. After earthquake and fire . opagatedmay mark the limit beyond which the seismic waves are, onaccount of certain physical properties of the underlying me- OBSERVATIONS OF DISTANT EARTHQUAKES. 145 dium, unable to penetrate; or there may be, as ProfessorNagaoka suggests, a maximum transit velocity. A roughcalculation, based on the relation of the duration of the istpreliminary tremor and the epicentral distance, and on thevalue of the different velocities, gives 600 kilometres or about400 miles as the probable depth of the layer along which thevibrations of the ist preliminary tremor are propag
. After earthquake and fire . opagatedmay mark the limit beyond which the seismic waves are, onaccount of certain physical properties of the underlying me- OBSERVATIONS OF DISTANT EARTHQUAKES. 145 dium, unable to penetrate; or there may be, as ProfessorNagaoka suggests, a maximum transit velocity. A roughcalculation, based on the relation of the duration of the istpreliminary tremor and the epicentral distance, and on thevalue of the different velocities, gives 600 kilometres or about400 miles as the probable depth of the layer along which thevibrations of the ist preliminary tremor are propagated. It is probable that the waves having velocities V2 and Vaare transmitted along layers at smaller depths within theearths crust. Propagation of the Seismic Motion Completely Around theEarth.—Let T be the observing station and C the earthquakeorigin. Then there are three sets of motion, which can be dis-tinguished; they may be denoted respectively as Wx, W2,and W3. The Wx waves are those propagated from C to T along the. shortest path, parallel to the surface, namely along the minorarc; the W2 waves are those propagated from C in the oppositedirection and arriving at T after passing through the antipodeof C, namely, along the major arc; and the W3 waves are theWx waves which are propagated beyond T in the same direc-tion, and again arrive at T after making one complete circuitof the earth. The identification of the W3 waves is possible only in a fewcases; that of the W2 waves is, however, more definite, beingcharacterized by the fact that their period is much slower than 146 AFTER EARTHQUAKE AND FIRE. those of the preceding vibrations, which form the end portionof the Wx waves. The average period of the W2 waves is, with a few excep-tions, uniform and gives a mean value of seconds, whichis identical with the predominating period in the 3rd phase ofthe principal portion; the period of the W3 waves is also nearlythe same as that of the W2 waves. These facts seem to ind
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