Advances in Conception about Earthquakes - Earthquake Engineering for Structural Design - page 133

Earthquakes have killed more than 3 million people during the period from
1900 to today. Earthquakes are so deadly because they strike without warning.
When more reliable methods for predicting earthquakes can be found, earthquake
warnings can be given in advance, saving human lives (Gray, 1996). There is a
great interest to predict location and time of large earthquakes, in order to mitigate
their devastating potential. A great deal is known about where earthquakes are
likely, due to the fact they occur in predictable areas such as at tectonic plate
boundaries. Contrary, there is not a current reliable way to predict the days or
months when an event will occur in any specific location and also the most
important aspect, the predicted magnitude remains an open question, about which
there are many debates among the specialists.
The Earthquake Prediction intends to forecast an earthquake of a given
magnitude range, occurring in a localized region and at a specific time window
(Wikipedia, nd). Earthquake prediction at the present time is not an exact science
and the forecast of the earthquake occurrence cannot be very accurate. There are
many controversies in trying to predict an earthquake, due to some successful
cases, followed at the same time by a greater number of unsuccessful cases of
failure in determining the occurrence of strong earthquakes.
Among the successes, one must mention the well-known earthquake prediction
of the 1975 earthquake of Haicheng, a very densely populated town in China,
where an evacuation warning was issued the day before the earthquake of M 7.3
magnitude. As expected, because of the great magnitude and the presence of a lot
of non-engineered constructions, the damage in Haicheng and in the neighboring
area was extensive, but very few lives were lost. The prediction was based on a
number of geophysical observations of precursor events as well as the abnormal
animal behavior. The increasing in foreshock activity triggered the evacuation
warning. This success gave hope that earthquake prediction was eventually
possible.
Unfortunately, most earthquakes do not have such obvious precursors. In fact,
one year after, the 1976 Tangshan earthquake with M 8.0 magnitude, with the
highest number of deaths from an earthquake last century, suddenly reduced the
enthusiasm of Chinese scientists. Although precursor events were observed as well
as geophysical and geochemical anomalies were detected, these precursor events
occurred over a very widely spread area, making it extremely difficult for scientists
to focus on a particular region and, therefore, to issue a short-term prediction or a
warning. There were some remarkable differences between the particular precursor
events of the Tangshan earthquake and those of other predicted earthquakes.
Furthermore, no significant foreshocks were observed. With this earthquake, the
hope in accurate predictability of earthquakes evaporated, showing that there is a
long way to setup a sure methodology to predict earthquakes.
The above two examples give a measure of the validity of the earthquake
prediction at the present time. However, some improvement is now in progress for
obtaining better predictions. After these two earthquakes, the main obvious lesson
is that there are some earthquakes which can be predicted and others not. The
target of scientists is to prepare adequate methodologies for both cases.