地震活動度と火山作用(3) : 有珠火山活動(1977)に出現した新相波-Low velocity surface waves及び大島→東京(1905)で見出された低速表面波

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Appearance of Low-velocity surface waves: the present writer found two remar-kable wave groups in the study of earthquake swarms which accompanied in the Usu volcanic activity in 1943〜1945, and he classified 3rd and 4th phases, named also K-phases in the paper Seismicity and volcanism (II) At the mori observatory JMA, 54km distant from the Usu volcanoe, these phases were observed in a lator portion of the seismograms. The greater part of the wave path from Vol. Usu to the Mori observatory run under the sea, and the velocity of the earlier K-phase was V_<K_3>=0.6 km/sec, while that of the later one was V_<K_4>=0.2〜0.3 km/sec. The dominant periods of these phases were arround 2 seconds. The earthquakes of this swarm continued for about two years from 1943 to 1945. More than two thousands earthquakes belonging to this swarm displayed, whithout exception, these two remarkable wave groups of maximum amplitudes at similar portions of the seismograms as observed at the Mori observatory. However, a contrasting phenomenon was observed at the Sapporo District meteolorogical observatory JMA, 69km in epicentral distance, where no particular low- velocity K-phases were observed in the seismograms of the same swarm, and similar phenomena observed at the Muroran LMO (Local Meteorological Ob-servatory) JMA, epicentral distance 25km. Such a difference in the seismogram types observed at Mori, Muroran and Sapporo are quite interesting. This difference may be attributable to the difference in geological structures along the paths of seismic waves from Usu to Mori, observatory, to Muroran LMO and to Sapporo DMO. A somewhat detailed study of the point has already been reported The activity of the Sakurajima volcano is almost invariably accompanied by a swarms of earthquakes. Such earthquakes have been observed at the kagoshima L.M.O (Local Meteorological Observatory), which is about 10 kilometers from the center of the volcano. In all the seismograms which took place in 1910, 1939 and 1946, the present writer found two remarkable K-phases similar to those mentioned previously. The velocities, calculated on an assumption that these earthquakes originated at the center of the volcano, turned out to be V_<K_3>=0.4km/sec and V_<K_4>=0.2km/sec. The wave path from the origin to the observing station crosses the Bay of Kagoshima, and the resemblance of the circumstances to those of the case at the Mori observatory should be particularly noted. Seismograms of the Ito earthquake swarm of 1930, as observed at Tokyo, Yokohama and Tomisaki, were also examined by the present writer. Some earthquakes were previously studied by Matuzawa and Fukutomi, although the seismograms used in their study were recorded by the seismographs at the Earthquake Research Institute of the University of Tokyo, whereas the data used by the writer are records at the local observatory of the Japan Meteorological Agency. Notwithstanding the difference in the type of seismographs, similar characteristic K-phases were observed in all the seismograms of this earthquake swarm which lasted for about six months, observed at the three stations, Tokyo JMA (Δ=97km), Yokohama LMO (Δ=69km) and Tomisaki (Δ=67km). The velocity of propagation of the 'K-phase was found to be V_K≑1.0km/sec, irrespective of the station's location. Miyakejima and Niijima earthquake swarms: A wave group exactly similar to that of the case Usu) was seen in each of the seismograms at Tokyo, Yokohama and Tomisaki stations, in the case of the Miyakejima earthquake swarms of 1962. The epicentral distance of these stations being 177 km, 147 km and 94 km respec-tively. The same was true in the seismograms recorded in Tokyo, in relation to the earthquake swarms occurring near Niijima island in 1957 and 1960. The velocity of these K-phases was K_3≒1km/sec in both Miyakejima and Niijima swarms, and the dominant period of the same phases was about 6 to 7 seconds. From the foregoing examples we have realized that even in short epicentral distances, from 10 to 200 kilometers, the largest amplitudes in the seismograms of shallow earthquakes are not always due to S-waves, and in some cases they must be assigned to the surface wave groups whose velocities are as low as 0.3 to 1.2km/sec or so. We must, therefore, take due precaution in the calculation of magnitude and energy of near earthquakes if we have to deal with K-phases of the present category. Although it is not yet the time for a full explanation of the waves we have been concerned with, we have found that the so-called low-velocity sedimentary layer subjacent to the sea-water plays an important role in the generation of these waves observed at Mori. Thus, we may possibly assign similar conditions to the cases in which submarine wave paths are involved. Such K-phases could be considered as surface waves due to the so-called liquid coupling. However, we have also recognized that similar low-velocity K-phases are present in the earthquake motions propagated through land paths across the Kanto Plain where diluvial and alluvial formations prevail. Whether or not the existence of a low-velocity layer is a necessary condition for the occurrence of such K-phases is an important question. It is, therefore, necessary to study this question theoretically from various angles. It is also necessary to collect, accumulate and assess the data related to the appearance of the K-phase as well as to make a detailed analysis of the materials already obtained.

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木澤綏
駒澤大学文学部

地震活動度と火山作用(3) : 有珠火山活動(1977)に出現した新相波-Low velocity surface waves及び大島→東京(1905)で見出された低速表面波

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