Advances in Conception about Earthquakes - Earthquake Engineering for Structural Design - page 131

time was very short for the accumulation of the plate flexion. The absence of
vertical blow explains the lack of tsunami at practically the same earthquake
magnitude
This is an example showing the great influence of initial conditions on the
development of an earthquake.
The earthquakes are produced by the local sudden movements of some tectonic
plates which belong to a very complex system of world tectonic plates. The
behavior of the Earth crust belongs to a nonlinear dynamic system, with many
similarities with the atmospheric dynamics. If for this climate problem one must
admit the limitation of the variability statistic, the same restriction must be
considered for seismic problem modeling.
The earthquake starts in an existing fault due to the slipping of the two borders
or for a new rupture. The source is characterized by the fault type, depth and area
of slipping or rupture. The main characteristic of the movement of the tectonic
plates is the shock acting as an impulse. Unfortunately, it is not possible to record
the event directly at the source. But it is clear that the restoring of the new
equilibrium produces some oscillations, with properties depending on the
characteristics of the source. They are longer with two or more important pulses in
case of slipping or with only one pulse in case of rupture. The source movements
generate the waves, which cross the crust and the site soil. The characteristics of
source, traveled path and site characteristic form the initial conditions of soil
vibrations. In accordance with the Theory of Chaos, the chaotic ground motion,
being the result of a nonlinear system very sensitive to the differences in these
initial conditions, will present a great variability of the main characteristics.
A very significant case for this variability is the Vrancea (Romania) seismic
zone, where frequent intermediate earthquakes (depth between 90 to 160 km) occur
practically in the same place. The last very important ones happened on 4 March
1977, M = 7.2, 16 August 1986, M = 5.0 and 30 August 1986, M = 7.0, 30 May
1990, M = 6.7 and 31 May 1990, M = 6.1 (Pustovitenko et al, 1994). The different
positions of epicenters of these earthquakes were situated at some kilometers
distance from each other, but gave rise to very different rupture directions. The
1977 and 1986 earthquakes were oriented mainly in the Southwest direction, while
in 1990 the rupture orientations were to Northeast. The most important records of
these earthquakes were registered at the INCERC in Bucharest, situated at 160 km
from the source, on the site with very bad soil conditions. Figure 4.19 presents the
spectra respectively obtained for these events on the same site (Gioncu, 1995). One
can see that the soil conditions did not have very important effects during low
magnitude earthquakes, the maximum spectral accelerations being obtained for T <
1.0 sec. Contrary, the characteristics of spectral accelerations for the 1977 and
1986 earthquakes, having a greater magnitude than the other ones, were completely
changed, a very important amplification being obtained for T = 1.6 sec. The 1986
and 1990 earthquakes were very similar in magnitude (M = 7.0 for 1986 and 6.7
for 1990), but some differences in initial conditions dramatically changed the
ground motion effects. Thus, in the light of Theory of Chaos, one can undersand