S il A lifi ti dSoil Amplification and Topographic Effectsp g p

Prof. Ellen M. Rathje, Ph.D., P.E.

Department of Civil Architectural andDepartment of Civil, Architectural, and Environmental Engineering

University of Texas at Austin

18 November 2010

Seismic Design FrameworkSource Characterization

Locations of sources (faults)Magnitude (M )Magnitude (Mw)

RecurrenceGround Motion Characterization

Closest distance fault to site (Rcl)Closest distance fault to site (Rcl)Local site conditions

RSoil conditions and

topographic conditionsRrup

Soil conditions

topographic conditions can affect ground

shakingSoil conditions

Topographic conditions

Local Effects• Soil Amplification

– Increase in ground motionIncrease in ground motion intensity due to dynamic response of local soil players

• Topographic Amplificationp g p p– Increase in ground motion

intensity due to focusing of y gwaves within hillsides

These effects typically applied to “rock” ground motions defined by seismic hazard assessment

1985 Michoacan EarthquakeExample of soil amplification (“Site effects”)

• Mw 8 along t lcoastal

subduction zone• ~300 km west of

Mexico City

Damage Patterns

• Some damage near coast• Most damage in Mexico City

– Unusual to have severe damage 300 km from gearthquake

• Ground shaking was significantly affectedGround shaking was significantly affected by soil conditions in Mexico City– Mexico City built on ancient lake bedMexico City built on ancient lake bed– Very soft clays underlie much of the city

Mexico City

NEQ waves

Mexico City

Mexico City

Ground Shaking

Ground Shaking

A lifi ti

10x amp

Amplification is different at each period

amp

4x amp

Selective Building Damage

• Dynamic soil response in damaged areasS il it i d T 2– Soil site period, Ts ~ 2 s

– Ts = 4 H / Vs 4(35 m)/70 m/s 2 s. = 4(35 m)/70 m/s = 2 s

• Damaged Buildings Soft SoilVs~70m/s

H~35 m

– Mostly taller buildings– Tbldg ~ 2 s

Vs~70m/s

• Areas east with deeper soil, Ts >> Tbldg Hard Soils bldg

Soil Amplification

• Amplification Factor (AF) = Sa,soil / Sa,rock

2 5

3

ock)

Soil Profile

1.5

2

2.5

 Factor (Soil/R

o

0.5

1

Amplification

Site Period (Ts)

Rock0

0.01 0.1 1 10

Period (s)

T = 4 H / VsTs = 4 H / VsH: thickness of soilVs: avg Vs over H

Soil Amplification

• AF’s are period dependent

2.5

3/Rock)

1 5

2

actor (So

il/

1

1.5

ification

 Fa

0

0.5

Ampli

0.01 0.1 1 10

Period (s)

Soil Amplification

• AF’s are influenced by Vs profileSofter soil (smaller Vs) larger AF (generally)– Softer soil (smaller Vs) larger AF (generally)

4

5

/Rock)

4

5

/Rock)

2

3

tion

 Factor (Soil/

2

3

tion

 Factor (Soil/

Vs (m/s)

0

1

0.01 0.1 1 10

Amplifica

0

1

0.01 0.1 1 10

Amplifica

0

20

0 250 500 750 1000s ( )

Period (s)Period (s) 40

60

80

Dep

th (m

)

80

100

120

Soil Amplification

• AF’s are influenced by level of rock motionSoil is nonlinear– Soil is nonlinear

PGArock = 0.1 g PGArock = 0.4 g

2.5

3

l/Ro

ck)

2.5

3

l/Ro

ck)

1.5

2

on Factor (So

i

1.5

2

on Factor (So

i

0

0.5

1

Amplificatio

0

0.5

1Am

plificatio

0

0.01 0.1 1 10

Period (s)

0

0.01 0.1 1 10

Period (s)

Accounting for Site Effects

• Simplified MethodsQuantify site conditions based on simple– Quantify site conditions based on simple parameters (e.g. Vs30)Develop estimates for amplification based on– Develop estimates for amplification based on these parameters

• Wave Propagation Analysis (Site Response)• Wave Propagation Analysis (Site Response)– Model full Vs profile of soil from bedrock

