Comparison of Site Fundamental Period Estimates Using Weak-Motion Earthquakes and Microtremors

Abstract eng:
This paper presents a comparison of the site period estimated with different techniques which use weak motion measurements recorded at seismic stations in Mayagüez, Puerto Rico. The stations are located in sites with soil profiles ranging from NEHRP classes B to E. The literature reports different degrees of success when estimating site periods based on low-intensity earthquakes and microtremor measurements. Some of the issues reported are related to effects due to differences in the seismic source, travel path, etc. This paper compares the dominant site periods computed using three well known methods, namely the Fourier amplitude spectra, the Fourier spectral ratios between the spectra at the site and at a reference station, and the spectral ratios between the horizontal and the vertical components of ambient vibrations (i.e., the Nakamura's technique). The paper also presents site period estimates based on two new techniques. The first approach proposed by Montejo and Suarez (2006) consists in the application of the wavelet transform to strong motion accelerograms. This paper applies for the first time this new technique to weak-motion earthquakes. The other new approach, proposed by Cano (2008), is based on random vibration theory applied to ambient vibrations. The paper evaluates the applicability of the different techniques for various site conditions and weak motion earthquakes with different amplitude and frequency content and ambient vibration signals recorded at strong motion stations located in Mayagüez, Puerto Rico. The results show that weak-motion measurements can be used to estimate the dominant period of sites with acceptable reliability.

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