286 PHYSICAL GEOLOGY 



The bending of railroad tracks (Fig. 284) and the zigzag position 

 of rows of trees which were straight before the earthquake were pro- 

 duced by the lateral shifting of earth blocks. 



Duration. — The duration of a severe earthquake is very short. 

 As has been stated, the shock which destroyed San Francisco lasted 

 about one minute, and the movement along the 300 to 400 miles of 

 fault rift probably did not consume two minutes. The great Assam 

 (India) earthquake lasted only two and a half minutes, and the 

 destruction was accomplished during the first 15 seconds. The de- 

 structive shocks of the Charleston earthquake lasted a little more 

 than half a minute. Between December 16, 181 1, and March 16, 

 1 8 12, at least 1874 shocks were felt in the Mississippi Valley, of which 

 eight were severe. No individual shock, however, was of long 

 duration. 



Frequency. — Delicate instruments (seismographs) show that the 

 earth is continually trembling in all parts, and it is probable that 

 quakes severe enough to be felt are shaking the earth in some regions 

 at all times. Certain portions of the world as, for example, parts of 

 Japan and southern Italy are subject to frequent shocks. In the 

 former, a severe earthquake occurs on an average of every two and 

 a half years, and minor shocks four times a day. A careful record of 

 the aftershocks of the earthquake at Messina, Sicily, in 1908, shows 

 that 87 shocks were felt during the first four days and 862 during the 

 following year, four of which were severe. 



All definite predictions as to the time and place of earthquakes are 

 of little value. This is illustrated in the case of San Francisco. The 

 earthquake rift or fault line was known before the earthquake. It 

 was believed that renewed faulting might occur at any time, but 

 whether within one year or many years could not be foretold. Since 

 earthquakes are the result of a relief of strain, it is evident that a 

 region is likely to be immune from severe shocks for some years after 

 it has been shaken, since the strains which produced the shock have 

 been partly or entirely relieved, and a shock will not occur until strains 

 have again accumulated. 



Areas Affected by Certain Earthquakes. — The areas affected by 

 earthquakes vary greatly in size. A region four times the size of 

 Europe is said to have been affected by the Lisbon (Portugal) earth- 

 quake of 1755 ; that shaken by the great Assam (India) earthquake 

 of 1897 was 1,750,00x5 square miles, of which 150,000 were laid in ruins. 

 An earthquake in 1891 shook three fifths of the entire area of Japan. 



