On the Propagation Characteristics of Microtremor

Abstract

Fundamental natures of microtremors have already been discussed by many investigators. The grouth mechanism of microtremor is not yet known. In this paper, the propagation characteristics of microtremor was investigated. The experimental field is Hakodate City in Hokkaido. The propagation of microtremor was confirmed from the tripartite observation, and by the orbital motions, directions and velocities of microtremors, it was found that these waves were Rayleigh type wave, Love type wave and a combination of both. Phase velocity observed at the place where confirmed the propagation of microtremors agreed partly with the dispersion curves of fundamental mode of Rayleigh and Love wave. A ditch with a depth of 1 m was made at the place confirmed the propagation of microtremor. However, it didn't affect the vertical component of microtremor. Nextly, microtremors were observed on the moat with a depth of 3 m at Goryokaku in Hakodate City, by means of tripartite and 3-component observation. From results of observation, it was found that the attenuation of the frequency characteristics of microtremors with vertical component between two places across the moat were more rapid with higher frequency and microtremors on the isolated island around the moat were only composed of Love type waves. The attenuation of energy ratio for k(wave number) x d (depth of the moat or slit) in case of the moat was well in accordance with results of model experiments which made in order to show the attenuation of Rayleigh wave for a slit. From a point of view of the propagation characteristics of microtremors, the vertical component of microtremors may be almost composed of Rayleigh wave. •˜1. ‚Í ‚ ¶ ‚ß ‚É "÷ "®‚Ì Šî 'b "I •« Ž¿ ‚É ‚±‚ ‚¢ ‚Ä‚Í, ‚· ‚Å‚ÉOEÂ-‚© ‚çŠô '½‚Ì •l •X‚É ‚ae‚‚Ä'² ‚× ‚ç‚ê ‚Ä‚¢ ‚é.