HoGBEN. — On Earthquake Motion. 593 



Wanganui earthquake of the 8th December, 1897, was about 

 eighty-five miles per minute, or, say, 225,000 cm. per second. 

 The vahie, according to Gray and Mihie (calculated from ex- 

 . periments on Japanese rocks), for normal waves in granite is 

 395,000 cm. per second.* 



Approaching the question of the motion of an earth-par- 

 ticle from another pomt of view, and regarding it as in most 

 cases approximately simple harmonic vibration, we have 27ra 

 = VT, where a = amplitude, V = maximum velocity of par- 

 ticle, T = pei'iod in seconds, and the maximum acceleration 



= mtensity = — • 



Now, from cases in which the number of vibrations per 

 second have been noted, I should judge that in the case of the 

 most violent New Zealand earthquakes these have been about 

 three per second. If we take T = ^ in the case of the Wanga- 

 nui earthquake, and the intensity to be between viii. and ix. 

 on the Eossi-Forel scale, or, say, 900 mm. per second accord- 



V2 



mg to Dr. Holden's equivalents, then we have — = 900, and 



^ V = 2Tra, .'. a = 2-5 mm. nearly, or the displacement of any 

 earth-particle was about ^l in. 



This is about one-third of the maximum amount of hori- 

 zontal displacement in the severe Japanese earthquake of the 

 15th January, 1887, according to the calculations of Professor 

 Sekiya, based upon seismograph tracings. Our estimate would 

 not, therefore, seem altogether improbable — possibly rather too 

 high. 



In the absence of accurate data, further speculation would 

 be unprofitable ; but I trust enough has been said to show 

 that future investigation and more accurate observation of 

 earthquakes is likely to lead to interesting results, 



* Quart. Jour. Geol. So., vol. 39, p. 139. 



38 



