Challenges for the Next Generation of Seismic Codes - Earthquake Engineering for Structural Design - page 494

Another important effect of near-source earthquakes is the influence of the
superior vibration modes , especially for short pulse periods, introducing
irregularities in the bending moment diagrams, especially at the middle high of the
frame (Fig. 10.18a). The situation induces a strong reduction of the available
ductility in the middle or top part of the frame. As the required ductility has a
maximum just in these places, the local collapse of the building may occur due to
insufficient ductility. This was a common phenomenon during the Kobe
earthquake, where many buildings were damaged just in the middle stories. In case
of large pulse periods, the reduction of available ductility occurs at the lower
levels, producing a global collapse (Fig. 10.18b).
Due to the characteristics of the pulse seismic actions, developed with great
velocity, and especially due to the lack of important restoring forces, the ductility
demand could be very high, so the potential of the inelastic properties of structures
for seismic energy dissipation has to be carefully examined. The short duration of
the ground motions is a very important factor. A balance between the severity of
the ductility demand, due to the pulse action, and the effects of the short duration
must be seriously analyzed.
These aspects have an important consequence on the establishment of the
reduction factor, because, due to short duration, the structure has not time enough
to dissipate a corresponding input seismic energy. Therefore, these factors must
have different values in case of near-source ground motions, in comparison with
the most common far-source cases.
10.6 STRUCTURAL RESPONSE TO INTRAPLATE CRUSTAL
EARTHQUAKES: LOW-TO-MODERATE GROUND
MOTIONS FOR CRUSTAL FRACTURE
10.6.1 Main Characteristics of Intraplate Crustal Earthquakes
Much attention has been paid to seismic risk for interplate earthquakes and the
results are consistent. In contrast, a relative interest has been shown for intraplate
earthquakes, even if their consequences can be disastrous. As it was presented in
10.4.3, there are two intraplate types: (i) the collision crustal earthquakes ,
produced in the suture zones of the continental plates. These earthquakes have the
characteristic of interplate subduction (thrust), presented in the previous section;
(ii) contrary, the crust fracture earthquakes have very peculiar characteristics, with
very important consequences on the structure responses. In the following, only the
corresponding structure responses are presented
The earthquakes produced by crust fractures, due to their reduced rupture
surfaces, have moderate magnitudes, generally under M 5.5. In very rare cases, a
magnitude higher than M 7.0 has been recorded, but only in some special
conditions (very bad soil or liquefaction phenomena), which can be identified by
geotechnical studies. Due to this fact, the areas, where intraplate earthquakes occur
by crustal fractures, are characterized by low to moderate ground motions.
.