小金井における脈動観測結果について (I)

Abstract

Observation of microseisms is being performed by the tripartite network, ΔPQS, which is established in the compound of Tokyo Gakugei University in Koganei-shi, Tokyo. This report deals, in the first place, with the graphical method to obtain the direction from which microseisms came, then it discusses the relationship between the propagating direction of microseisms and period and the meteorological and oceanographical conditiions, on the basis of the microseismograms recorded during the period of July to November, 1962, as tabulated in Table 1.In each of Figs. 6 to 10, the upper figure shows the directions from which the microseisms arrived. The direction is divided into sections of 45° each, and the number of microseismic waves that reached the respective sections is represented by percentage. The middle figure shows the frequency distribution of period. The lower figure is the weather map of the day of observation.The above results are summerized in the following.1. Microseisms observed in Tokyo were closely related to the oceanographical and meteorological conditions of the Pacific coast and its offing, north of the Boso Peninsula. When a typhoon, or a cyclone or a cold front, occurred over the sea north of the Boso Peninsula and the sea was more or less rough, the microseisms reaching Tokyo were predominantly from E or NE. In case the typhoon, or cyclone or front, was located near the coast, the period of the arriving microseisms was mostly 4 to 5 seconds, but if the typhoon existed in the distant offing (near or on the outer side of the Japan Trench), the period and the amplitude of the microseisms were apt to become remarkably large.2. In case a typhoon occurred over the southeastern ocean and a front in the northeastern offing, microseisms coming from SE were observed along with those predominantly from E or NE.3. When a typhoon occurred in the southwestern or western offing, with a cyclone or a front in the northeastern offing, and the sea along the Pacific coast of Japan was rough, the microseisms arriving at Tokyo were fairly distinctly classified into two directions, SW and NE or E.4. From the 16th to 17th of November, 1962, the sea north of the Boso Peninsula was comparatively calm, and Typhoon No. 28 at about 800km southwest of the mainland of Japan was moving eastward at a considerable speed. The microseisms then observed in Tokyo were predominantly from the direction in which the typhoon's center was located at the time of 19 to 20 hours earlier than the observation time, and the direction changed with the typhoon's movement. The occurence of these microseisms may be explained as follows: The swells, having the period averaging about 14 seconds, were caused by the wind of the velocity about 25m/sec in the storm zone (within the 150km radius); these swells, traveling at the velocity about 15m/sec, approached the mainland, and there they gave to the microseisms due to the Longuet-Higgins mechanism.The writer intends to accumulate more data in order to confirm the abovementioned results.