Earthquakes and Ground Motions - Earthquake Engineering for Structural Design - page 282

some exceptional cases the magnitude can be higher (Pezeshk, 2004, Gioncu,
2006).
- The same observations result by examining the European earthquakes recorded
in the so-called stable regions. As documented in the historical records (Ambraseys
and Bommer, 1991, Gioncu and Mazzolani, 2002), these regions usually
experienced earthquakes with magnitude under M 6 and only in a relatively small
number of cases a magnitude M 6.5. The majority of earthquakes have magnitude
under 4.5. But this observation does not exclude the possibility that some more
important earthquakes occurred in the preceding millennia (Woo, 1996). This
remark is also valuable for the future.
- The rare occurrence of severe seismic loading at a site situated in “Stable
continental regions” suggests that the philosophy for the design of buildings must
be different from the one used in areas where interplate earthquakes occur.
- Buried thrust or rift fault types, caused by compression or tension of the crust,
produce the great majority of intraplate earthquakes. Only in very rare cases do
these earthquakes result from a strike-slip fault. Local factors such as the strength
of crustal rock, stress concentration, stress drop and level of stress play a dominant
role in the earthquake process (Jankulovski et al, 1996).
- The duration of intraplate earthquakes are shorter than the one of interplate
earthquakes. Intraplate records show that the strongest ground motions last just for
a few seconds. The motions start with one or two relatively large amplitude cycles,
followed by 5 or 6 cycles of reduced amplitude, then rapidly decay towards the
end.
- The ground motions for intraplate earthquake are characterized by higher
frequency content (which corresponds to short natural vibration periods) and very
short pulses (Chandler et al, 1992). Therefore, the shapes of interplate and
intraplate earthquakes are very different (Fig. 7.57).
Figure 7.57 Normalized spectra for interplate and intraplate earthquakes
(Gioncu and Mazzolani, 2002)