244 HOVEY— EARTHQUAKES : [April 24, 



Scandinavia on the north and to the east coast of North America 

 on the west, an area estimated by Baron von Humboldt at four 

 times that of the whole of Europe. The Andean earthquake of 

 1868 shook severely a strip of country 2,000 miles long. The 

 modern seismographs have given pronounced records of earth- 

 quakes whose origin was certainly not less than 8,000 miles distant — 

 truly world-shaking events. 



The depth of the origin of the shocks below the surface of the 

 earth probably never exceeds thirty geographical miles and usually 

 is not more than from five to fifteen miles. The geological struc- 

 ture of the region through which the earth waves are propagated 

 affects the rate of advance of the same earthquake in different direc- 

 tions and produces many changes in the direction of movement and 

 great differences in the destruction wrought upon buildings. 

 Heavy earthquake shocks are transmitted through the earth at a 

 greater velocity than light ones and the same shock shows different 

 rates in different materials. 



In the case of distant quakes three disturbances are recorded 

 instrumentally. The first set of waves to arrive comes on a direct 

 course through the earth's mass ; the second set comes along the 

 shortest route on the surface, while the third set arrives by the 

 opposite and longest surface route. The last are comparatively 

 feeble, and they may arrive three and one half hours behind the 

 second set. The first set of waves, those coming through the earth, 

 are propagated with the greatest velocity, which is practically uni- 

 form and is about ten kilometers (6^ miles) per second. These 

 direct waves have been shown by Marvin to be longitudinal in 

 character, and this character combined with their velocity is sup- 

 posed to cause them to give out the musical sounds which are the 

 premonitory rumblings of an earthquake. The second set are the 

 surface waves due to the " principal portion " of the earthquake, and 

 the increased use of delicate instruments of measurement has led to 

 the acceptance of 3.3 km. per second as their normal rate of propaga- 

 tion. The determination of these various velocities leads to the 

 conclusion that the crust of the earth is practically uniform in con- 

 stitution to a depth of at least thirty miles. 



The duration of an earthquake and the number of shocks in it 



