Site effects on ground motion

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1 NATIONAL TECHNICAL UNIVERSITY OF ATHENS LABORATORY OF EARTHQUAKE ENGINEERING Site effects on ground motion Ioannis N. Psycharis

2 Site effects Ground motion propagation from source to site (Kramer, 1996)

3 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

4 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

5 Local ground response

6 Effect of soil on response spectra Average normalized response spectra for 107 earthquake records grouped in four soil categories (Seed et al. 1976)

7 Ground motion amplification Average spectral amplification vs. V s-30 recorded during the 1989 Loma Prieta earthquake (Borcherdt & Glassmoyer, 1994) Amplification of SA Amplification of SV (average for T= s) (average for T= s)

8 Soil classification (EC8) Typically based on the average shear wave velocity at the top 30 m of the soil profile (e.g. EC8) v Ground type h i = thickness of layer (m) v i = elastic shear wave velocity Ν = no. of layers at the top 30 m of soil deposit Description Parameters ν S,30 (m/s) Ν SPT c u (kpa) A Rock or other rock-like geological formation > 800 B C D E S,30 30 h i1,n v i i 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 v s values of type C or D and thickness varying between about 5-20 m, underlain by stiffer material with v s > 800 m/s > 50 > < 180 < 15 < 70

9 S e / a g EC8 Elastic response spectrum 2.5Sη 2.5SηT C /T S 2.5SηT C T D /T 2 0 Τ Β Τ C T D Περίοδος, T (sec) Ground type Τ Β (sec) Τ C (sec) Τ D (sec) S Α Β C D E

10 EC8 Elastic response spectrum Type A Type B Type C Type D Type E S e / a g Period, Τ (sec)

11 Soil response for weak earthquakes Small shear strains (γ < 10-5 ) elastic response The elastic shear modulus, G max =ρv s2, is used for the calculation of the response Profiles of V s can be obtained from in situ measurements (downhole, crosshole, geophysical techniques)

12 Elastic response (homogeneous soil) Shear cantilever behavior Equation of motion 2 u V 2 t 2 2 u s x 2 g z where V s G ρ max H G max, ρ, V s z u(z,t) Eigenperiods 4 H Τi, i 1, 2,... ( 2i 1)V s Bedrock x g Eigenmodes ( 2i φ i( z) sin 1) π z 2 H Participation factors Γ i ( 2i 4 1) π π 2 G max max ω ρ H 3π G 5π Gmax ω ω3 2 2 ρh 2 ρh

13 Elastic soil response Surface amplification (Kanai 1962) A( T ) 1 1 κ κ 1 T T soil T soil T T soil 2 Soil ρ soil, V soil, T soil T = period of seismic waves T soil = predominant period of soil ρ κ ρ soil rock V V soil rock Bedrock ρ rock V rock x g (period T)

14 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/G max and damping as functions of the shear strain γ are given in the literature for common soil types.

15 Nonlinear soil response Definition of secant shear modulus and hysteretic damping ED ξ 4πE S E D E S = ½ G s γ 2

16 Nonlinear soil response G/G max Sand: Seed & Idriss Average Clay: Seed and Sun, 1989 Damping (%) Sand: Seed & Idriss Average Clay: Idriss, 1990

17 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

18 Equivalent linear analysis Soil layers Equivalent MDOF lamped-mass model (From Park & Hashash, 2004)

19 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/G max 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)

20 Verification Comparison of recorded ground surface accelerations and predictions by SHAKE E-W component N-S component (Borja et al. 1999)

21 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)

22 Basin effects

23 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

24 Basin effect (Stewart et al. 2001, Graves 1993)

25 Basin effect Amplification factors, defined relative to the prediction for the site class, vs. basin depth (Field et al. 2000)

26 Effect of surface topography

27 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

28 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)

29 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)

30 EC8 Topographic amplification factors EN , 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.

Canyon Geometry Effects on Seismic SH-Wave scattering using three dimensional BEM

Proceedings of the rd IASME / WSEAS International Conference on GEOLOGY and SEISMOLOGY (GES'9) Canyon Geometry Effects on Seismic SH-Wave scattering using three dimensional BEM REZA TARINEJAD* and MOHAMMAD