EARTHQUAKES 



291 



as a result of an earthquake shock. It is evident that the cause of 

 this disarrangement of the underground water is the opening and 

 closing of fissures 

 leading to water- 

 bearing strata or 

 joints, and to fault- 

 ing which may open 

 a water-bearing 

 stratum to a fissure. 



It is not unusual 

 to find sand or mud 

 cones and " crater- 

 lets " after earth- 

 quake shocks (Fig. 

 289). These are 

 formed by jets of 

 water which were 

 forced through fis- 

 sures during the 

 disturbance. The 

 water forming these 

 jets originated in a 

 water-bearing stra- 

 tum or in water-bearing strata, or in fissures and caverns. 



Gases. — Gases, usually containing large amounts of sulphureted 

 hydrogen (H 2 S), are also sometimes discharged, with or without 

 water. These gases were imprisoned in the soil and escaped either 

 as a result of the Assuring of the ground or by being forced out by the 

 shaking together of the loose material. The sulphureted hydrogen 

 was doubtless formed, for the most part, by the decomposition of 

 animal and vegetable matter in the soil, just as is that which rises 

 from the mud on the bottom of ponds when it is stirred with a stick. 

 The escape of the sulphureted hydrogen was especially noticeable in 

 the Mississippi Valley and Charleston earthquakes. 



Construction of Buildings in Earthquake Regions. — A study of 

 the effects of earthquakes on buildings has led to certain recommenda- 

 tions concerning the location and construction of houses in earth- 

 quake regions. (1) Artificially filled ground and deep alluvial soils 

 should be avoided, since these are likely to be badly fissured and are, 

 moreover, thrown into large waves by a shock. (2) A firm and 



Fig. 



— Craterlet formed during the Charleston 

 earthquake. (U. S. Geol. Surv.) 



