Stationarity and Degree of Stationarity Norden Huang Research Center for Adaptive Data Analysis National Central University
Need to define the Degree Stationarity Traditionally, stationarity is taken for granted; it is given; it is an article of faith. All the definitions of stationarity are too restrictive. All definitions of stationarity are qualitative. Good definition need to be quantitative to give a Degree of Stationarity
Definition : Strictly Stationary
Definition : Wide Sense Stationary
Definition : Statistically Stationary If the stationarity definitions are satisfied with certain degree of averaging. All averaging involves a time scale. The definition of this time scale is problematic.
Degree of Stationarity
Degree of Statistical Stationarity
An Example Ocean Wind Wave Data
Ocean Waves Water waves are nonlinear. Crests of breaking waves need many harmonics to fit Waves are nonstationary Spectrum full of Harmonics; it is hard to separate free from bound wave energy
Ocean Waves : data
Ocean Waves : IMF
Ocean Waves : Hilbert Spectrum
Ocean Waves : Hilbert Spectrum x10
Ocean Waves : Hilbert Spectrum x100
Ocean Waves : Degree of Stationarity
Earthquake Data Chi-Chi, Taiwan September 21, 1999 An Example Earthquake Data Chi-Chi, Taiwan September 21, 1999 Huang, N. E. , et al. 2001 : A new spectral representation of earthquake data: Hilbert Spectral analysis of station TCU129, Chi-Chi, Taiwan, 21 September 1999, Bulletin of the Seismological Society of America, Volume 91, pp 1310-1338.
Earthquake Earthquake is definitely transient; therefore, nonstationary. For near field locations, the earth motion is also highly nonlinear. Traditional treatment of earthquake data by response spectral analysis is not adequate.
Response Spectrum The response spectrum of a earthquake signal is defined through the maximum displacement of a linear single degree of freedom system with predetermined damping driven by the given earthquake signal. The displacement is given by the Duhamel Integral:
Response Spectrum
Response Spectrum
Response Spectrum As the Duhamel Integral gives a quantity with the dimension of velocity, the response spectrum is also known as the pseudo-velocity spectrum. The linear single degree of freedom system is a linear filter; therefore, There is a definitive relationship between the Fourier Spectrum and Response spectrum.
Chi-Chi Earthquake : Data
Chi-Chi Earthquake : F & RS ; E
Chi-Chi Earthquake : F & RS ; N
Chi-Chi Earthquake : F & RS ; Z
Chi-Chi Earthquake : Hilbert E
Chi-Chi Earthquake : Hilbert N
Chi-Chi Earthquake : Hilbert Z
Chi-Chi Earthquake : MH & F : E
Chi-Chi Earthquake : MH & F : N
Chi-Chi Earthquake : MH & F : Z
Chi-Chi Earthquake : Hilbert : E200
Chi-Chi Earthquake : Hilbert E1000
Chi-Chi Earthquake : DS E
Chi-Chi Earthquake : DS N
Chi-Chi Earthquake : DS Z
Chi-Chi Earthquake : DS All
Chi-Chi Earthquake : DSS200 All
Chi-Chi Earthquake : DSS1000 All
Chi-Chi Earthquake : DS Hilbert spectral analysis reveals a ‘damaging-causing’ low frequency band of energy not properly shown in the Fourier Analysis. The strongest component, EW, is also the most nonstationary one. The weakest component, Z, is also the most stationary one. The Hilbert and Fourier spectra agree well for the most stationary case.
Heart Rate Variability : HRV Normal heart rate is chaotic
Quiz on physiologic dynamics Heart Failure Heart Failure Heart Rate (bpm) Heart Rate (bpm) Normal Atrial Fibrillation Heart Rate (bpm) Heart Rate (bpm) Time (min) Time (min) Loss of dynamical fluctuations is bad Not all dynamical fluctuations are good
Heart Rate Variability : 8 hours
Degree of Stationarity
Data White Noise
Data White Noise
Degree of stationary for nonlinear data Inter- and intra-wave modulations
Duffing Chip Data
Duffing Chip : Hilbert ZC
Duffing Chip : Hilbert Quad
Duffing Chip : Hilbert Hilbert
Duffing Chip : Degree of Stationarity
Duffing Chip : Degree of Stationarity
Duffing Chip : Normalized Intra-wave Modulation
Conclusions The high frequency range of the spectrum is highly intermittent. Even the Statistical Degree of Stationarity cannot smooth the variations. Before invoke the stationarity assumption, we should check the Degree of Stationarity.