Earthquakes and Ground Motions - Earthquake Engineering for Structural Design - page 275

- Amplification of horizontal accelerations (Fig. 7.52) is 0.4 for thrust
earthquake, at 2.6 sec period, and 0.82 for strike-slip, being maximum at
around 3 seconds. In both cases, the amplification is more severe in site S1
than in site S2, due to the effect of directivity.
Considering all these aspects, a very important conclusion results: in
comparison with the thrust earthquakes, the strike-slip earthquakes are the most
damaging, explaining the dramatic effects of the strike-slip 2004 Kobe earthquake,
in comparison with the effects of the thrust 2004 Northridge earthquake.
7.6 GROUND MOTIONS PECULIARITIES OF CRUSTAL INTRAPLATE
EARTHQUAKES
7.6.1 Intraplate versus Interplate Earthquakes
The intraplate earthquakes occur in the so-named Stable continental regions , away
from the boundaries between tectonic plates. The occurrence of these earthquakes
apparently violates the plate tectonic model, which considers that the earthquakes
occur only at these boundaries. Unfortunately, the design for earthquake resistance
has only recently been required by codes in regions of low-to-moderate seismicity,
contrary to the areas of high seismicity. The research works on the seismic
response of structures are mainly concentrated on the cases of recent earthquakes
that have occurred in zones with important seismicity, where the seismic activity is
very frequent and can strongly affect human lives. Therefore, such research is not
very interested to study the moderate seismicity effects.
There are many zones in the world where the earthquakes do occur, but either
rather infrequently or with a moderate intensity, so that they tend to be perceived
more as “accidental” than regular (Pinto, 2000). In Europe, the crustal intraplate
earthquakes are the most frequent and this finding is very important, due to their
particularities. The most frequent ground motions have magnitudes from 4 to 5
(Fig. 7.53a), framing into the category of low-to-moderate earthquakes . The
majority of the records are from 0 to 25 km, 60 percent of them being in the range
from 4 to 14 km (Fig. 7.53b). Intermediate earthquakes occur in Romania, Greece
and Italy only (Gioncu and Mazzolani, 2002).
Figure 7.54 reports the histograms of seismic intensities of all the seismic
events in Germany, Switzerland and Greece, updated to 1985 (Pinto, 2000). All
these countries are situated in diffuse seismic zones, but Germany is far from the
collision seismic zone of the Alps, Switzerland is near to this zone, and Greece is
situated in a special diffuse zone, having various fault types. The number of events
is very small for Germany, greater for Switzerland, but reduced in comparison with
Greece. The maximum earthquakes' intensity in Germany and Switzerland is about
VI to VII, while for Greece the intensities reach IX. Therefore, the comparison
with Greece clearly suggests that in the former two countries the seismic hazard
should be treated in a different way than in Greece.