4. Data Processing/Signal Processing
Rayleigh Wave Group Velocity in Japan Revealed From the Cross-correlation Analysis of Microseisms Excited by Typhoons
Nobumasa Takagi, Haruo Sato and Takeshi Nishimura
Tohoku University, Japan.
Abstract
Cross-correlation analysis of microseisms has been recently recognized as a new method to measure the group velocity of surface waves and used for the velocity tomography in various areas in the world. We note that microseisms are mostly excited by oceanic waves and the propagation direction of microseisms observed on land is usually unilateral; however, the necessary condition of this method is theoretically the establishment of an equi-partition state, that is, an isotropic distribution of microseism propagations. There have been few studies which paid attention to this condition for the practical analysis yet. In the cross-correlation analysis, it is ideal that each station pair is isotropically surrounded by seismic noise sources. In order to realize an idealized condition, here, we select records of microseisms in four periods of different weather conditions: three typhoons and one low pressure area are located at different positions surrounding Japan island. Microseisms registered by 709 short-period seismic stations of Hi-net in Japan are used for the analysis, where a time window of 36-48 hour length is long enough for stable measurement in each period. From the cross-correlation analysis, we find a good correlation between the location of the center of typhoon and the propagation direction of microseisms for periods of 4-16s; however, we recognize that microseisms of 2-4s periods are generated from the nearest coast. By stacking all the microseisms recorded for the four different periods, we obtain reliable group velocities of Rayleigh waves, which show normal dispersion. We further apply the tomography technique to group velocities for station pairs in order to obtain a 2D velocity distribution in Japan. The results well reflect the geological structure of each area. For example in a 4-8s period band, granitic mountain areas show high velocities near 3 km/s while the Kanto plain shows low velocities near or under 1 km/s reflecting thick deposits.
Last modified: Wed May 10 17:01:06 2006