à 360 m/s avec l’épaisseur de 5 à20 m
soulignée par une roche dure. Au travers de cette recherche, nous essayons de trouver comment les stratifications et les sous-couches
peuvent influer sur le ratio d'amplification sismique. La profondeur du substratum rocheux, ses critères de détermination et de
l’épaisseur des sous-couches ont été étudiés par analyse complète en plusieurs étapes. Dans cette procédure, le logiciel lEERA a été
utilisé basée sur une théorie de la propagation unidimensionnelle des ondes de cisaillement dans les couches de sol. En outre, l’effet
d’épaisseur de sous-couches a été étudié dans des analyses détaillées sensibles. Les analyses sen ce qui concerne les ratios
d’amplification dans les sous-couches qui sont classés comme Code d'Euros et Code de Tremblement de terre iranienne (2800) basées
sur leur vitesse de l’onde de cisaillement, a montré la relation importante entre l’épaisseur de couches de solde type E, B et D et les
ratios d’amplification sismique.
KEYWORDS: amplification ratio, shear wave velocity, EERA software.
1 INTRODUCTION.
In theory, the term of site amplification refers to the increase in
the amplification of seismic waves as they pass through soft soil
layers near the earth surface. The increase is due to the low
impedance of soil layers near the surface, where impedance is
defined as the product of mass density of soil and the wave
propagation velocity (Sefak2001).
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 modeled as
nonlinear or elastic (Zhao 2010, Zhang 2010, Irikura 2010) .
The evaluation of site amplification effects is recognized as
one of the most important activities of the seismology and
earthquake engineering (Ferrari et all 2010)and the existence of
soil amplification was amply demonstrated in many past
destructive earthquakes (Tezcan 2002,Kaya2002,Ozdemir2002).
Joint evaluation of Athens(Greece), earthquake of 7
September 1999 shows that very stiff soils of the Athens basin
compared to the nearby outcropping soft rocks have amplified
the peak horizontal acceleration (Bouckovalas 2001, Kouretzis
2001).
Site effect-like amplification- is one of the most important
factors in both seismology and earthquake engineering (Zhao
2009, Zhang 2009, Irikura 2009, Graizer 2009) .
The distribution of damage caused by earthquake ground
shaking commonly reflects real differences in local soil
conditions. Regional site conditions relevant for seismic hazard
studies can be derived from various geologic, seismologic and
geotechnical source (Kockar 2010, Akgun 2010,Rathje 2010).
Incoherences between seismic amplification that predicted
by Earthquake Codes and observations after Bam earthquake
(destructions of earthquake, primary analysis) show the
importance of detailed analysis due to find blind spots. Field
boring, soil sampling and several comprehensive analyses based
on one dimensionally shear wave propagation theory with
EERA software were done in this study.
