Faults and Earthquakes - Earthquake Engineering for Structural Design - page 179

Chapter 6
Faults and Earthquakes
6.1 ANATOMY OF AN EARTHQUAKE
6.1.1 Uniqueness of Earthquakes
An earthquake is a phenomenon resulting from the sudden release of stored energy
in the Earth's crust which creates seismic waves. At the Earth's surface,
earthquakes may manifest themselves by a shaking or displacement of the ground
and sometimes tsunamis, which may lead to loss of life and destruction of property
(Wikipedia, nd).
The great number of observations and ground motion records for past
earthquakes worldwide, together with ground response analyses, have indicated
that this tremendously complicated natural phenomenon of earthquakes is very
difficult to understand. Despite the considerable amount of data available so far
and a relevant literature on these topics, nobody can say with enough confidence
that exact knowledge and rigorous statement exist about these events which occur
below the Earth's crust due to unforeseeable forces at an unpredicted time.
Each earthquake is unique, being the result of the effect of many factors
(defined by type and value), influencing the ground motions and generating very
complex phenomena. Examining the recorded ground motions, a chaotic
movement can be observed, without showing any rule at first glance. But, like a
cardiologist who understands the heart movements looking at the cardiogram, the
seismologist can detect the anatomy of an earthquake. So, contrary to the first
impression, the ground motions are the result of the overlapping of the effects of a
limited number of basic factors, having different importance and giving their
variety in the recorded movements. Due to the fact that an interaction between
these factors exists, this complex aspect of ground motions can be identified.
The human and economic losses are generally considered as the main
characteristic of an earthquake, which is classified according to its magnitude; but
we can observe that this aspect cannot only be considered as a representative
factor. In fact, there are some cases in which very strong earthquakes in magnitude
have not produced important losses, like for example the 1960 Chile earthquake,
with magnitude 9.5, and the 1964 Alaska earthquake, with magnitude 9.2, because
they both occurred in zones with low population density. Contrary there are other
cases, in which even moderate magnitude earthquakes caused very large damage
which was foreseeable, because they occurred in urbanized zones: the 1994
Northridge earthquake, with magnitude 6.7, the most damaging earthquake in the
US history, and 1995 Kobe earthquake with magnitude 6.9, the most devastating
earthquake in world history. Considering these different events, one can look to an
earthquake as a local phenomenon having some distinct features, which differ from