On Pulsatory Oscillations of Ground at Tomisaki on the Pacific Coast

[摘要] This paper deals with the pulsatory oscillations of ground observed at Tomisaki where situates at the top of Bôsô peninsula projecting into the Pacific Ocean. It must be indisputable that the microseisms are caused by the meteorological agents. The author has tried to investigate the occurrence and decay of microseisms consulting daily weather charts. As the generating agent following four are considered-typhoon, cyclone, strong wind (mainly monsoon) and discontnuity line. The former two are specially treated for three cases according to the travelling path of their center.Thus the microseisms at Tomisaki are treated in many cases taking into account of the kind of meteorological agent and the relation between the maximum amplitude, its time of occurrence and the position of center of cyclone (or typhoon), their barometric depression, the maximum wind velocity etc. is precisely investigated. Moreover the comparison between the microseisms at Tomisaki and Tôkyô, the studies on the mode of oscillation etc. are made. The main results thus obtained are as follows:-(1) The cyclone and typhoon are the most powerful agent to cause microseisms, and the monsoon, discontinuity lines are next to them. By cyclones larger microseisms are caused at Tomisaki than typhoons, while the reverse seems to be the case at Tôkyô.(2) The maximum amplitude of microseisms ordinarily occurs when the depression comes to the nearest place and sometimes when it travels on the Pacific Ocean, east to the northern part of Japan Proper. The latter case is caused by the rapid development of the depression at that place(3) When the depression travels on sea, it gives far larger microseisms compared with the similar case it travelling on land.(4) Microseisms caused by the strong wind blowing there becomes proportionally large according as the wind velocity increases.(5) Extraordinarily large microseisms, such as more than 50μ in amplitude, are observed only when the remarkable depression exists near the observing place. Merely strong wind causes microseisms less than 30μ even when its velocity becomes as 25m/s.By the above results the author has come to the following conclusion about the productive mechanism of microseisms at the coast that the sea waves are the most powerful agent to cause microseisms, and especially powerful when they are generated in the neighbourhood of the center of cyclone or typhoon.

[学科分类] 大气科学