7he Days of a Man D906 



Again, the tectonic earthquake produces two very 

 different sets of phenomena — the one the break or 

 "faulting" in which disturbance centers, the other 

 the spread of interfering waves set in motion by the 

 parting and grinding of sundered rock walls in the 

 fault. It is the conflict of jarring waves in widening 

 and diverging circles which does the harm to man 

 and his affairs, but the shifting of the mass starts 

 them on their mission of destruction. 

 jiarth- Every tectonic earthquake implies some fault or 



quake rifts j-jft^ with somc sort of displacement, permanent or 

 temporary, in the relation of the two sides. In ex- 

 treme cases the break extends miles in a straight line, 

 tearing up through the surface soil and passing down- 

 ward to a depth only to be guessed at, probably as far 

 as the crust is rigid. Moreover, in all severe dis- 

 turbances of this sort, subsidiary changes which have 

 no direct connection with the fault itself take place. 

 These slumps or landslides signify but little geologi- 

 cally; they simply mean that loose soil has been shaken 

 down by the shock. The true earthquake rift moves 

 on in straight lines, broadly speaking, careless of 

 topography, though topography is necessarily modi- 

 fied by it. For on either side, it may be hundreds of 

 feet, rock is crushed to flinders by the impact and 

 grinding of sundered walls. 



An old fault is consequently marked by excess of 

 erosion. Streams choose it for their basins, and where 

 it crosses a mountain the softened strata yield to form 

 a saddle or other depression. Valleys resulting from 

 it are fertile and well watered, and often marked in 

 California by dairies and reservoirs. For the most 

 part, also, in much-faulted regions such as rim the 

 Pacific, each new rift follows the line of an old fault, 



c 180 :i 



