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PHYSICAL GEOLOGY 



stable foundation is of paramount importance, and particularly on 

 soft and " made " ground. (3) Low structures, especially when well 

 braced, with the beams and rafters attached firmly to the walls, are 

 most desirable, because if a building does not vibrate as a whole, the 

 parts act as battering-rams to throw over or break the walls. (4) 

 Since the fires which almost invariably accompany earthquakes are 

 often more destructive than the earthquakes themselves, it is im- 

 portant that there should be ample fire protection. It is estimated 

 that had the buildings at Messina been properly constructed at the 

 time of the earthquake in 1908, 998 deaths out of every thousand 

 would have been prevented. 



Effect of Earthquakes on the Sea. — One of the most disastrous 

 effects of earthquakes on low coasts is produced by the great sea waves 

 (tsunamis) which sometimes follow the shocks. After the first severe 

 trembling which shook Lisbon in 1755, the sea retreated from the 

 shore, laying bare the bottom of the harbor, and then returned in a 

 wave 60 feet high which completed the devastation of the city. This 

 wave was destructive along the coasts of Portugal and Spain and was 

 felt on the coasts of countries far distant. A great sea wave cost 

 the lives of 27,000 people in Japan in 1896. 



The velocity of great sea waves and the distance to which they are propagated is 

 well-known. In the Japanese earthquake of 1896 the wave which reached Honolulu, 

 3500 miles away, was 8 feet high at that place, and its mean velocity between these 

 points was 681 feet a second. It was also recorded at San Francisco, to which point 

 its mean velocity was 664 feet a second. The great sea wave from an earthquake in 

 Peru, South America, in 1868, reached Honolulu, 5500 miles away, in 12 hours, and 

 Japan, over 10,000 miles away, the next day. Because of their great wave length 

 (sometimes 200 miles), great sea waves may not be sensible to vessels in mid-ocean and 

 are never destructive until they reach a shallowing shore. 



Great sea waves are apparently not all due to the same cause. 

 Some are probably produced by a sudden depression of a portion of 

 the ocean bottom by faulting and a consequent drawing in of the ocean 

 water. This causes the withdrawal of the water from the land, and 

 the wave set in motion by the meeting of the water then spreads in all 

 directions, devastating low-lying coasts. A sudden shock on the sea 

 bottom is probably also competent to give rise to a great sea wave. 

 Explosions of submarine volcanoes set waves in motion which may 

 work great havoc on low coasts. 



Evidence that a Region has been Free from Severe Earthquakes. 

 — It is not always possible to tell whether or not a region has been 





