7.2 
ECONOMIC GEOLOGY 
EcoNoMIc SEISMOLOGY 
Earthquakes are of interest to academic geology from the 
light they throw on the internal structure of the earth. 
They concern economic geology as regards building design, 
the recognition of situations liable to them, and their possible 
prediction. 
A. EarTHQUAKE ProBaBILITY AND PrEDIcCTION—The 
imminence of earthquakes may ultimately be foreseen from 
their distribution in time and space, or observation of their 
causes. A great shock results from the release of the crust 
from gradually increasing stress, which might be recognized 
by slight movements before the catastrophe. The slow 
tilting of a waterpipe across a line of suspected movement 
might give warning of an approaching shock. 
Earthquakes are most numerous in winter when atmos- 
pheric disturbances are usually most violent, and the breaking 
of a strained belt in the crust may be precipitated by a sudden 
change in air pressure. Thus the earthquake which devastated 
Yokohama and Tokyo in 1923, occurred during the passage 
of a violent cyclone which cannot have caused, but may have 
hastened the disaster. 
A long period without serious earthquake shock need not 
imply permanent immunity. The eastern coast of North 
America experiences severe shocks at distant intervals, such 
as those at Charleston in 1886, and on the Lower St. Law- 
rence in 1925; and any part of that line may be badly 
shaken after centuries of rest. 
A map of the world showing the distribution of earth- 
quakes shows that they are most frequent in areas of recent 
earth-movement, and along persistent faults. 
Earthquake warnings may be of great service since 
secondary consequences that might be avoided if foreseen 
are often the most disastrous. The Lisbon Earthquake of 
1st November, 1755, was not of the greatest severity, for 
parts of the city built on limestone and basalt were not 
damaged (Sharpe, Proc. G. Soc., 1838, No. 60, p. 36). The 
heavy death-roll was due to a wave 50 feet high, which swept 
up the Tagus and drowned the 60,000 people who had taken 
shelter on the mole. According to tradition the mole was 
swallowed by an unfathomable abyss: but probably its