Learning from Earthquakes - Earthquake Engineering for Structural Design - page 73

about the cause of earthquakes varies between mythology and natural phenomena.
In the early period (1910-1950), the causes were related to the presence of faults
and the theory of tectonic plates of Wegener was elaborated and confirmed by in
situ observations. During the maturated period (1950-1975) the development of a
world network of instrumentations and the scientific examination of produced
earthquakes began. The modern period, after 1975, when the large network of
instrumentations and the use of GPS allow studying the tectonic plate movement
and the effect of these movements on the Earth, gave the possibility to understand
the rupture mechanism and to study the different earthquake types.
The post-earthquake examination of damaged buildings has succeeded during
the same periods, from primitive to modern methodologies. In the last period it is
recognized that, although there are tragic aspects, a damaging earthquake provides
a unique laboratory for multi-disciplinary studies to document, understand and
explain what happened. Such studies are beneficial to minimize the potential
impacts of future earthquakes and to provide information, which can be transferred
to other communities, in order to mitigate seismic risk (Hays, 1996). This
information is derived from seismological, geotechnical, architectural, structural
engineering and socio-economical studies. The result of these multi-disciplinary
investigations has provided critical data, showing the vulnerability of a community
versus an earthquake, depending on physical factors (magnitude, focal depth,
proximity to the urban center, directivity, soil properties, etc.), as well as social
factors (degree of prevention, mitigation, preparedness measures, etc.). Each
damaging earthquake provides unique experiences and data, increasing the basic
understanding of the earthquake complex phenomena and the social dimensions of
an earthquake disaster.
By post-earthquake investigations of damaging earthquakes produced during
both historical and modern periods, some very important lessons can be learned.
These lessons refer to the earthquake hazard, construction vulnerability and
mitigation of seismic risk.
3.13SeismicRiskMitigation
Unfortunately both scientists and engineers have no means to warn of or to
accurately forecast an earthquake; the only possibility they have is to try to
mitigate its consequences.
The last important earthquakes produced both in old and new urban areas have
demonstrated that the social and economic impact of building damages, loss of
functions and business interruptions were very huge, stressing the necessity to
develop a transparent seismic design methodology to be applied to build new safe
constructions, as well as to strengthen existing ones (Fajfar and Krawinkler, 1997).
In the past, for a long time, among the builders there was a defeatist attitude to
consider an earthquake as a fatal force, not possible to resist. After the great 1906
San Francisco earthquake, Professor Charles Derleth (University of California)
wrote (Roesset, 1992):