1959 Volume 9 Issue 3-4 Pages 204-239
The formation of Showa-Shinzan, the world-famous volcano in the history of volcanology, was accompanied by marked swarms of earthquakes. The author, aiming to study in detail the characteristics of these earthquakes recorded by the seismographs at three weather stations around Showa-Shinzan, namely, Sapporo (DMO) (Δ=69km), Mori (Δ=54km) and Muroran (LMO) (Δ=25km), as well as to obtain clues for predicting volcanic activities and further in order to clarify their mechanisms, has continued his research since he first reported on it in Part I of this paper. The principal results attained this time are summarized below:
1) The A-type earthquake swarms (December 1943-May 1944) which preceded the eruption had various amplitudes, but as the time of eruption drew near the amplitudes diminished exponentially. The B-type earthquake swarms accompanying the formation of the lava dome occurred when the eruption was over and lasted about one year, with nearly the same amplitudes throughout the period. This interesting fact suggests that there are some differences between the two types in hypocentral depth.
2) The waves of the maximum amplitudes are fairly distinctly polarized, and at all the three stations we observed that the ratio (AN/Ax) of the maximum amplitudes of the two components of horizontal motion is AN/AE>1 for the A-type earthquakes and, reversely, AN/AE<1 for the B-type earthquakes. This marked contrast between the two groups would be attributed largely to differences in the depths of hypocenters and the mechanism of occurrence of earthquakes.
3) During the early stage of the volcanic activity (December 28,1943-January 30,1944), the principal ten earthquakes of A-type distinctly showed the initial motions of P-waves. They appeared as compressional waves at Sapporo and Mori stations, and dilatational waves at Muroran station. Thus, a noteworthy result was obtained as compared with the migration of the hypocenters in the volcano.
4) There were recognized two phases which have str i k ingly slow group velocities. They were observed distinctly on the seismograph at Mori station (Δ=54 km). This tendency is particulary marked in the B-type earthquake swarms. The phases are tentatively named the third and the fourth phase. The velocity of the third phase appears to be less than 1 km/sec and that of fourth to be 340 m/sec. I t is safe to conclude from many evidences that the third is an Airy phase, whereas the fourth phase has characteristics of an Air-coupled Rayleigh wave.
5) The mode of variation of the amplitude ratio of the third and the fourth phase suggests the tendency of the hypocenters of the A type earthquakes to become shallower as the time of eruption draws near, while those of the B-type earthquakes deepen with the time elapsed after the eruption.
