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by
I. M. Idriss
Professor Emeritus
University of Californ ia at Davis
e-mail: [email protected]
EARTHQUAKE GROUND MOTIONS
Presented at the Session on
"Earthquake Effects on Soils and Foundations"
Part of the Refresher Course on
"Seismic Analysis and Retrofitting of Life line Buildings in Delhi, India"
May 26, 2005
Audi torium of the Delhi Secretariat Building
Delhi, India
PROCEDURES AVAILABLE FOR ESTIMATING
EARTHQUAKE GROUND MOTIONS
1) Accumulation of strong motion data & correlation with
identified local subsur face conditions -- empirical approach.
2) Use of analytical procedures to calculate ground motions
at a given site semi-empirical approach.
These analytical procedures include:
a) Source Path Site
b) Local Site Response
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EMPIRICAL RELATIONSHIPS
Instrumental records at a multitude of local site conditions
have clearly shown the influence of local site conditions
on the characteristics of earthquake ground motions.
Therefore, empirical relationships typically incorporate
the influence of local site conditions.
Some derive separate sets of equations for each generalized
site condition.
Some introduce a parameter (e.g., Vs30
) that is used to
reflect these local site conditions.
EMPIRICAL RELATIONSHIPS
Geotechnical Subsurface Classification
Devised by Geomatrix
A -- Rock (Vs30 > 600 m/sec)
B Shallow sti ff soil (up to 20 m thick over rock)
C Deep narrow soil (more than 20 m over rock & narrow val ley)
D Deep broad soi l (more than 20 m over rock & broad valley)
E Soft deep soi l
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EMPIRICAL RELATIONSHIPS
NEHRP Site Classification
One Letter: Geotechnical Subsurface Classification
Average shear wave veloci ty, Vs30, in top 30 m
A -- Vs > 1500 m/sec
B Vs = 760 to 1500 m/sec
C Vs = 360 to 760 m/sec
D Vs = 180 to 360 m/sec
E Vs < 180 m/sec
F Liquefiable Soils
EXAMPLES
RECORDED DATA
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1994 Northridge
Earthquake
EMPIRICAL APPROACH
Closest Distance to the Rupture Surface - km
3 4 5 6 8 20 30 40 50 60 80 200 30010 100
Peak
HorizontalAcceleration-g
0.02
0.03
0.04
0.05
0.06
0.08
0.2
0.3
0.4
0.5
0.6
0.8
0.01
0.1
1
Soil Sites
Rock Sites
1994 Northridg e Earthquake
Strong & Weak HorizontalComponents Recorded at
Rock Sites
Soil Sites
Derived MedianRelationships
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Period - sec
0.01 0.1 1 10Spec
tral
Acce
lera
tion
(Sa
)/Pea
kGroun
dAcce
lerati
on
(pga
)
0.0
0.5
1.0
1.5
2.0
2.5
3.0
Motions Recorded at SoilSites -- Northridge Earthquake
Motions Recorded at RockSites -- Northridge Earthquake
1994 Northridge
Earthquake
Period sec0.01 0.1 1
Sp
aA
eaoz
0.0
0.5
1.0
1.5
2.0
2.5
3.0
0.3 g 0.7 g
Soil SitesNorthridge Earthquake
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Longitude120.0 120.5 121.0 121.5 122.0Lt
22.0
22.5
23.0
23.5
24.0
24.5
25.0
Approx RuptureSurfaceCategory B SitesCategory C SitesCategory D SitesCategory E Sites
1999 Chi-Chi
Earthquake
Distance - km1 10 100
Pe
akHor
izon
talAcce
lera
tion-g
0.01
0.1
1
Motions Recorded at
Category B SitesCategory C SitesCategory D SitesCategory E Sites
1999 Chi-Chi
Earthquake
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Peak Horizontal Acceleration at Category B Sites -g
0.0 0.1 0.2 0.3 0.4 0.5
P
eakHorizontalAccelerationatSoilSites-g
0.0
0.1
0.2
0.3
0.4
0.5
:
Deep Soil Sites(San Fernando )
Category D Sites
Category E Sites
Category B Sites
1999 Chi-Chi
Earthquake
Period - sec0.01 0.1 1
Spec
tra
lAcce
lera
tion
/zpa
0.0
0.5
1.0
1.5
2.0
2.5
3.0
0.3g 0.4g 0.7g
Category B SitesChi-Chi Earthquake
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OBSERVATIONS RE: EMPIRICAL APPROACH
1. Local site conditions do affect earthquake ground motions
2. The observed trends in zpa & in spectral shapes indicate
a strong nonlinear response of soil sites
3. These trends are reasonably well accommodated in the current
Code provisions
4. The on-going work (NGA) to derive updated attenuation
relationships shou ld shed more light on the influence oflocal site conditions on earthquake ground motions.