(Vs~760 m/s) to the ground surface(Vs~760 m/s) to the ground surface– Apply motions at base of soil and compute

expected amplification at ground surfaceexpected amplification at ground surfaceBoth methods assume a one-dimensional soil profile

Simplified Methods• Parameters

– Vs30: average Vs over top 30 mVs30: average Vs over top 30 m– Z1.0: depth to Vs=1.0 km/s

Vs (m/s) Vs (m/s)

0

5

0 500 1000 1500Vs (m/s)

0

5

0 500 1000 1500Vs (m/s)

Vs30 = 345 m/s

Z1 0 ? ( 30 )5

10

15h (m

)

5

10

(m)

Z1.0 = ? (> 30 m)

15

20

Dep

th

V 30 625 /

15

20

Dep

th 

25

30

Vs30 = 625 m/s

Z1.0 = 16 m 25

30

Influence of Vs30: GMPEs

0 8

1

1.2

ion (g)

Vs30 = 760 m/s

Vs30 = 300 m/s0 8

1

1.2

ion (g)

0.4

0.6

0.8

pectral A

ccelerati

0.4

0.6

0.8

pectral A

ccelerati

Vs 30 = 760 m/s

0

0.2

0.01 0.1 1 10

Sp

Period (s)

0

0.2

0.01 0.1 1 10

Sp

Period (s)

/

Vs30 = 300 m/s

( )

2

2.5

3

(Soil/R

ock) Rock PGA = 0.22 g

Rock PGA = 0. 45 g

Moderate Rock PGA High Rock PGA

0 5

1

1.5

lification Factor 

0

0.5

0.01 0.1 1 10

Amp

Period (s)

Influence of Z1.0: GMPEs

Site Response Analysis

Advantages:• Model detailed velocity• Model detailed velocity

profile• Model local soil types0

0 500 1000 1500Vs (m/s) Model local soil types

Increased Complexity:• Measuring Vs down to

5

10

15

20

25

Depth (m

)

bedrock• Selecting input motions

D fi i li il

30

• Defining nonlinear soil properties

Site response program Strata available for free at:http://nees.org/resources/strata

Integration with PSHA

• Define hazard in terms of an acceleration response spectrum on rock (Vs30 ~ 760response spectrum on rock (Vs30 ~ 760 m/s)Apply soil amplification to rock response• Apply soil amplification to rock response spectrum− Building code procedure− GMPE amplification Increasing

Complexity

− Site response analysisp y

Topographic Amplification

• Increase in ground motion intensity due to focusing of waves within hillsidesfocusing of waves within hillsides

L=half-widthCrest

H=heightBase

Amplification = Crest Motion/ Base MotionSh R ti H / LShape Ratio = H / L

Topographic Amplification

• Amplification increases with increasing Shape RatioShape Ratio

Theoretical Values

H/L Slope PGA Amp0.2 11 1.00.4 22 1.50.6 31 1.5

Geli et al. (1988)

Topographic Amplification

• Frequencies of maximum amplification: where wavelength equals mountain widthwhere wavelength equals mountain width

2L=width

W l th f ti V / fWavelength of motion = Vs / f Mountain width = 2LAmplification frequency f* ~ Vs / 2LAmplification frequency, f ~ Vs / 2L

Larger Vs or smaller L f* increases

Topographic Effects

• Field measurements of topographic effects generally larger than theoretical predictionsgenerally larger than theoretical predictions– PGA: Theoretical ~ 1.2 to 1.5; field ~ 1.5 to 3.5

At f*: Theoretical 2 0 to 4 0; field 4 0 to 10– At f*: Theoretical ~ 2.0 to 4.0; field ~ 4.0 to 10• Reasons for inconsistency

– Complexity of natural ridges vs. theoretical models– Interaction of adjacent ridges– Underlying velocity structure– 3D geometry

No standard procedure to predict topographic amplification

Summary

• Soil AmplificationAmplification depends on soil properties and– Amplification depends on soil properties and input intensityAmplification is period dependent– Amplification is period-dependent

– Apply soil amplification factors to rock acceleration response spectrumacceleration response spectrum

• Topographic AmplificationRidges can amplif motions– Ridges can amplify motions

– Complex problem with no standard procedure for estimationfor estimation