DEPTH OF EARTHQUAKE-FOCUS. 131 



curves, and Z>, c, d, three points on the curve ; a is the epicentrum. 

 This is Seebach's method. It gives good results if the coseismals be 

 not too irregular. 



Determination of the Focus. — The normal wave is a wave of longi- 

 tudinal-oscillation. The direction of oscillation, therefore, is the same 

 as the direction of transmission (wave-path), which is the radius of the 

 agitated sphere. If, therefore, the direction of the ground-motion be 

 followed into the earth, it carries us back along the wave-path to its 

 origin, the focus. Two such wave-paths by their intersection would 

 determine its position. Thus, in Fig. 110, if c and b be the position of 

 two seismomet- s e 1) ol 



ric observato- 

 ries, the angles of emergence, 

 x c a and x b a, being given by 

 observation, and the distance, 

 c b, being known, we have all 

 the elements necessary to de- _ „ n 



J Fig. 110. 



termine either by calculation 



or by accurate plotting the wave-paths c x and b x, and their point of 



intersection x, and therefore of the depth a x. 



We have assumed the earth-waves as normal. We are justified in 

 so doing, because this is the most decided wave, and soon outruns the 

 transverse wave. 



Although seismometers, such as we have described, are necessary 

 for accurate results from few observations, yet by multiplying the ob- 

 servations, even by rough methods, approximative results may be ob- 

 tained. We will mention several examples : 



In 1857 a terrible earthquake shook the territory of Naples, destroy- 

 ing many towns and villages, and killing about 10,000 people. The 

 scene of destruction was visited soon after by Mr. Mallet. By careful 

 examination of overthrown objects, many lines of transit of the surface- 

 wave were determined, which, protracted, carried him with considera- 

 ble certainty to the epicentrum ; similarly many lines of emergence, or 

 paths . of the spherical wave, protracted back, conducted to the focus. 

 This focus was determined to be not a point, but a fissure, nine 

 miles long and through three miles of solid rock. The center of this 

 rent was about six miles beneath the surface.* By somewhat 

 similar methods the focus of the Japan earthquake of February 

 22, 1880, was found by Milne to be only three to five miles deep.f 

 By a different and new method, viz., the law of decrease of intensity 

 of the shock-motion, the focus of the Charleston earthquake of Au- 

 gust, 1886, was found by Captain Dutton to be about twelve miles 



* Mallet, Principles of Seismology. 



f Seismological Society of Japan, vol. i, Part II, p. 1. 