5. The available recorded data are not suffi cient to cover all site
conditions
6. Therefore, it is necessary to utilize analytical procedures to
augment the recorded data & to evaluate response at soil
sites for which little or no recorded data exist
OBSERVATIONS RE: EMPIRICAL APPROACH (Cont 'd)
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ANALYTICAL PROCEDURES
SEMI-EMPIRICAL APPROACHES
SOURCE PATH SITE
Excellent for assessing relative effects o f variousparameters on earthquake ground motions
Has proven to be a valuable tool for calculatingearthquake ground motions at rock & stiff so il sites.
Not well suited for calculating effects of local site
conditions on earthquake ground motions
LOCAL SITE RESPONSE
Has proven to be a valuable tool for calculatingeffects of local site conditions on earthquake
ground motions
Al lows for incorporat ing nonlinear soil behaviorunder dynamic loading conditions
Equivalent Linear Representation
Non Linear Representation
ANALYTICAL PROCEDURES
SEMI-EMPIRICAL APPROACHES
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The equivalent linear procedure has been & continues
to be the most widely used procedure in practice for
calculating site response & for developing si te specific
earthquake ground motions and design parameters.
There are several non-linear techniques that have been
and continue to be developed.
These procedures are needed to make reasonably
accurate estimates of displacement / deformationsof earth structures.
LOCAL SITE RESPONSE
Factors that Influence Site Response Calculations
Input Rock Motion
Soil Profile
Soil Properties
Method o f Analysis
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Treasure Island & Yerba Buena Island Sites
SF-Oakland Bay Area
Example No. 1
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Shear Wave Velocity - ft/sec
0 500 1000 1500 2000 2500
DepthBelowG
roundSurface-ft
0
40
80
120
160
200
240
280
320
Best Estiamte Velocities usedin the analyses
Measured Velocities:
Downhole (Redpath, 1991)
SCPT (Rollins et al, 1992)
Downhole (Gibbs et al, 1992)
Treasure
Island Site
Wave Velocity - ft/sec0 1000 2000 3000 4000
Dept
hBe
low
Groun
dSur
face-
ft
0
20
40
60
80
Yerba Buena Island
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Time - sec
0 5 10 15 20 25
Acceleratio
n-g
-0.16
-0.08
0.00
0.08
0.16EW Component of Recording
at Treasure Island
Time - sec
0 5 10 15 20 25
Acceleration-g
-0.16
-0.08
0.00
0.08
0.16EW Component of Recording
at Yerba Buena Island
Period - sec
0.1 1 10
Spec
tra
lAcce
lera
tion
-g
0.0
0.2
0.4
0.6
0.8
Spectrum for MotionRecorded at TI
Spectrum for motionRecorded at YB
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Period - sec
0.1 1 10
Spec
tra
lAcce
lera
tion
-g
0.0
0.2
0.4
0.6
0.8Spectrum for Motion
Recorded at TI
Spectrum for motioncalculated at TI usingas input motion:recording at YB
Spectrum for motioncalculated at TI usingas input motion:ALL excep t YB
EFFECTS OF SOIL PROPERTIES
ON SITE RESPONSE
Treasure Island Site
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Period - sec
0.01 0.1 1 10
Spec
tra
lAcce
lera
tion-g
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
damping = 5 percent
Spectrum for MotionRecorded at TI
Spectrum for motioncalculated at TI usingas input motion recordingat YB & best estimate v
s
Spectrum for motioncalculated at TI usingas input motion recordingat YB & range of vs
San Francisco Ai rport & Sierra Point Sites
Example No. 2
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Shear Wave Velocity - ft/sec
0 500 1000 1500 2000 2500 3000
DepthBelowG
roundSurface-ft
0
50
100
150
200
250
300
350
400
450
500
Best Estimate VelocitiesUsed in the Analyses
Gibbs et al (1992)Redpath (1991)
Shear Wave VelocityMeasurements b y:
San Francisco
Airpor t Site
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3535
Period - sec
0.01 0.1 1 10
Spec
tra
lAcce
lera
tion-g
0.0
0.2
0.4
0.6
0.8
1.0
1.2
damping = 5 percent
