NATIONAL TECHNICAL UNIVERSITY OF ATHENS
LABORATORY OF EARTHQUAKE ENGINEERING
Site effects on ground motion
Ioannis N. Psycharis
Site effects
Ground motion propagation from source to site
NATIONAL TECHNICAL UNIVERSITY OF ATHENS
LABORATORY FOR EARTHQUAKE ENGINEERING
(Kramer, 1996)
Ioannis Psycharis
Site effects on the ground motion
Site effects
● Local ground response
Influence of the soil response on the seismic motion at the
ground surface.
Usually it is considered through the nonlinear one-dimensional
response of a soil column.
● Basin effects
Influence of two- or three-dimensional sedimentary basin
structures on ground motions, including wave reflections and
surface wave generation at basin edges.
● Effect of surface topography
♦ Ridges
♦ Canyons
♦ Slopes
NATIONAL TECHNICAL UNIVERSITY OF ATHENS
LABORATORY FOR EARTHQUAKE ENGINEERING
Ioannis Psycharis
Site effects on the ground motion
Basic definitions
● Free surface motion
= the motion at the surface of a soil deposit
● Bedrock motion
= the motion at the base of a soil deposit
● Rock outcropping motion
= the motion at a location where bedrock is exposed at the
ground surface
Free surface
Rock outcropping
Bedrock
NATIONAL TECHNICAL UNIVERSITY OF ATHENS
LABORATORY FOR EARTHQUAKE ENGINEERING
Ioannis Psycharis
Site effects on the ground motion
Local ground response
Effect of soil on response spectra
Average normalized response spectra for 107 earthquake records
grouped in four soil categories (Seed et al. 1976)
NATIONAL TECHNICAL UNIVERSITY OF ATHENS
LABORATORY FOR EARTHQUAKE ENGINEERING
Ioannis Psycharis
Site effects on the ground motion
Ground motion amplification
Average spectral amplification vs. Vs-30 recorded during the
1989 Loma Prieta earthquake (Borcherdt & Glassmoyer, 1994)
Amplification of SA
Amplification of SV
(average for T=0.1-0.5 s)
(average for T=0.4-2.0 s)
NATIONAL TECHNICAL UNIVERSITY OF ATHENS
LABORATORY FOR EARTHQUAKE ENGINEERING
Ioannis Psycharis
Site effects on the ground motion
Soil classification (EC8)
Typically based on the average shear wave velocity at the top 30
m of the soil profile (e.g. EC8)
30
v S ,30 
hi

i1 ,N v i
▪ hi = thickness of layer (m)
▪ vi = elastic shear wave velocity
▪ Ν = no. of layers at the top 30 m of soil deposit
Ground
type
Description
A
Rock or other rock-like geological formation
Deposits of very dense sand, gravel, or very
stiff clay
Deep deposits of dense or medium-dense sand,
gravel or stiff clay
Deposits of loose-to-medium cohesionless soil
or of predominantly soft-to-firm cohesive soil
A soil profile consisting of a surface alluvium
layer with vs values of type C or D and
thickness varying between about 5 - 20 m,
underlain by stiffer material with vs > 800 m/s
B
C
D
E
NATIONAL TECHNICAL UNIVERSITY OF ATHENS
LABORATORY FOR EARTHQUAKE ENGINEERING
Parameters
νS,30 (m/s)
ΝSPT
cu (kPa)
> 800
_
_
360 - 800
> 50
> 250
180 - 360
15 - 50
70 - 250
< 180
< 15
< 70
Ioannis Psycharis
Site effects on the ground motion
EC8 – Elastic response spectrum
Se / ag
2.5Sη
2.5SηTC/T

