عنوان مقاله [English]
Rayleigh waves can be more destructive in comparison with other types of waves during earthquakes. Identification and extraction of Rayleigh waves from seismic records can be more exactly performed in time-frequency domain. In the present study, Continuous Wavelet Transform and Stockwell Transform have been used to transfer three-components of seismic signals to time-frequency domain. Rayleigh waves can be extracted based on elliptical characteristics of these waves. On the other hand, Retrograde and Prograde Rayleigh waves and their propagation azimuth can be separately extracted with phase transferring of vertical component as well as normalized inner product technique. In the present study, the mentioned two algorithms have been programmed. Accuracy of the methods has been verified with using synthetic and Chi-Chi earthquake data. Finally, Rayleigh waves and their seismic parameters have been extracted from Tabas, Bam and Manjil earthquake records. Rayleigh waves of the earthquakes have been analyzed and compared with using different seismic parameters. According to the results, Retrograde Rayleigh waves are more predominant in comparison with Prograde Rayleigh waves in earthquake signals.
Extraction of Rayleigh waves is one of the important tasks in seismology and applied geophysics due to their destructive effects. A lot of study has been conducted in order to develop an effective method for the extraction of Rayleigh waves. Most of these methods have been presented based on the ellipticity characteristics of Rayleigh waves. Each of them has their advantages and disadvantages. In this study, two of them have been investigated, programmed and compared with each other.
Methodology and Approaches
Rayleigh waves have been extracted from three-component signals in time-frequency domain in both of the investigated methods. In algorithm 1, continuous wavelet transform has been used to transfer three components of signals from time to time-frequency domain. However, Stockwell transform has been used to perform this approach in algorithm 2. Stockwell transform retain the absolute phase of each localized frequency component, the characteristic of which is required in algorithm 2 for time-frequency transform. On the other hand, Rayleigh waves have been extracted based on the instantaneous reciprocal ellipticity and phase difference in algorithm 1 and 2 respectively. The most important advantages of algorithm 2 is that retrograde and prograde Rayleigh waves and their propagation azimuth can be separately extracted while it is not possible in algorithm 2.
In this study, both of these algorithms have been clearly illustrated by flowcharts and programmed in Matlab. Synthetic signals and Chi-Chi earthquake signals have been used for verification. After verification, results of both algorithms have been compared, and finally they have been employed in extracting Rayleigh waves of Tabas, Bam and Manjil earthquakes. Finally, maximum displacements of Rayleigh waves and total earthquake signals have been investigated and compared in these three earthquakes. In addition, energy discharge of extracted Rayleigh waves during the earthquakes have been investigated.
Results and Conclusions
Results of the study showed that algorithm 1 can be effectively used if there is not required to extract retrograde and prograde Rayleigh waves and their propagation azimuth separately because the algorithm is simpler and faster in comparison with algorithm 2. However, algorithm 2 should be employed on the condition that the extraction of retrograde and prograde Rayleigh waves and their propagation azimuth have been required. In addition, it has been concluded that Rayleigh waves have more energy in earthquake signals compared to other types of waves. Consequently, elliptical polarization is more than linear polarization in earthquakes. In comparison of retrograde and prograde Rayleigh waves, it has been concluded that retrograde Rayleigh waves have more energy and therefore they can be more destructive. Finally, it has been resulted that ratio of Rayleigh waves are different in earthquakes. Induced displacement by Rayleigh waves in Manjil earthquake was more than Bam and Tabas earthquakes.