534 GEOLOGY. 



The Geologic Effects of Earthquakes. 



Earthquakes are of much less Importance, geologically, than many 

 gentler movements and activities. Disastrous as they sometimes are 

 to human affairs, they leave few distinct and readily identifiable marks 

 which are more than temporary. 



Fracturing of rock. — During the passage of notable earthquake 

 waves, the solid rock is probably often fractured (see p. 509), though 

 where it is covered by deep soil the fractures are rarely observable at 

 the surface. Elsewhere the crevices are readily seen, especially if they 

 gape. In a few instances surface-rock has been seen to be thoroughly 

 shattered after the passage of an earthquake, as in the Concepcion 

 earthquake of 1835.^ Joints which were before closed are often opened 

 during an earthquake. Thus in northern Arizona, not far from Canyon 

 Diablo, there is a crevice traceable for a considerable distance, which 

 is said to have been opened during an earthquake. Locally, it gaps 

 several feet. Other notable earthquake fissures have been recorded in 

 India, ^ Japan, and New Zealand. During a]i earthquake which shook 

 the South Island of New Zealand in 1848, '' a fissure was formed averag- 

 ing 18 inches in width, and traceable for a distance of 60 miles, parallel 

 to the axis of the adjacent mountain chain.^' ^ The development of 

 fractures or the opening of joints is sometimes accompanied by faulting. 

 This was the case in Japan during the earthquake of October 28, 1891, 

 when the surface on one side of a fissure, which coukl be traced for 40 

 miles, sank 2 to 20 feet. In this case there was also notable horizontal 

 displacement, the east wall of the fissure being thrust locally as much 

 as 13 feet to the north.* 



Changes of surface. — Circular surface openings or basins are some- 

 times developed during earthquakes. This was the case during the 

 Charleston earthquake of 1886,^ and similar effects have been noted 

 elsewhere. These openings often serve as avenues of escape for ground- 

 water, gases, and vapor. They are commonly supposed to be the result 



* Darwin. Journal of Researches, 1845, p. 303. 



2 Gldham. Quar. Jour. Geol. Soc, Vol. XXAHLII, p. 257. 



3 Geikie. Text-book of Geology, 4th ed., p. 372. 



< Koto. Jour. Coll. Sci., Japan, Vol. V, Pt. IV (1893), pp. 329, 339. Cited by Geikie, 

 loc. cit/y p. 373. 



^ An elaborate account of this earthquake is given by Dutton, Ninth Ann. Rept., 

 U. S. Geol. Surv., pp. 209-528. 



