PARAMETRIC STUDY OF SEISMIC SITE RESPONSE UNDER SHEAR WA VE VELOCITY VARIATION OF BEDROCK

In site effects investigations, the only part of subsurface layers are considered cause amplify or reduce the seismic motions.This part of subsurface layers is located on hard layer that earthquake-induce waves are not amplified. This layer called seismic bedrock is defined based on shear wave velocity and may not absolutely geologic bedrock and alluvial layer with high elastic stiffness can be. According to the Uniform Building Code (UBC-1997), the seismic bedrock is defined as an environment that shear wave velocity is about 760 to 1500 m/sec. According to Ishihara and Ansal (1982) the shear wave velocity of the soil layers between 600 to 1000 m/s as seismic bedrock. The second (1999) and third (2005) edition of Iranian seismic code (Standard No. 2800), is introduced an environment with the shear wave velocity greater than 750 m/sec as seismic bedrock. According to the TC4 micro zonation guide, soil layers with shear wave velocities greater than 600 m/sec is considered as seismic bedrock. This code has recommended that shear wave velocity in seismic bedrock is greater than 3000 m/sec for tall building. In this research, profiles with the same average shear wave velocity of the top 30 m soil () value but in different shear wave velocity structures proportional with soil type (II, III, IV) Standard No. 2800 were produced. Profiles depth are extended for five different shear wave velocity (750, 1000, 1500, 2500 and 3000 m/sec) as seismic bedrock using the baseline profiles shown in Figure 1. The profiles in the top of 30 m that have VS30 constant with the profiles below 30 m continued similar to baseline values used from linear equation. The shear wave profiles for each VS30 the profiles all have the same average shear wave velocity Equation 1 as controlled by the baseline velocity profile at 30 m.

کلیدواژه ها:

Seismic Site Response ، Shear Wave Velocity ، Bedrock