226 EARTHQUAKES. 



CHAPTER XIT. 



DISTRIBUTION OF EARTHQUAKES IN SPACE AND TIME. 



General distribution of earthquakes — Occurrence along lines— Ex- 

 amples of distribution— Italian earthquake of 1873 — In Tokio — 

 Extension of earthquake boundaries — Seismic energy in relation to 

 geological time ; to historical time — Relative frequency of earth- 

 quakes — Synchronism of earthquakes — Secondary earthquakes. 



General distribution of earthquakes, — The records of 

 earthquakes collected by various seismologists lead us 

 to the conclusion that at some time or other every 

 country and every ocean in the world has experienced 

 seismic disturbances. In some countries earthquakes are 

 felt daily, and from what will be said in the chapter on 

 earth pulsations it is not unlikely that every large earth- 

 quake might with proper instrumental appliances be 

 recorded at any point on the land surfaces of our globe. 

 The area over which any given earthquake extends is 

 indeterminate. The area over which an earthquake is 

 sensible is sometimes very great. The Lisbon shock of 

 1755 is estimated as having been sensible over an area of 

 3,300 miles long and 2,700 miles wide, but in the form 

 of tremors and pulsations it may have shaken the whole 

 globe. 



The regions in which earthquakes are frequent are 

 indicated in the accompanying map, which, to a great 

 extent, is a reproduction of a map drawn by Mallet. The 

 regions coloured with the darkest tint are those where 



