Applications of Wavelet Transform on The Analysis of Strong-Motion Data
Date Issued
2012
Date
2012
Author(s)
Chen, Shen-Ming
Abstract
The influence of local site geological and soil condition on the intensity of ground shaking and earthquake damage has been known for years, which plays an important role in earthquake-resistant design and must be accounted for on a site-by-site basis. From the response spectra of the recordings in the Taipei basin, unusual spectral ordinates can be found in the range of period between 2 – 3 second and such ground motion would be very harmful to tall buildings. Although many efforts work on the site effects of Taipei basin and derive many important results, however, the origins of the unusual response spectra have been rarely discussed. This study tries to find the origins which result in unusual spectral ordinates in the recordings of the soil sites. Through the application of Wavelet Packet Transform (WPT), the complicated acceleration trace of a record can be decomposed into many components with very narrow frequency band. By the adequate time window and comprising the strong shaking stage signals of the E-W and N-S component, we can derive a very simple decomposed acceleration trace (DAT) with a distinct shaking direction and maximum amplitude. Through the new defined index CEI, the common low-frequency components corresponding to the low-frequency basin effects of the soil sites can be found. The 3rd (fc=0.244 Hz.) to 5th (fc=0.439 Hz.) decomposed ground motions are identified to be the main components due to the basin effects. From the figure associates the Taipei basin map with the DATs of all soil sites, we can observe the response of the Taipei basin at a certain period of shaking. The low-frequency strong shakings usually occur at those sites at the central and the western deepest area of the basin. The systematic behaviors of soil sites including horizontal oscillations and rotations seem to be due to the resonance of the basin sediment. Because of the lack of phase information in the DATs and results obtained from limited recordings of limited events, the oscillations and rotations due to resonance of the sediment is still a speculation. More studies are needed to verify. The amplifications of those sites at the east and south edge of the basin are found to be small in these three low-frequency components. For higher frequency bands (0.6 – 1 Hz.), the large amplifications can be found at the central area and the east edge of the basin. By the comparison of the SAs of the original and the extracted ground motions, we can find that the SAs in the range of period 2 – 5 second are comparable with each other. This approach can extract the low-frequency ground motions from a record effectively, and can be helpful to the improvement of design spectrum corresponding to the Taipei basin in the future. This study also proposes a three-phase approach includes the calculation of constant-level coda trace, which helps to estimate residual ground displacement. The obtained permanent displacement values are strongly consistent with the values obtained by GPS measurements.
Subjects
Taipei basin
site effect
basin effect
near-fault
permanent ground displacement
wavelet
Type
thesis
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