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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
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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
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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
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1985 Michoacan EarthquakeExample of soil amplification (“Site effects”)
• Mw 8 along t lcoastal
subduction zone• ~300 km west of
Mexico City
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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
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Mexico City
NEQ waves
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Mexico City
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Mexico City
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Ground Shaking
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Ground Shaking
A lifi ti
10x amp
Amplification is different at each period
amp
4x amp
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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
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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
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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)
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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
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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)
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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
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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
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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)
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Influence of Z1.0: GMPEs
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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
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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
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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
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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)
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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
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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
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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