Seismic Ground Motion - Dynamic Behavior of Concrete and Seismic Engineering - page 113

a Q of 20 on a 200 m deep layer, with 200 m/s speed values, we calculate loss
factors of exp (- S/20) at 1 Hz (negligible), and of exp (- S) at 20 Hz (significant!).
This indicates that the high frequency decrease effect is due either to propagation
over long distances within poorly attenuating media or to a very superficial effect
within highly attenuating materials. We note that the values of Q found in the
literature generally reflect two phenomena: on the one hand, the intrinsic inelastic
attenuation, and on the other, energy losses through diffusion on short wavelength
heterogenities - this often results in a frequency dependency of Q that is thought to
be linked to the size of the heterogenities. This dependency is expressed generically
as:

Į

Q = Q f with 0 Į 1 dd

0

3.4.3. Site effects

Seismic motions can also be noticeably modified by close surface geological
conditions. This is evident by comparing their wavelengths (O = c/f, for c varying
from 100 m/s to 2 km/s, and f varying from 0.5 to 10 Hz, i.e. for O varying from 10
m to 4 km), with the dimensions of surface heterogenities that are in the same range.
Because such heterogenities can be quite important, both mechanically and
geometrically, the interferences between the incident wave fields can be very intense
and cause modifications of the spectral characteristics of seismic motions.

Seismic effects are thus wave propagation effects leading to amplifications (or
de-amplifications) located in space which can reach very high factors (greater than
10 in some extreme cases). They mainly affect topographic reliefs and sedimentary
fillings. In the next section, we briefly explain the basic physics for these typical
configurations. The curious reader will find a more comprehensive description in
[BAR 99], which discusses the remaining unsolved questions, and gives an account
of the diverse estimation methods used.

We also feel it is important to mention an additional site effect category - those
characterized by ground located failures caused by intense vibrations. These include
liquefaction affecting saturated sandy soils (causing subsidence) and slope
instability (landslips, sliding, both superficial and deep). If the latter arise in static
conditions, the earthquake is only a starting element, but the former are specific to
earthquakes. Taking both of these phenomena into account depends more on
particular dispositions (especially for foundations or ground improvement) than
structure calculation, so they are not covered in detail here.