Challenges for the Next Generation of Seismic Codes - Earthquake Engineering for Structural Design - page 496

The depth of crustal fracture sources lays in the category of surface earthquakes.
Therefore, the effects of near-source ground motions, presented in the previous
section, are valid also here, but with reduced magnitude and some specific features,
different from the interplate sources. In the following the main differences will be
emphasized.
10.6.2 Elastic Spectra
In regions of low to moderate seismicity, local data are very scarce and,
consequently, it is difficult to obtain reliable data necessary to establish proper
elastic spectra. In this situation, the theory of multi-source data fusion, presented in
Section 4.6, can be very useful. Therefore, the data from analogous regions with
similar geological and seismo-tectonic features must be used for regions lacking in
earthquake records (Chandler et al, 2001).
The spectra for crustal fracture earthquakes are very different in comparison
with the spectra of interplate earthquakes. A comparison among the corresponding
spectra in the US Eastern territory (intraplate earthquakes) and Western territory
(interplate earthquakes) is presented in Figure 10.19. One can see that the
difference consists in an important amplification of peak acceleration for very short
periods (0.1 to 0.2 sec).
In Europe, a very significant situation is the case of Portugal, Italy and Greece,
where the two source types, both interplate and intraplate, are present. In Portugal,
the first type, located offshore, corresponds to the boundary between the African
and Euroasian plates and it is capable of generating large magnitudes earthquakes
(M 8.5), as well as effects at large distance from epicenter, long duration and long
periods. The second type corresponds to seismic source located inland, capable of
generating smaller, but still significant magnitudes (M 7.0), with effects at short
distance, short duration and short periods (Azevedo and Guerreiro, 2007). In Italy
and Greece, the interplate earthquakes are situated in Southern zones, due to the
subduction of African plate under the European plate. The rest of the territories are
subjected to intraplate earthquakes (mainly due to collision).
Earthquakes occurring in Australia are the result of intraplate movements
(Chandler et al, 1992). The spectra for some Australian and Eastern Canadian
earthquakes are plotted in Figure 10.44. All these spectra show very important
amplifications for very short periods. Nevertheless, the influence of soft soil
conditions is very important, reducing the peak amplification and increasing the
pulse period (Fig. 10.45). This aspect was present in many recorded earthquakes in
low and moderate seismic areas.