56 



ooc'urs at tin' point c on tlnj outer cirilc. The outer eirele i?^ the liue traeed hy 

 the ])niiiter aft<'r flie cessation of tlie disturlianee. 



It nin.^t not lie supposed tliat tlie want of eoiiici<lenre lietween tiie circles 

 trac!(l before and after the earthnuake is evidence of a permanent dis])laeeiuent 

 of tlu: ground, since during a prolonged disturbance the accunuilated effects of 

 frictional A)rc'os are almost sure to ])roduee some resulting motion of the so-called 

 steady line, especially when the eijuilibrium is ni'arly neutral. 



This rei'oid exemjdities very well the characteristics mentioned at the end of 

 tlie ])i-eceding jaragraph. The motions are at first so small that it is difticult to 

 l>oiiit out the beginning definitely, and the undulations die out so gradually that 

 it is c(|nally «lifficult to determine the end. Again, though the amplitudes are 

 far ficim regular, we cannot ])oint to any one displacement as the principal shock. 



At li, where; the motion is greatest, the displacement (from West to East) is 

 (1 mm. on tlie record, that is 1 mm. of actual earth motion. The motion there 

 is not very far from being simple harmonic, with a period of 1.4 sec. TJiis gives 

 a maximum velocity of 2.3 mm. per .sec. and a maximum acceleration of 0.6 mm. 

 jier sec. per sec, or say j^Vo' "^ *'"-^ acceleration due to gravity. In registering 

 earthijuakes such as this, the necessity of avoiding friction in the seismometer 

 will be a]>j»arent (§ 38). 



§ 54. Earthquake of March Sih, ISSl* 



Plate XIII is the lecord of this eartliquake, which was one of unusual 

 violence compared with the minute disturlxvuces to wliich residents in Tokio are 

 accu.-^tomed. 



The record, wliich is in some respects the best the writer has obtained of 

 any earth([uake, was given by the horizontal pendulum seismograph of § 23. 

 The multiplying ratio was again six to one. Of the two records on Plate XIII, 

 the inner is N-S motion, the outer E-W motion. Tlie cross lines d and d' .show 

 the di.-;tance measured circumferentially between tlie marking ends of the two 

 ])ointers. ( )ue revolution of the plate corresponds to about 80 .seconds of time. 

 The beginning of motion is at a on the E-AV circle (at the to]) of the Plate). At 

 a', whicit is the corresponding jioint on the N-S circle, and for some time later, 

 scarcelv any displarenient oeciiis. 'I'lie earliest considerable N-S motion is at b, 

 which is synchiduous with the E-W displacement /-'. IMidway between a and b 

 tliere is a considerable E-W motion which has nothing to correspond with it on 

 the N-S cin'le. 



The most violent motions occur a littli' later than the points marked b and 

 //. The record extends over nearly two com]»lete revolutions of the plate, up to 

 the point c in the outer circle, where it will be observed to leave off abruptly, 

 'fhis is because the writer, who had the good fortune to be ])re.sent in the obser- 

 vatory during this earth<|iiake, withdrew the jilate at the point marked c, in 



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* See also 'i'l.uis. of the Seismologicivl Society of Japan, \u\. Ill, \i. 121. 



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