ground motion variability: comparison of surface and downhole ground motions adrian rodriguez-marek,...
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GROUND MOTION VARIABILITY: GROUND MOTION VARIABILITY: COMPARISON OF SURFACE AND COMPARISON OF SURFACE AND DOWNHOLE GROUND MOTIONSDOWNHOLE GROUND MOTIONS
Adrian Rodriguez-Marek, Washington State University, USA
Fabrice Cotton, LGIT, Grenoble, FranceFabian Bonilla, IRSN, Fontenay aux Roses, France
Presented at
The Next Generation of Research on Earthquake-induced Landslides: An International Conference
in Commemoration of the 10th Anniversary of the Chi-Chi Earthquake
Taiwan, September, 2009
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OUTLINE
Motivation – site specific estimates of ground
motion uncertainty Ground motion database Regression analyses
Uncertainty at surface vs. depth Analysis of residuals
Single station uncertainty Implications for seismic design
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Ground Motion Predictions
SOURCE (e.g. magnitude, style of faulting)
PATH (e.g. decay rate)
SITE (e.g. Vs30, depth to bedrock)
Ground Motion Parameters
(e.g. – Spectral Acceleration)
ln(Ground Motion Parameter)
MOTIVATION
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Ground Motion Predictions
ymean = f (M,R,etc) + f(site) +
is a zero mean, standard normal random variable variability is typically broken down into intra- and
inter-event components:
= intra intra + inter inter
accounts for inherent or aleatoric variability
MOTIVATION
intra2
inter2
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Ground Motion Predictions
ymean = f (M,R,etc) + f(site) +
Question: Is from attenuation relationships usable in all cases?
Atkinson (2006) – Lower when considering single stations Morikawa et al. (2008) – Lower for single source, path
estimates are function of parameterization Consideration of multiple-stations, multiple source-paths
combinations implies a mixing of epistemic uncertainty in estimates (ergodicity assumption, Anderson and Brune 1999)
MOTIVATION
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Ground Motion Predictions
ymean = f (M,R,etc) + f(site) + s-s s-s + e-e e-e
+o o
(e-e Event to event variability (intra-event)
(s-s Site-to-site variability
(o Remaining (unexplained) variability
MOTIVATION
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Ground Motion Predictions
ymean = f (M,R,etc) + f(site) + s-s s-s + e-e e-e
+o o
For a given site i, (s-s assumes a given value i (no more a random variable), hence ...
ymean = f (M,R,etc) + f(site) + i s-s + e-e e-e
+o o
MOTIVATION
Standard normal random variables
Site Term
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Ground Motion Predictions
ymean = f (M,R,etc) + f(site) + s-s s-s + e-e e-e +o o
For a given site i, (s-s assumes a given value i (no more a random variable), hence ...
ymean = f (M,R,etc) + f(site) + i s-s + e-e e-e
+o o
MOTIVATION
Standard normal random variables
Site Term
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Site-specific ground motion predictions
ymean = f (M,R,etc) + f(site) + i s-s + e-e e-
e +o o
With site specific information: i can be estimated Analytically (Site Response Analyses) Multiple recordings (rarely) Estimates of i must include epistemic uncertainty
(hopefully lower than (s-s
The remaining uncertainty can be captured with single-station estimates of variability
MOTIVATION
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Site Response Analysis
When site-specific site amplification studies are conducted, the starting point is bedrock ground motion, then ...
Questions: Are amp and input uncorrelated (implicit assumption)? What is input? amp ?
Ampinputsurface22
MOTIVATION
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OPPORTUNITY!!OPPORTUNITY!!
KiK-net database: Large number of digital strong motion records
320 Events, 3784 Records Records screened and processed (G.Pousse, F.
Bonilla) Surface and Downhole recordings Shear-wave measurements at each site
DATABASE
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KiK-net DatabaseKiK-net Database
100
101
102
103
4
4.5
5
5.5
6
6.5
7
7.5
Distance, km
Mag
nitu
de
DATABASE
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OBJECTIVES
Develop a GMPE from Kik-Net data using both surface and downhole recordings
Compare different estimates of uncertainty Surface vs. downhole Single-station estimates
Obtain an estimate of standard deviation for site specific analyses
OBJECTIVES
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REGRESSION METHODOLOGY- Functional form: Boore and Atkinson (2008)
ln(y) = Fm + Fd + Fsite + Fborehole
Fd = [c1 + c2(M-Mref)]ln(R/Rref) + c3(R-Rref)
R = sqrt(R2 + h2)
Fm = e1 + e5(M-Mh) + e6(M-Mh) for M<MhFm = e1 + e7(M-Mh) for M>Mh
Fsite = blin*ln(Vs30/Vref)
Fborehole = a + b*ln(Vs30/Vref) + c*ln(Vshole/3000)
- Separate uncertainty into intra-event and inter-event (Random effects)ln(yij) = mean_estimateij + ij intra + i inter
REGRESSION ANALYSIS
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Notes on regression methodology Use a single magnitude and distance scaling for surface and
downhole data (regression from all the data)
Inter-event terms are assumed to be the same for the surface and the borehole.
The dependency of the intra-event term on depth, Vs30, or magnitude is obtained from an analysis of residuals.
