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Statistical analysis on geotechnical and geosciences data
J.P. WangDept Civil, Hong Kong University of Science and Technology (HKUST)
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Introduction
• It is very interesting that some mathematical functions could fit our data nicely….
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Mid
-term
gra
de
1st - 158th student
Mean = 70
SD = 10.5
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35~40 40~45 45~50 50~55 55~60 60~65 65~70 70~75 75~80 80~85 85~90 90~95 95~1000
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Freq
uenc
y
Range
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35~40 40~45 45~50 50~55 55~60 60~65 65~70 70~75 75~80 80~85 85~90 90~95 95~1000
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Freq
uenc
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Normal distribution
• Like the “mid‐term” example, several statistical studies on earthquake and debris‐flow data will be introduced in this presentation.
• In addition to those findings, some applications will be discussed on the use of the statistical inferences from samples.
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a. Earthquake and Poisson?
• It is common to see such a sentence in textbooks: “Using the Poisson distribution (a function) to model earthquake probability in time….”
• Pr(x) = e‐v * vx / x! ; v = mean rateex: v = 1 per year; Pr(one earthquake) = 37%
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• It seems that the model and observation are in good agreement.
• Next, we changed the “boundary condition” to examine the same hypothesis.
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Summary (Wang et al., 2014)
• From earthquake data around Taiwan, the Poisson model and the observation are in good agreement, and the hypothesis was not rejected by Chi‐square tests, on some conditions
• A rule of thumb: mean rate = 0.1; if the mean rate < 0.1, earthquake temporal randomness could be modeled by the Poisson
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b. Earthquake magnitude and Weibull
• Earthquake‐magnitude probability distribution is a key input to probabilistic seismic hazard analysis.
• Currently, the McGuire‐Arabasz (1990) method/algorithm is commonly used for the modeling:
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• The Weibull distribution (a function) was discovered and proposed in the late 1930s:
• Many applications to biology, finance, and engineering (e.g., Weibull, 1939, 1951; Islam et al., 2013)
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Summary (Wang, 2015)
• Earthquake magnitude should be a Weibull random variable, which is supported by earthquake data around Taiwan
• The Weibull approach provides a better modeling on magnitude probability distributions, than the conventional method
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c. Bi‐variate modeling• The examples above are focused on one single variable; but more often we deal with bi‐variate statistics and aim to develop their joint probability distribution:
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3X
Y
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• 139 debris flows occurred in the Jiangjia Ravine, China since the 1960s
• Similar to those shown previously, a (uni‐variate) statistical study on the debris‐flow data was reported in Hong et al. (2015):
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• Since P and Q are not statistically independent, their (bi‐variate) joint probability function is not equal to the product of two (uni‐variate) marginal functions:
*)(*)(*)*;Pr(
*)Pr(*)Pr(*)*;Pr(
qFpFqQpP
qQpPqQpP
QP
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• Therefore, in order to achieve a better modeling on the bi‐variate data, one logical approach is to adopt bi‐variate models, such as bi‐normal distribution, to fit the observation from scratch:
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(observation data)
• From methodology to calibration, the difficulty and effort are much increased from uni‐variate to bi‐variate statistical works....
• The copula method is increasingly applied to bi‐variate or multi‐variate modeling, for the method being more “user‐friendly”
• Li and Tang (2014) applied the copula method to model the joint probability function of soil friction angle and cohesion (the two are dependent), among many others (e.g., Goda, 2010)
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What is copula?
• Based on Sklar’s theorem (1959), a joint function could be decomposed into: i) several marginal functions, ii) a copula function
• Let’s see an example in the next slice
);(),(),();Pr( , yFxFCyxFyYxX YXYX
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i) X following uniform distribution between a and b:
ii) Y following uniform distribution between a and b:
iii) X‐Y dependence structure can be modeled by the Clayton copula:
Therefore, the X‐Y joint function could be modeled by:
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YXClayton FFC
abaxxFxX X
)()Pr(
abayyFyY Y
)()Pr(
/1
, 1),(
abay
abaxyxF YX
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• Using the copula approach, here is our best bi‐variate model for P‐Q simulation:
2.31/1.62
( ) ( )1.62 1.62249.4 237, ( , ) (1 ) (1 ) 1
p q
P QF p q e e
(Weibull + Weibull + Clayton)
Observation Model
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• We also applied the copula method to PGA and CAV, the two most parameters for earthquake‐resistant design
PGA
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CAV = cumulative absolute velocity= ∫ |a| dt
• With data from Taiwan, here is our best bi‐variate model for PGA‐CAV (Xu, 2015):
(Lognormal+ Lognormal + Gaussian)
Observation Model
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ln * 4.44 ln * 2.84 2 2( ) ( )1 1 2 21.22 1.19
, 1 20 0
exp( 1.73 )( *, *)22.67
PGA CAV
PGA CAVr r r rF PGA CAV drdr
Summary and conclusion
• Some mathematical functions can well capture a variable’s randomness or distribution, such as using the Poisson distribution to model earthquake temporal randomness
• The copula method seems a good solution to multi‐variate statistical modeling
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