Challenges for the Next Generation of Seismic Codes - Earthquake Engineering for Structural Design - page 475

not mean that this value was not overcome in another points of epicentral area. The
velocity spectra for Northridge earthquake shows velocities exceeding 350cm/sec
(Krawinkler and Alavi, 1998). Therefore, the incertitude in this problem is very
high and, for design purposes, a very high velocity must be assumed. Considering
the number of important pulses, generally, this earthquake type is characterized by
one velocity pulse. The period of pulses is very long, the maximum being recorded
during the Chi-Chi earthquake from 6 to 11 seconds. Regarding the spectrum type,
it corresponds to the interplate type (type T1), with moderate amplification and
long corner period (0.7 to 1.2 sec). This earthquake type has, generally, short
duration, with the exception in case of bad soil conditions, when the duration
increases significantly.
Strike-slip crustal earthquakes. In comparison with the subduction faults, the
number of important strike-slip faults is reduced: San Andreas (USA), North
Anatolia (Turkey) and Alpine (New Zealand). But, due to the great earthquakes
produced along the San Andreas and the North Anatolian faults in very urbanized
areas, this earthquake type has been widely studied. The recorded magnitudes were
not as strong as for subduction earthquakes, but the spectra show that the velocities
can exceed the ones of subduction earthquakes. For instance, the spectra for for
Loma Prieta, can exceed 500cm/sec, and for Kobe, even over 650cm/sec
(Krawinkler and Alavi, 1998). The near-source effects are very important, due to
the special characteristics of ground motions: large normal components, the
possibility of two or three rupture sources (as in the Kobe earthquake), forward
directivity effects, very long velocity pulse period, long duration and important
effects of site soil conditions. The spectrum corresponds to the T1 type. Due to the
particularities of this earthquake type, it cannot be used as background for the
elaboration of codes in zones having other earthquake types .
10.4.3 Intraplate Crustal Earthquakes
There are two intraplate earthquakes:
Collision crustal earthquakes, produced by the suture of two ancient tectonic
plates, such as the ones occurring along the Pyrenees, Alps, Apennines and
Himalayan Mountains. The characteristics of ground motions are the same as those
for subduction, with the exception of the magnitude, which the maximum recorded
values not exceeding M 8.0.
Crust fracture earthquakes. This earthquake type has the most large spreading
in the world, occurring in the areas considered stable, but able at any time to
produce low to moderate magnitude ground motions (the maximum values, in
normal soil conditions, do not exceed M 6.0). Many countries located in these
regions are still in process to elaborate provisions for the seismic protection of
buildings, which need to be very different from the ones of high seismicity regions.
Due to the crustal position of source, the near-source moderate effects occur, with
moderate velocity pulse of one, maximum two, cycles. The periods of pulse are
very short and the corresponding spectrum is T2 type. The duration in normal
conditions is very short, but the influence of soil conditions is very high, increasing
both magnitude and duration of ground motions.