2.5SηTCTD/T 2
0

S
ΤΒ
ΤC
TD
Περίοδος, T (sec)
Ground
type
ΤΒ (sec)
ΤC (sec)
ΤD (sec)
S
Α
0.15
0.40
2.50
1.00
Β
0.15
0.50
2.50
1.20
C
0.20
0.60
2.50
1.15
D
0.20
0.80
2.50
1.35
E
0.15
0.50
2.50
1.40
NATIONAL TECHNICAL UNIVERSITY OF ATHENS
LABORATORY FOR EARTHQUAKE ENGINEERING
Ioannis Psycharis
Site effects on the ground motion
EC8 – Elastic response spectrum
4.0
Type A
Type B
3.5
Type C
3.0
Type D
Type E
Se / ag
2.5
2.0
1.5
1.0
0.5
0.0
0
1
2
3
4
5
Period, Τ (sec)
NATIONAL TECHNICAL UNIVERSITY OF ATHENS
LABORATORY FOR EARTHQUAKE ENGINEERING
Ioannis Psycharis
Site effects on the ground motion
Soil response for weak earthquakes
Small shear strains (γ < 10-5)  elastic response
● The elastic shear modulus, Gmax=ρVs2, is used for the
calculation of the response
● Profiles of Vs can be obtained from in situ measurements
(downhole, crosshole, geophysical techniques)
NATIONAL TECHNICAL UNIVERSITY OF ATHENS
LABORATORY FOR EARTHQUAKE ENGINEERING
Ioannis Psycharis
Site effects on the ground motion
Elastic response (homogeneous soil)
Shear cantilever behavior
● Equation of motion
 u
2  u

V
  xg
s
2
2
t
z
2
2
Gmax, ρ, Vs
where Vs 
Gmax
ρ
u(z,t)
H
z
● Eigenperiods
Τi 
4H
,
( 2i  1)Vs
xg
Bedrock
i  1, 2, ...
● Eigenmodes
 ( 2i  1) π z 
φi( z )  sin 

2H


● Participation factors
Γi 
4
( 2i  1)π
ω1 
NATIONAL TECHNICAL UNIVERSITY OF ATHENS
LABORATORY FOR EARTHQUAKE ENGINEERING
π Gmax
2 ρ H2
ω2 
3π Gmax
2 ρH2
ω3 
5π Gmax
2 ρH2
Ioannis Psycharis
Site effects on the ground motion
Elastic soil response
Surface amplification (Kanai 1962)
1
A(T ) 
1  κ

1  κ

● T
2
2
2 






T
0
.
3
T



   

