Living with Earthquakes - Earthquake Engineering for Structural Design - page 38

Because of the marked loss of life and mass destruction, this earthquake remains
one of the most devastating tragedies from Europe's history. This earthquake is
considered as the birthday for the science of Seismology, due to the first studies
concerning the wave propagation from the source to the site (Agnew, 2002).
A series of three earthquakes centered near NewMadrid,Missouri, US , in 1811
and 1812 marked the strongest intraplate earthquakes to occur in North America
(Wikipedia, nd). Although the magnitude estimates for the pre-instrumentally
recorded earthquakes are uncertain, there is no question that these events were
unique for the Eastern North America and rare elsewhere. Similar-sized
earthquakes are known to have occurred only in China and India. These
earthquakes greatly affected the topography of the North America continent. Lake
St. Francis, 64 km long and 1 km wide, formed when a piece of sunken ground
flooded with groundwater, and additional shifts in surface structure caused the
Mississippi to change its course.

2.3GREATEARTHQUAKESOF 20 th CENTURY

The 20 th Century can be divided into three very distinct periods. The first one (until
1950) is characterized by very timid attempts to develop a consistent explanation
of the nature of earthquakes. During this period no records are available (with the
exception of the historical record of 1940 El Centro earthquake, the first recorded
one). Due to the absence of anti-seismic rules for the protection of buildings
situated in seismic areas, the number of deaths was very large. The second period
(1950-1980), is characterized by a coherent theory development, but due to a
reduced number of instrumented seismic stations, limited information concerning
the characteristics of ground motions and damage exists for these earthquakes. A
dense network of instrumental seismic stations characterizes the last period of 20th
Century, where the developed theory was supported by a large number of records.
At the same time, the tremendous development of Seismology, as a Science of the
Earth behavior, and Engineering Seismology, as a branch of Seismology, devoted
to supply the structural designer with information to be used in practical
application, have had a great influence in the developing of anti-seismic concepts.
The main earthquakes produced in the 20

th

Century are presented in Table 2.4,
where date, places, magnitude, deaths, earthquake type and estimated damage (in
US $) are given.
Examining the earthquakes presented in Table 2.4, one can see that,
fortunately, the very strong events (Chile, 1960, Alaska, 1964, Aleutian Islands,
1957, Kamchatka, 1923 and 1957, etc.) occurred in less populated areas. Also, it is
very clear that the great number of deaths occur in developing countries, while the
great damage is recorded in the developed ones. This means that the sensitivity for
seismic protection and consequently the problems of seismic design are very
different in these countries.