Linear Vs30 dependency, no nonlinear effects
REGRESSION ANALYSIS
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Comparison with Boore and Atkinson NGA Relationship
101
102
103
10-5
100
Distance, km
PG
A (
g)
BA
This Study
REGRESSION ANALYSIS
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Vs30 dependency
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2-1.2
-1
-0.8
-0.6
-0.4
-0.2
0
0.2
Period (s)
linea
r V
s30
term
Surface
-100m-200m
REGRESSION ANALYSIS
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-4
-2
0
2
4SURFACE (Mean = -0.0018 StdDev = 0.6746)
-4
-2
0
2
4BOREHOLE at 100m (Mean = -0.0015 StdDev = 0.5522)
-4
-2
0
2
4BOREHOLE at 200m (Mean = -0.0006 StdDev = 0.5543)
Raw results : 3784 intra-events residuals
REGRESSION ANALYSIS
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Can the site term be improved? Surface Residuals at T = 1.0 s plotted versus site
period from H/V of record itself
intra = 0.608 intra = 0.580 (4.6% reduction)
10-2
10-1
100
101
-4
-2
0
2
4
Site Period (s)
log r
esid
uals
REGRESSION ANALYSIS
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Can the site term be improved? Surface Residuals at T = 1.0 s plotted baserock
shear wave velocity
intra = 0.608 intra = 0.581 (4.4% reduction)
REGRESSION ANALYSIS
0 500 1000 1500 2000 2500 3000 3500-3
-2
-1
0
1
2
3
Baserock Shear Wave Velocity (m/s)
log
resi
dual
s
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Surface standard deviations
Vs30Vs30 and
H800Vs30 and
To Vs30 and
Vshole
PGA .675 .671 .667 .674
T=0.1 s .774 .770 .764 .774
T=1.0 s .608 .585 .580 .581
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Surface standard deviations
Vs30Vs30 and
H800Vs30
and To
Vs30 and
Vshole
PGA .675 .671 .667 .674
T=0.1 s .774 .770 .764 .774
T=1.0 s .608 .585 .580 .581
About 4% decrease (only significant effect)
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Surface vs. Downhole comparison: Total Residuals
Average reduction of
11.5% at depth (over all periods from 0.01s to 1.36s)Maximum reduction of
16.2% for a period near 0.1s.
ANALYSIS OF RESIDUALS
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Surface vs. Downhole comparison: Intra-Event Residuals
Average
18% reductionMaximum of
24% at 0.1s
ANALYSIS OF RESIDUALS
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Implications
Lower residuals at depth: Various hypothesis1) Site effects are responsible for larger residuals
at surface2) Site parameterization is better at depth than
at surface (more site variability at surface)3) Surface-waves, non 1-D effects responsible for
larger residuals
ANALYSIS OF RESIDUALS
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Single-Station Residuals
44 stations with at least 15 recordings(995 records)
ANALYSIS OF RESIDUALS
100
101
102
103
4
4.5
5
5.5
6
6.5
7
7.5
Distance (km)
Mag
nitu
de, M
w
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The strong reduction in standard deviation from surface to depth is not observed when looking at single-station
estimates
Stations with more than 15 recordings
0 5 10 15 20 25 30 35 40 450
0.5
1
1.5single station standard deviations
surface
borehole
0 0.2 0.4 0.6 0.8 1 1.2 1.40
0.5
1
surface std
bore
hole
std
Mean Res: Surf = 0.63190 BH = 0.62551
Results for PGA
ANALYSIS OF RESIDUALS
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Single Station Standard Deviation
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 20.5
0.6
0.7
0.8
0.9
1
Period (s)
Tota
l Sig
ma
(log
units
)
Surface, Regression
Borehole, Regression
Surface, Single StationBorehole, Single Station
Atkinson (2006)
ANALYSIS OF RESIDUALS
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Single Station Standard Deviation
ANALYSIS OF RESIDUALS
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Single Station standard deviation removing event term
0 0.5 1 1.5 2 2.5 3
0.4
0.5
0.6
0.7Single Station Sigmas Removing Event Term
Period (s)
Tot
al S
igm
a (lo
g un
its)
Surface
Borehole
Atkinson
0 0.2 0.4 0.6 0.8 1 1.2 1.40
2
4
6
8
Period (s)
Per
cent
age
Red
uctio
n at
dep
th
Average reduction of
4.9% at depth (over all periods from 0.01s to 1.36s)Maximum reduction of
7.6% for a period near 0.1s.
ANALYSIS OF RESIDUALS
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Implications
Surface and Depth single-station residuals are more or less equal1) Site effects are responsible for larger residuals
at surface2) Site parameterization is better at depth than
at surface (more site variability at surface)3) Surface-waves, non 1-D effects responsible for
larger residuals4) Surface estimates of single station variability
can apply to borehole variability
IMPLICATIONS
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Implications for site-specific analyses: use of single station residuals
ymean = f (M,R,etc) + f(site) + i s-s + e-e e-e +o o
IMPLICATIONS
Single Station Residuals
Single Station Residuals extracting
event term
e-e and o o
Surface Borehole Surface Borehole
PGA 0.63 0.63 0.48 0.47
T=0.1s 0.65 0.63 0.52 0.48
T=1.0 s 0.63 0.64 0.44 0.41
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For site response analyses
Ln F = ln(Sa)surf – ln(Sa)bh
Assumes independence of bh and F
bh = o (single station variability removing event
terms)
F = ??
FBHsurface22
IMPLICATIONS
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Single station estimates of the amplification function standard deviations vary between 0.23 and 0.30, with an average value across periods of 0.26.
ANALYSIS OF RESIDUALS
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For a selected period (T=0.1s)
o= 0.477
F= 0.280
bh = sqrt(0.477^2 + 0.28^2) = 0.553
From database: surf = 0.517 6% difference
o and F are not independent ... or o and inter-event terms are not independent
(path effects on both)
ANALYSIS OF RESIDUALS
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Conclusions
There is an average reduction of 11.5% reduction in total sigma values at depth over all periods (from 0.01s to 1.36s), with a maximum reduction of 16.2% for a period near 0.1s (20% reduction in intra-event sigmas)
The large reduction in standard deviation from
surface to depth is not observed when looking at single-station estimates
Single station estimates of standard deviation at the surface are in general applicable to larger depths.