1  


T


 Tsoil Tsoil 
  soil  

= period of seismic waves
● Tsoil = predominant period of soil
●
Soil
ρsoil , Vsoil , Tsoil
ρ V
κ  soil soil
ρrock  Vrock
NATIONAL TECHNICAL UNIVERSITY OF ATHENS
LABORATORY FOR EARTHQUAKE ENGINEERING
Bedrock
ρrock Vrock
xg (period T)
Ioannis Psycharis
Site effects on the ground motion
Soil response for strong earthquakes
Large shear strains  inelastic response
● The secant shear modulus and the hysteretic damping are
used for the calculation of the response through an iteration
procedure.
● Relations of G/Gmax and damping as functions of the shear
strain γ are given in the literature for common soil types.
NATIONAL TECHNICAL UNIVERSITY OF ATHENS
LABORATORY FOR EARTHQUAKE ENGINEERING
Ioannis Psycharis
Site effects on the ground motion
Nonlinear soil response
Definition of secant shear modulus and hysteretic damping
ξ
ED
4πES
ED
NATIONAL TECHNICAL UNIVERSITY OF ATHENS
LABORATORY FOR EARTHQUAKE ENGINEERING
ES = ½ Gs γ2
Ioannis Psycharis
Site effects on the ground motion
Nonlinear soil response
G/Gmax
Sand: Seed & Idriss – Average
Clay: Seed and Sun, 1989
Damping (%)
Sand: Seed & Idriss – Average
Clay: Idriss, 1990
NATIONAL TECHNICAL UNIVERSITY OF ATHENS
LABORATORY FOR EARTHQUAKE ENGINEERING
Ioannis Psycharis
Site effects on the ground motion
One-dimensional analysis
● Considers effects of soil response on one-dimensional (nearly
vertical) wave propagation
● Assumptions:
♦
All soil layers are horizontal
♦
SH-waves that propagate vertically from the bedrock
● Cannot model:
♦
Slopping
♦
Irregular ground surfaces
♦
Basin effects
♦
Embedded structures
In such cases 2-D and 3-D analyses are required
NATIONAL TECHNICAL UNIVERSITY OF ATHENS
LABORATORY FOR EARTHQUAKE ENGINEERING
Ioannis Psycharis
Site effects on the ground motion
Equivalent linear analysis
Soil layers
Equivalent MDOF lamped-mass model
(From Park & Hashash, 2004)
NATIONAL TECHNICAL UNIVERSITY OF ATHENS
LABORATORY FOR EARTHQUAKE ENGINEERING
Ioannis Psycharis
Site effects on the ground motion
Equivalent linear analysis
Iterative procedure
●
Make an initial estimation of shear
modulus and damping
●
Calculate the strain time-histories for
each layer for the given seismic motion
at the bedrock
●
Obtain the maximum strain values for
each layer and calculate the
corresponding effective shear strain
(~65% of peak strain)
●
Use the G/Gmax and ξ curves to obtain
better estimation of shear modulus and
damping
●
Repeat until convergence is reached
Limitation: Constant shear modulus and
damping is used during each iteration for the
whole time-history (overestimates stiffness)
NATIONAL TECHNICAL UNIVERSITY OF ATHENS
LABORATORY FOR EARTHQUAKE ENGINEERING
Ioannis Psycharis
Site effects on the ground motion
Verification
Comparison of recorded ground surface accelerations and
predictions by SHAKE
E-W component
N-S component
(Borja et al. 1999)
NATIONAL TECHNICAL UNIVERSITY OF ATHENS
LABORATORY FOR EARTHQUAKE ENGINEERING
Ioannis Psycharis
Site effects on the ground motion
Verification
Comparison of acceleration response spectrum for Treasure
Island strong motion (Loma Prieta, 1989 earthquake)
Recorded motion
Calculated
motion using
nearby rock
recordings as
control motion
(Idriss 1993)
NATIONAL TECHNICAL UNIVERSITY OF ATHENS
LABORATORY FOR EARTHQUAKE ENGINEERING
Ioannis Psycharis
Site effects on the ground motion
Basin effects
Basin effect
Flat soil layer case
(Stewart et al. 2001)
The seismic waves may resonate in the layer but cannot become
trapped
Basin case
The seismic waves become trapped within the basin if incidence
angles larger than the critical are developed
NATIONAL TECHNICAL UNIVERSITY OF ATHENS
LABORATORY FOR EARTHQUAKE ENGINEERING
Ioannis Psycharis
Site effects on the ground motion
Basin effect
(Stewart et al. 2001,
Graves 1993)
NATIONAL TECHNICAL UNIVERSITY OF ATHENS
LABORATORY FOR EARTHQUAKE ENGINEERING
Ioannis Psycharis
Site effects on the ground motion
Basin effect
Amplification factors, defined relative to the prediction for
the site class, vs. basin depth (Field et al. 2000)
NATIONAL TECHNICAL UNIVERSITY OF ATHENS
LABORATORY FOR EARTHQUAKE ENGINEERING
Ioannis Psycharis
Site effects on the ground motion
Effect of surface topography
Effect of ridges
(Stewart et al. 2001,
Geli et al. 1988)
● Crest amplification maximizes
at wave lengths
corresponding to the ridge
half-width
● Maximum spectral
amplification is about 1.6 for
this case
NATIONAL TECHNICAL UNIVERSITY OF ATHENS
LABORATORY FOR EARTHQUAKE ENGINEERING
Ioannis Psycharis
Site effects on the ground motion
Effect of canyons
● Maximum amplification
near the canyon edge
at wave lengths similar
or smaller to the
canyon dimension
● Maximum amplification
is about 1.4 for this
case
(Trifunac 1973)
NATIONAL TECHNICAL UNIVERSITY OF ATHENS
LABORATORY FOR EARTHQUAKE ENGINEERING
Ioannis Psycharis
Site effects on the ground motion
Effect of slopes
Spectral amplification at crest of
a 21 m tall, 3:1 (h:v) slope
● Maximum crest
amplification is about 1.2
(Stewart & Sholtis 1999)
NATIONAL TECHNICAL UNIVERSITY OF ATHENS
LABORATORY FOR EARTHQUAKE ENGINEERING
Ioannis Psycharis
Site effects on the ground motion
EC8 – Topographic amplification factors
EN-1998-5, Appendix A
● Isolated cliffs and slopes
A value ST > 1,2 should be used for sites near the top edge
● Ridges with crest width significantly less than the base width
A value ST > 1,4 should be used near the top of the slopes for
average slope angles greater than 30° and a value ST > 1,2
should be used for smaller slope angles
● Presence of a loose surface layer
In the presence of a loose surface layer, the smallest ST value
given above should be increased by at least 20%
● Spatial variation of amplification factor
The value of ST may be assumed to decrease as a linear
function of the height above the base of the cliff or ridge, and
to be unity at the base.
NATIONAL TECHNICAL UNIVERSITY OF ATHENS
LABORATORY FOR EARTHQUAKE ENGINEERING
Ioannis Psycharis
Site effects on the ground motion