地震活動と火山作用(2) : 有珠火山と三原山の地震群

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It is suggested that there are some kind of Low-Velocity surface waves which display maximum amplitudes in some seismograms of near earthquakes that have been observed at certain localities. We must, therefore, be well aware of this fact in the estimation of magnitude and energy of near earthquakes. Evidence of such waves has been found time and again in a number of earthquake swarms and aftershock groups of shallow origin. It has thus been revealed that such kinds of K-Phases are observed very clearly even at very short epicentral distances ranging from some ten or twenty kilometers up to two hundred kilometers. It has also been demonstrated that the velocity of such K-Phases varies with locality or wave path, and with the focal depth of the earthquake. The velocities of these waves are low, within a range of 0.2 km/sec to 1.9 km/sec, and therefore these phases form solitary groups with predominant amplitudes considerably later than those of the S-Phases. It is not seldom that the amplitudes of these later phases overwhelm those of the S-Phases, as is indicated in the seismograms shown here. From the well known theory of dispersion of surface waves, it may naturally be presumed that these K-Phases belong to certain kinds of Airy phases which are related to the surface waves along the sedimentary layers near the earth's surface in the region concerned. Since the presumption should not be ignored in all cases, and since such a geological structure may be found in other parts of the world, the appearance of these Low-Velocity surface waves which give maximum amplitudes to seismograms may also be expected in other countries of the world.

著者

木澤綏
駒澤大学文学部

地震活動と火山作用(2) : 有珠火山と三原山の地震群

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