EARTHQUAKES 281 



Summary of the Causes of Earthquakes. — The earth may be 

 caused to tremble in many ways. (1) Severe earthquakes have been 

 produced by volcanic eruptions, but the disturbances thus caused 

 are confined to comparatively small areas, as they are the result of 

 steam explosions and of the fracturing of the rock as lava rises in the 

 earth's crust. 



(2) The falling of the roof of a cave may produce a jar which will 

 cause some damage. The earthquake shocks at Visp, Switzerland, 

 which fissured buildings and caused landslides, were due to the col- 

 lapse of cavern and tunnel roofs, and the earthquakes which are of 

 frequent occurrence in the Karst region (p. 72) on the east coast of 

 the Adriatic are of this origin. The jar produced by the fall of an 

 overhanging rock which formed the brink of a fall has been sufficient 

 to break windows several hundred yards distant. 



The above causes (1) and (2) are unimportant, and their effect 

 is small. 



(3) The great earthquakes of the world are a result of the fracturing 

 of the rock of the earth's crust, or of the vibrations produced during 

 faulting, (a) They may be due to the jolting of earth blocks whose 

 movement begins and ends suddenly ; and also when thick delta 

 deposits suddenly slump an earthquake may be produced, (b) They 

 are also due to the vibrations produced during faulting by the friction 

 of one block as it rubs against another. This method may be il- 

 lustrated by rubbing the closed fist on a table, or by rubbing two 

 blocks of wood together, (c) They may be produced by a simple 

 breaking of the rock. It has been suggested that some at least of 

 such fracturing " may have relation to sudden deformation by rock 

 flowage." (Leith.) x 



It is evident from the above that great earthquakes are most 

 likely to occur in growing regions ; for example in young mountains, 

 where the strains have not yet been relieved. 



Displacements. — The amount of the movement of the earth along faults in the 

 production of earthquakes varies greatly. After the California earthquake (1906) it 

 was found, as already stated, that the movement was horizontal and varied from 8 to 20 

 feet. The movements of the crust in the Sumatra (East Indies) earthquake of 1892 

 were also horizontal, the total slip of the fault amounting to from 11 to 13 feet. No 

 trace of the fault was visible at the surface, the proof of the movement being fur- 

 nished by geodetic measurements. Vertical movements are perhaps more common 

 than horizontal, although they are usually accompanied by some horizontal movement. 

 The Japanese earthquake of 1891, for example, was produced by a fault which has been 



1 Leith, — Structural Geology, 1913, p. 69. 