Spectrum for motionrecorded at Sierra Point
Spectrum for motionrecorded at SFO
3636
Period - sec
0.01 0.1 1 10
Spec
tra
lAcce
lera
tion-g
0.0
0.2
0.4
0.6
0.8
1.0
1.2
damping = 5 percent
Spectrum for motion calculatedat SFO using motion recordedat Sierra Point as input motion
Spectrum for motionrecorded at SFO
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EFFECTS OF INPUT ROCK MOTION
ON SITE RESPONSE
San Francisco Airport Site
3838
Period - sec
0.01 0.1 1 10
Spec
tra
lAcce
lera
tion-g
0.0
0.2
0.4
0.6
0.8
1.0
1.2
damping = 5 percent
Spectrum for MotionRecorded at SFO
Spectrum for motioncalculated at SFO usingas input motion:recording at Sierra Point
Spectrum for motioncalculated at TI usingas input motion:
ALL excep t Sier ra Poi nt
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EFFECTS OF SOIL PROPERTIES
ON SITE RESPONSE
San Francisco Airport Site
4040
Period - sec
0.01 0.1 1 10
Spec
tra
lAcce
lera
tion-g
0.0
0.2
0.4
0.6
0.8
1.0
1.2
damping = 5 percent
Spectrum for MotionRecorded at SFO
Spectrum for motioncalculated at SFO usingas input motion recordingat SP & best estimate v
s
Spectrum for motioncalculated at SFO usingas input motion recordingat SP & range of v
s
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La Cienega Downhole Array
Los Angeles
Example No. 3
4242
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4343
4444
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4545
EFFECTS OF SOIL PROPERTIES
ON SITE RESPONSE
La Cienega Site
4646
Shear Wave Velocit ies
La Cienega Site
Los Angeles
Shear Wave Velocity (ft/sec)
0 500 1000 1500 2000 2500 3000
Depth
Belowt
heGroundSurface(ft)
0
100
200
300
400
500
600
700
800
900
measured vsvalues
vsvalues used in b oth analyses
below a depth of 80 ft
vsvalues used in analysis 1
from ground surface to 80 ft
vsvalues used in analysis 2
from ground surface to 80 ft
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4747
Peak Horizontal A cceleration (g)
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8
DepthBelowt
heGroundSurface(ft)
0
100
200
300
400
500
600
700
800
900
Peak Recorded Values
Input: at depth = 830 ftAnaly sis 1
Input: at depth = 830 ftAnaly sis 2
La Cienega Site
Los Angeles
4848
Peak Horizontal A cceleration (g)
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8
Depth
Belowt
heGroundSurface(ft)
0
10
20
30
40
50
60
70
80
90
100
Peak Recorded Values
Input: at depth = 830 ftAnalysi s 1
Input: at depth = 830 ftAnalysi s 2
La Cienega Site
Los Angeles
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4949
Factors that Influence Site Response Calculations
Input Rock Motion
Soil Profile
Soil Properties
Method of Analysis
5050
ANALYTICAL APPROACH
Observations re: Analytical Approach
1. The input motion has a very significant influence on
calculated site response
2. The soil profile also has a very significant influence on
calculated site response
3. The soil properties also influence the calculated site response,but to a lesser extent than input motion or soil profile
4. The method of analysis can have a profound influence if
inappropriately applied
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5151
CLOSING COMMENTS
Equivalent Linear Procedure:
Limit depth of soil pro file to about 500 ft
Limit level of shaking to that resulting in no more than about
1 percent shear strain at any depth within the soil pro file
5252
CLOSING COMMENTS
Non Linear Procedure:
Select only those procedures & corresponding computer
programs that have been calibrated with a sufficient number
Of case histories
Limit level of shaking to that resulting in no more than about
A few percent shear strain at any depth within the soi l profi le
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5353
CLOSING COMMENTS
Al l Procedures:
Selection of input motion is a critical step
use at least 7 reasonably uncorrelated input motions
Al low for appropriate var iat ion in the soi l properties
& the soil profile
5454
THANK YOU