282 



PHYSICAL GEOLOGY 



traced 40 miles, on one side of which the ground sank from 2 to 20 feet, while on the 

 other side (the east) the wall of the fissure was moved 13 feet northward at the same 

 time. The portion of the Alaskan coast affected by the earthquake of 1899 was found 

 to have been displaced vertically in amounts varying from zero to 47 feet, the average 



being between 5 and 12 

 feet. The great earth- 

 quake of Owen's valley, 

 California, in 1872, was 

 produced by a fault which 

 has been traced 40 miles, 

 whose vertical displace- 

 ment at the surface 

 (throw) was from 5 to 20 

 feet. If a fault caused 

 the Charleston earthquake 

 (Fig. 280), no evidence of 

 such movement appears 

 at the surface. 



Fig. 280. — Diagram showing the absence of surface 

 evidence of a fault, because of the presence of a thick, 

 unconsolidated bed of sand above the solid rock. 



It is sometimes 

 found that the dis- 

 placement along a 

 fault not only varies 

 in amount at different places, but also that along the same fault 

 the downthrow side in one portion is on the right, for example, 

 and at another on the left. Such a fault is called a hinge fault. 



Many earthquakes originate beneath the sea, some of which have 

 been very destructive. Off the coast of Greece the telegraphic 

 cable broke at the moment of an earthquake in 1873, and upon sub- 

 sequent examination it was found that the break was seven miles 

 from land, and that the water which formerly had been 1400 feet deep 

 at this spot was 2000 feet in depth after the shock. Submerged preci- 

 pices 3000 to 5000 feet high occur in this region and are doubtless 

 fault scarps whose formation caused many earthquakes. Many 

 records are extant of vessels which were made to vibrate by submarine 

 earthquakes, to such a degree that the crew thought that they had 

 struck a reef; loose objects rattled about, and in some cases men 

 were thrown to the deck by the violence of the shock. 



Depth of the Plane or Point of Origin. — Wherever it has been 

 possible to determine the direction of the emergence of the waves of 

 greai earthquakes, it has been found that they converge at a depth 

 of less than 12 miles and usually less than 5 miles; that is within 

 the zone of fracture. The point or place of origin is called the focus. 



